Title page

JNCC Report

Seabird monitoring on Skomer Island in 2016

Edward Stubbings, Birgitta Büche and Elisa Miquel Riera

The Wildlife Trust of South and West Wales

The Welsh Wildlife Centre

Cilgerran, Cardigan

SA43 2TB

Ros Green and Matt J. Wood

School of Natural & Social Sciences

University of Gloucestershire

Cheltenham

GL50 4AZ

[IN PREPARATION]

This document should be cited as:

Stubbings, E.M., Büche, B.I., Miquel Riera, E., Green, R.M. & Wood, M.J. (2016). Seabird monitoring on Skomer Island in 2016. JNCC Report

Skomer Seabird Report 2016: WTSWW & UoG

The Wildlife Trust of South and West Wales and University of Gloucestershire wish to acknowledge the financial contribution of the JNCC Support Co

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ContentsSummary7Weather81Introduction101.1Introduction to capture-recapture survival estimates112General methods132.1Whole island counts132.2Study plot counts of Common Guillemots Uria aalge and Razorbills Alca torda132.3Breeding success143Northern Fulmar Fulmaris glacialis153.1Breeding numbers - whole island counts153.2Breeding success163.3Timing of breeding184European Storm Petrel Hydrobates pelagicus195Manx Shearwater Puffinus puffinus205.1Breeding study plots census205.2Breeding Success245.3Adult survival256Great Cormorant Phalacrocorax carbo276.1Breeding numbers276.2Breeding success277European Shag Phalacrocorax aristotelis287.1Breeding numbers287.2Breeding success288Lesser Black-backed Gull Larus fuscus308.1Methods for estimating breeding numbers308.2Breeding numbers – results308.3Breeding success358.4Adult survival379Herring Gull Larus argentatus399.1Breeding numbers399.2 Breeding success399.3Adult survival4010Great Black-backed Gull Larus marinus4210.1Breeding numbers4210.2Breeding success4210.3Diet study4311Black-legged Kittiwake Rissa tridactyla4611.1Breeding numbers4611.2Breeding success4711.3Timing of breeding4911.4Breeding adult survival4912Common Guillemot Uria aalge5112.1Breeding numbers - whole island counts5112.2Breeding numbers - study plot counts5112.3Breeding success5312.4Timing of breeding5512.5Adult and juvenile survival5613Razorbill Alca torda5713.1Breeding numbers - whole island counts5713.2Breeding numbers - study plot counts5813.3Breeding success6213.4Timing of breeding6313.5Breeding adult survival6414Atlantic Puffin Fratercula arctica6514.1Breeding numbers6514.2Puffin burrow occupancy and breeding success6714.3Feeding rates6814.4Timing of breeding6914.6Breeding adult survival6915 Acknowledgements7116References7117Appendices72Appendix 1Skomer Guillemot Report 2016 for WTSWW72Appendix 2Breeding adult survival rates from capture-recapture analyses73Appendix 3Mean seabird counts by section in 201675Appendix 4Guillemot and Razorbill Population Study Plots 201676Appendix 5Correction factor (k-value) when converting counts of individual Common Guillemots to breeding pairs77Appendix 6Kittiwake productivity78a)ERRATA Kittiwake productivity figure 201578

List of tables

Table 1Summary table6

Table 2Northern Fulmar whole island counts 2004-201614

Table 3Northern Fulmar productivity per AOS on Skomer Island 201616

Table 4Northern Fulmar productivity on Skomer Island 2012-201616

Table 5 Northern Fulmar phenology records 2010-201617

Table 6Manx Shearwater responses to playback in census plots 1998-201621

Table 7Manx Shearwater burrows in census plots 1998-201622

Table 8Manx Shearwater breeding success at The Isthmus study plot in 201623

Table 9Record of Lesser Black-backed Gull systematic counts in sub-colonies31

Table 10Lesser Black-backed Gull counts of AONs in 201632

Table 11Percentage of empty Lesser Black-backed Gull nests counted in May 201633

Table 12Lesser Black-backed Gull empty nests 1998–201633

Table 13Estimated number of Lesser Black-backed Gull fledglings in 201634

Table 14Estimated productivity of Lesser Black-back Backed Gulls in 201635

Table 15Productivity of Herring Gulls on Skomer in 201638

Table 16Black-legged Kittiwake whole island count details 2007-201644

Table 17 Black-legged Kittiwake productivity per AON on Skomer Island – 201645

Table 18 Black-legged Kittiwake productivity on Skomer Island 2011-201546

Table 19 Black-legged Kittiwake phenology records 2010 - 201647

Table 20Common Guillemot whole-island counts 2004-201549

Table 21 Common Guillemot study plot totals 2012-201650

Table 22Common Guillemot productivity on Skomer Island 1989-201652

Table 23Common Guillemot breeding success 2016 (Wick Corner plot)53

Table 24 Common Guillemot timing of breeding 2008-201454

Table 25Razorbill whole island count details 2006-201655

Table 26Razorbill study plot totals 2011-201657

Table 27Razorbill productivity on Skomer Island 201660

Table 28Razorbill productivity 1993-201661

Table 29Razorbill phenology records 2010, 2012–201662

Table 30Spring counts of individual Puffins in 2016: North Haven63

Table 31Maximum spring Puffin counts: Skomer & Middleholm 2004-201664

Table 32Burrow occupancy and breeding success of Atlantic Puffins65

Table 33Feeding rates of Atlantic Puffins, 201666

List of Figures

Figure 1Northern Fulmar breeding numbers 1963-201615

Figure 2Northern Fulmar productivity on Skomer Island 1986-201617

Figure 3Number of responses in breeding study plots 1998-201619

Figure 4Number of burrows in breeding study plots 1998-201620

Figure 5Annual variation in Manx Shearwater breeding success 1995-201624

Figure 6Survival rates of adult breeding Manx Shearwaters 1978-201525

Figure 7Great Cormorant breeding numbers 1960-201626

Figure 8European Shag breeding numbers 1960-201627

Figure 9Lesser Black-backed Gull breeding numbers 1961-201630

Figure 10Percentage of empty Lesser Black-backed Gull nests 1991–201634

Figure 11Productivity of Lesser Black-backed Gulls per AON 1981-201635

Figure 12Survival rates of adult breeding Lesser Black Backed Gulls 1978-201536

Figure 13Herring Gull: Number of AONs 1961-201637

Figure 14Breeding success of coast-nesting Herring Gulls, 1962-201638

Figure 15Survival rates of adult breeding Herring Gulls 1978-201539

Figure 16Great Black-backed Gull breeding numbers 1960-201640

Figure 17Great Black-backed Gull productivity on Skomer Island 1996-201641

Figure 18Great Blacked-backed Gull diet remains within a five metre radius cross-shaped transect around 25 Great Black-backed Gull nests 201642

Figure 19Frequency of occurrence of food items within a five metre radius cross-shaped transect around 25 Great Black-backed Gull nests in 201642

Figure 20Mean number of Shearwater carcasses found within 10m radius of 25 Great Black-backed Gull nests 1959, 1965, 1973, 1992, 2008-201643

Figure 21Black-legged Kittiwake breeding numbers 1960-201644

Figure 22 Black-legged Kittiwake productivity per AON on Skomer Island 1989-201647

Figure 23Survival rates of breeding adult Kittiwakes 1978-201548

Figure 24Common Guillemot numbers (whole island) 1963-201549

Figure 25 Common Guillemot numbers (study plot counts) 1973-201651

Figure 26Common Guillemot breeding success 1995-2014, 2016 (Wick Corner)53

Figure 27 Razorbill numbers 1963-201658

Figure 28 Razorbill study plot count 2016, totals per day.59

Figure 29 Razorbill study plot count 2014 onwards, totals per day.59

Figure 30Survival rates of adult breeding Razorbills 1970-201562

Figure 31Spring counts of individual Puffins in 2016: North Haven64

Figure 32Maximum spring counts of Puffins: Skomer 1989-201665

Figure 33Number of feeds per hour for Atlantic Puffins, 201667

Figure 34Survival rates of adult breeding Puffins 1972-201568

Summary

This document reports on the 2016 breeding season for seabirds on Skomer Island, drawing together the work of The Wildlife Trust of South and West Wales (WTSWW) staff, volunteers, and research institutions including the University of Gloucestershire (coordinated by Dr. Matt Wood) and The University of Sheffield (Prof. Tim Birkhead). The report includes whole island population counts, study plot counts, estimates of breeding success from fieldwork this year, and breeding adult survival estimates from long-term capture-recapture studies. Part of this work is funded by the Joint Nature Conservancy Council (JNCC), part of the UK Government’s Department for the Environment, Farming and Rural Affairs (DEFRA), for the monitoring of Skomer’s seabird populations as a key site for the Seabird Monitoring Programme.

The table below summarises population counts for nine species in 2016 and makes comparison with the previous year as well as giving a five year percentage change.

Whole island seabird population counts for 2016

Table 1Summary table

Species and count units

Totals for 2016

Totals for 2015

% Change from 2015

5 Year % Change

Northern Fulmar (AOS)

675

584

+15.58

+49.01

Great Cormorant (AON)

4

7

-42.86

-

European Shag (AON)

6

4

+50

+20

Lesser Black Backed Gull (AON)

6936

7630

-9.10

-19.75

Herring Gull (AON)

321

377

-14.85

-19.95

Great Black-backed Gull (AON)

108

123

-13.8

+12.5

Black-legged Kittiwake (AON)

1477

1546

-4.46

-7.34

Common Guillemot (IND)

*

23746

-

-

Razorbill (IND)

7250

7489

-3.19

+45.85

Atlantic Puffin (IND)

22539

21349

+5.57

+96.04

Count units used in this report

AOS-Apparently Occupied Site

AON-Apparently Occupied Nest

AOT-Apparently Occupied Territory

IND-Individual

*Not censused in 2016 (see 2.1 Whole island counts)

Northern Fulmar: 675 AOS represents a 15.58% increase on the previous year and brings the population nearly back up to the historical highs of the 90s and early 2000s. Productivity was also up from 0.35 to 0.43 chicks per AOS.

Manx Shearwater: Within the shearwater census plots the number of responses was up slightly on 2015 as were the total number of burrows. Breeding success in 2016 was 0.63 per pair in the Isthmus study plot, slightly above the 20 year average, while adult survival over the winter 2014-15 dropped sharply to just 71% compared with the average of the previous ten years of 87%.

Lesser Black-backed Gull: 2016’s estimate of 6,936 breeding pairs is 9.1% lower than 2015. This is a 66% decrease since the population was at its peak in 1993, returning to levels last seen in the mid-1960s. Productivity was just 0.36 chicks per breeding pair. Adult survival, at 81%, dropped nearly 9% on the ten-year average.

Herring Gull: Numbers are at an all-time low at 321 AON, a 14.85% decline on the previous year, whilst productivity was also 25% down on the previous year at 0.52. Survival of breeding adults remains low at 80%.

Great Black-backed Gull: Great Black-backed Gull numbers were down slightly on the previous year (13% down at 108 AONs) as was productivity at 1.44 chicks per AON (12% down on 2015).

Black-legged Kittiwake: This year’s count of 1,477 nests is a drop of 4.5% on the previous year and leaves cause for concern. Productivity was 0.65 chicks per AON, which is 15% lower than 2015. Survival was about average since 1978 at 80%.

Common Guillemot: No whole island count of Common Guillemots was made in 2016. However, study plot counts, which are thought to be representative of the whole island population were 3.68% higher than last year’s plot counts at 7,097.8 IND. Productivity of Common Guillemots within plots monitored by WTSWW was 0.63 and was 0.89 at the study site at the Amos (Sheffield University).

Razorbill: A mean total of 7,250 IND was counted in 2016 (3.19% less than 2015). Adult survival of 95% the previous winter (2014-15) shows this species recovering from a sharp drop in survival seen after the seabird wreck of 2013-14.

Atlantic Puffins: 22,539 IND Atlantic Puffins were counted in spring 2016 which is an increase on the previous year (+5.57%) and the highest total since current records began in 1988. Feeding rates were similar to 2015, with productivity bouncing back to 0.78 chicks fledged per occupied burrow in 2016, 12% higher than that of 2015, and much higher than 2014 following the severe winter storms of 2013-14. Survival of breeding adults (2014-15) was 0.905.

Weather

Yet another stormy winter with back to back storms – Storms Gertrude, Henry, Imogen, Jake and Kate all arriving in the new-year between January and March. The rest of spring saw more unsettled weather (although not as severe) which continued into June. The next spell of rough weather was towards the end of August by which time most birds had fledged.

March – Strong winds on the 2nd and 9th and plenty of rain at the start of the month, turning more settled towards the end of the second week, becoming changeable again during the last week. Mean temperatures and rain fall around the seasonal average.

April – The month again began with strong winds and rain, turning more settled with some warm and sunny spells. Wintery showers then fell on the 28th and 29th. Temperatures and rain fall again average for the time of year.

May – May started wet and windy with westerly winds, and there was another unsettled interlude between the 17th and 22nd. Otherwise it was a generally warm month with frequent easterly winds, and plenty of dry sunny weather, but also some thundery downpours.

June – The month started warm and dry but became unsettled, often with low pressure dominating, from the 10th. Winds were between north-west and north between the 15th and 18th. Mean temperatures provisionally 1.3C above the long term average, but the positive anomaly was much larger by night than by day. Rain fall was almost double the long term average.

July – The month began breezy and showery with westerly winds dominant, turning warm and sunny between the 17th and 23rd (being particularly hot on the 19th), then becoming unsettled again during the last week.

August – Began wet with low pressure in charge with particularly strong south westerlies and rain on the 19th and 20th (force 6 and 8 respectively). There were some more significant settled spells during the second half of the month.

September – September began unsettled, turning warmer and more settled in the second week before returning to more unsettled weather during the second half of the month.

October – Almost completely dominated by easterly winds and settled sunny weather.

1Introduction

Seabirds are a significant component of the marine environment and Britain has internationally important populations of several species. A recent census (Perrins et al. 2011) of the Manx Shearwater population on Skomer estimated 316,070 breeding pairs. This affords Britain’s (and Skomer’s) seabird populations even greater importance and probably makes Britain’s Manx Shearwater population(s) a higher proportion of a world population than is the case for any other bird species breeding in the Britain and Ireland. Skomer is believed to hold the largest Manx Shearwater colony in the world. Other seabird species that breed on Skomer in important numbers include Northern Fulmar, Lesser Black-backed Gull, Black-legged Kittiwake, Common Guillemot, Razorbill and Atlantic Puffin. A national Seabird Monitoring Programme, co-ordinated by the Joint Nature Conservation Committee (JNCC), includes a small number of "key site" seabird colonies where detailed monitoring of breeding success, annual survival rates and population trends is carried out. These sites are geographically spread to give as full coverage of British colonies as possible.

Skomer Island is the most suitable site for this work in south-west Britain. It is a National Nature Reserve managed by The Wildlife Trust of South and West Wales (WTSWW) under a lease from Natural Resources Wales (NRW). Not only is Skomer the most important seabird colony in southern Britain, but the waters around the island were designated as a Marine Nature Reserve (MNR) in 1991 and became Wales’ first Marine Conservation Zone (MCZ) in 2014. Seabird monitoring fits within a broader framework of monitoring marine and terrestrial organisms on and around the island.

There is an impressive data set for seabirds on Skomer. This is especially important for species such as seabirds with long periods of immaturity and high adult survival rates. The Wildlife Trust has been monitoring seabirds on the island since the early 1960s. Additional detailed studies (annual adult survival rates, breeding success and other aspects of seabird ecology) of particular species have been carried out for many years by other bodies, including Dr Matt Wood’s long-term studies of shearwaters, auks and gulls (begun by Prof Chris Perrins), Prof Tim Birkhead’s long-term study of Common Guillemot population dynamics, and Prof Tim Guilford’s studies on the migration strategies of seabirds.

In 2016, the study plot counts of Common Guillemot and Razorbill, the whole island counts of Northern Fulmar, all breeding gulls (including Black-legged Kittiwake) and Razorbill, breeding success rates of Northern Fulmar, Herring Gull, Great Black-backed Gull, Black-legged Kittiwake and Common Guillemot were partially-funded by JNCC. This work is carried out by the island staff (including volunteers) and a contract Field Worker. Elisa Miquel Riera was the JNCC-WTSWW Field Worker in 2016, for the second year running. In 2016 a Seabird Monitoring Volunteer (Tracey Ann-Hooley) was recruited to provide additional support between 25th May and 30th June.

This report includes other seabird monitoring studies undertaken on Skomer. Dr Matt Wood from The University of Gloucestershire coordinates long-term studies of six seabird species, also partially-funded by JNCC. The JNCC-UoG Field Assistant in 2016 was Ros Green.

The studies of Lesser Black-backed Gulls require a significant amount of field work and careful coordination between the JNCC Field Workers, the island staff and volunteers. Systematic nest count areas were rotated again in 2016, where practicable, to build up a picture of correction factors across sub-colonies throughout the island, to improve the accuracy and efficiency of this work while minimising disturbance.

1.1Introduction to capture-recapture survival estimates

The survival rates presented here have been calculated in the same way as in the other years since 1978: they are estimates of survival rates of adult breeding birds, from analysis of long-term encounter histories of individual birds, some of which have been alive, and part of these analyses, for many years. These long-term databases are an invaluable ecological record of the fluctuating fortunes of six seabird populations on Skomer Island dating back to 1970 (Razorbill), 1972 (Atlantic Puffin), 1977 (Manx Shearwater) and 1978 (Herring Gull, Lesser Black-backed Gull and Black-legged Kittiwake).

1.1.1Methods

Estimates of annual survival and re-sighting probabilities are derived from Multi-Event Mark-Recapture (MEMR) analysis of long-term ringing and re-sighting data, using the software programs UCARE and ESURGE. For the purposes of monitoring annual variation in survival rates between years, a model is fitted to allow both survival and encounter probability to vary annually (Cormack-Jolly-Seber model), with more sophisticated analyses taking place in support of other projects as they emerge.

At least two years of observations are needed to obtain an accurate survival estimate for a given year, so the survival estimate for the last year of the study is not comparable with the others and produces an unreliable estimate, and is not presented. The survival estimate becomes reliable with two or more years’ data, so we await the return of birds next year, for example to distinguish death from temporary absence from the colony. Similarly, the estimates for other more recent years are likely to change (hopefully not much) with the addition of further years of data.

Graphs showing estimated survival rates of the species over the course of the study are presented under each species account. Years for which survival rates are not given are those in which estimates were not sufficiently reliable to be presented (see notes accompanying Figures). A table listing survival estimates of all six species is given in Appendix 2. For those species where a trend is apparent, this is highlighted in the text. Field observations were made from April – August 2016 by Ros Green and analyses carried out by Matt Wood (University of Gloucestershire).

1.1.2The value of long-term capture-recapture studies

This approach requires more resources than simpler techniques (in terms of fieldwork, database management, and analytical expertise), but the approach is well worthwhile because it brings three considerable benefits:

· Firstly, by monitoring the same individually-marked seabird colony, variation can be controlled between individuals and sites. In other words, it makes the survival estimates much more accurate if the same birds are followed, in the same place, over many years.

· Secondly, the analytical approach can correct for birds that are tricky to see, or a year of challenging field conditions (like bad weather). Just because a bird hasn’t been seen in the past year it doesn’t mean it has died: it may not have been possible to find it in its burrow or re-sight it on a cliff ledge, because it’s shy or awkward to see, or because this year’s weather made telescope re-sightings more difficult. Long-lived seabirds sometimes have gaps in breeding, so it may also be taking a year off! This ‘unseen’ bird might come back in future years, and correcting for this ‘encounter probability’ greatly increases the accuracy of survival estimates, if you have data over a sufficiently long period.

· Thirdly, and most importantly, if a trend is observed that is of concern from a conservation perspective or a pattern that might facilitate discovering more about seabird ecology, the improved accuracy of this approach over more simplistic estimates gives a much better chance of finding out why survival rates (or encounter probabilities, or frequency of gaps in breeding) might be changing.

That, after all, is the point of monitoring seabirds in the first place, and why long-term projects are an invaluable resource for this and future generations of people who care about seabirds, their island breeding colonies, and the wider marine environment.

2General methods

2.1Whole island counts

Whole island counts of the cliff nesting species (apart from Common Guillemot) were carried out in May/June (28th May-23rd June) and two complete counts were made. Bad weather meant that counts were not possible on the 31st May and 1st June but they commenced again on the 2nd June. Counts were disrupted again between the 16th and 18th but all counts were completed by the 23rd.

As the numbers of cliff-nesting seabirds on Skomer has increased, whole-island counts from a boat have become more logistically challenging. To maintain a sustainable workload whilst retaining the long-term whole-island counts, it was decided last winter to rotate whole island counts of Razorbills and Fulmars with those of Guillemots. This was approved by JNCC and endorsed by the Seabird Sub-group of the Islands Conservation Advisory Committee. Razorbills and Fulmars were counted in 2016, meaning that there will be no whole island count of these two species in 2017.

In mid-June 1999, black-and-white photographs were taken of all count sections and these are filed on the island. Since 2013 new photographs of most of the sections have been taken in order to update the existing ones, as vegetation and the cliffs themselves have changed over the years. In 2016 the Seabird Monitoring Volunteer (Tracey-Ann Hooley) updated the last few photographs and created a new folder for use in the field. Additionally, all section photographs are stored digitally on the island computer and a spare copy of the folder is kept in the island office.

The Lesser Black-backed Gull colonies were counted by eye from established vantage points between the 28th May and 2nd June. An attempt was then made to ground truth a sample of colonies (between the 23rd and 30th May) to produce a correction factor (for missed nests) with which to calculate an island population.

Count units (explained under summary) and methods follow those recommended by Walsh et al (1995) but note that the Lesser Black-backed Gull census methodology has been developed on the island (see Sutcliffe 1993).

Graphs showing whole island populations since the 1960s are presented for each species. Note that in past years different counting units and methods have been used for some species, although those in recent years have been standardised. General trends can nonetheless be identified with some confidence.

2.2Study plot counts of Common Guillemots Uria aalge and Razorbills Alca torda

All ten recommended counts were made during the first three weeks of June of the same study plots used in previous years, using methods outlined in Walsh et al. (1995). In mid-June 1999, black-and-white photographs were taken of all study plot sites and these are filed on the island. In the intervening years new plot photographs have been taken to update the existing ones where vegetation and the cliffs have changed over the years. Edits were also made to the colony sub divisions to remove gaps between them which caused ambiguous boundaries.

2.3Breeding success

Methodology follows that of Walsh et al. (1995). Brief details are given separately in each species account. Black-and-white photographs of the breeding success plots were taken in mid June 1999 and are filed on the island. Only one of these images is now in use, at Wick Corner ledge, with all others having been replaced by new photographs when required. All occupied Common Guillemot, Razorbill and Black-legged Kittiwake breeding plots were re-photographed in 2014 as vegetation and the cliffs themselves have changed over the years. Of particular note is a large cliff collapse at South Stream cliff over the winter of 2013/2014.

3Northern Fulmar Fulmaris glacialis

3.1Breeding numbers - whole island counts

It was decided last winter to rotate whole island counts of Razorbills and Northern Fulmars (hereafter Fulmar) with those of Common Guillemots (see 2.1 Whole island counts). Razorbills and Fulmars were counted in 2016, meaning that there will be no whole island count of these in 2017. Two counts of apparently occupied sites were conducted in May/June. The mean of the two counts was 675 AON (Range 680 – 669). This represents another increase (15.58% higher than 2015) in numbers and is almost up to the historical highs of the 90s and early 2000s. Study plot counts and whole island counts suggested a decline in population since 1990. However, since 1998 there has been a gradual increase in productivity, suggesting the population may have stabilised with regards to food availability and competition for both nest sites and food. Moreover, despite two years (2011 and 2012) where whole island counts dropped below 500 AOS, the number of breeding sites counted has steadily risen over the last four years.

Table 2Northern Fulmar whole island counts 2004-2016

Year

Total

% Change on previous year

5 Year % Change

10 Year % Change

2004

730

+15.1

+5.6

2005

726

-0.5

-0.5

2006

595

-18.0

-6.3

2007

611

+2.7

-3.6

2008

565

-7.5

-22.6

2009

527

-6.7

-27.4

2010

530

+0.6

-10.92

2011

474

-10.57

-22.42

2012

453

-4.43

-19.82

2013

503

+11.04

-4.55

2014

556

+10.54

+4.91

-23.42

2015

584

+5.04

+23.21

-1.85

2016

675

+15.58

+49.01

+10.47

Figure 1Northern Fulmar breeding numbers 1963-2016

3.2Breeding success

3.2.1 Methods

Three visits were made to each of the seven Fulmar study plots between the 28th May and 17th June to observe site occupancy as described in Walsh et al. (1995) productivity-monitoring method 1 (nest-site mapping). The sample size for breeding success consists of sites where an egg is seen or a bird appears to be incubating on two or more consecutive checks, these visits being made five to ten days apart. A last visit was made on the 6th of August to determine the presence or absence of large chicks at all sites. All large chicks were assumed to have fledged.

The final productivity for the island is given as the total number of fledged chicks divided by the number of AOS’ across all plots.

3.2.2Results

106 AOSs were identified between late May and mid-June. The productivity for the island was 0.43 (Table 3), which is higher than the last five years overall mean (2011 – 2015) of 0.39 as well as last year’s productivity of 0.35 (Table 4). However, it is slightly below the historical mean (1986 – 2016) of 0.49 (Figure 2). The most successful site was Matthews Wick, with 0.68 chicks fledging per AOS (Table 3). Tom’s House was not included in the final productivity figure as this year it did not have any occupied sites.

Table 3Northern Fulmar productivity per AOS on Skomer Island 2016

Site

No. sites monitored

No. sites occupied

Chicks fledged

Productivity per site

Tom’s House

2

0

0

-

Basin (West)

35

23

11

0.48

Basin (East)

17

14

4

0.29

North Haven

49

33

11

0.33

South Haven

17

9

2

0.22

Castle Bay

19

11

6

0.55

Matthew’s Wick

22

16

11

0.69

Totals

161

106

45

Productivity

0.43

Table 4Northern Fulmar productivity on Skomer Island 2011-2016

Year

No. Sites monitored

No. Sites occupied

No. Large chicks fledged

Productivity

2011

235

129

44

0.34

2012

266

124

55

0.44

2013

298

146

48

0.33

2014

165

112

51

0.46

2015

170

124

44

0.35

2016

161

106

45

0.43

Mean

0.39

SD

0.06

SE

0.02

Notes: The historical productivity data in this table is given as the total number of fledged chicks divided by the number of AOS’ across all plots.

Figure 2Northern Fulmar productivity on Skomer Island 1986-2016

Notes: The historical productivity data in this table is given as the total number of fledged chicks divided by the number of AOS’ across all plots.

3.3Timing of breeding

The phenology records in 2016 are detailed in table 5. Eggs were first seen on 24th May and chicks on 9th July (all records from the Basin).

Table 5Northern Fulmar phenology records 2010-2016

2010

2011

2012

2013

2014

2015

2016

First egg

22th May

Unknown

20th May

Unknown

15th May

23rd May

24th May

First chick

6th July

Unknown

13th July

Unknown

10th July

9th July

9th July

4European Storm Petrel Hydrobates pelagicus

To contribute towards the next national Seabird Count, funding was secured from Natural Resources Wales to undertake a whole-island census of European Storm Petrels (hereafter Storm Petrel) on both Skokholm and Skomer in 2016. The Skomer population was estimated at 220 Apparently Occupied Sites (95% CL: 195-252), which seems stable since 2003/4 when the last whole-island census took place (Brown 2006). Funding became available just weeks before the census period, requiring considerable effort by WTSWW staff and volunteers, and an impressive amount of coordination and extra work during a busy field season. The full account of the census will be reported soon by NRW (Wood et al. in prep).

As part of a continuing project to estimate survival of breeding adult Storm Petrels on Skomer, a number of ringing visits were made to the small breeding colony at Tom’s House in mid-late July and August, after the incubation period to minimise disturbance. A total of 51 birds were trapped, of which 28 were new and 22 were retraps from previous years. There was also one control which is yet to be traced.

Preliminary analyses of ringing data from 2006-16 indicate a low recapture probability of ringed, and a large number of birds encountered once and never recaptured (nearly 87% of individuals are ‘transient’, most likely non-breeding birds prospecting for nesting sites). These factors hinder the estimation of annual survival rates, but survival estimates averaged over longer time periods (e.g. five years) will remain valuable, especially if combined with periodic census of Apparently Occupied Sites in this breeding colony. The value of this project will increase as it becomes more long-term, therefore the continued ringing of adult Storm Petrels at Tom’s House is recommended, to further understanding of their ecology. The project is a team effort between Matt Wood and Ros Green (University of Gloucestershire) and WTSWW staff and volunteers.

5Manx Shearwater Puffinus puffinus

5.1Breeding study plots census

Within the shearwater census plots, the number of responses (our index of population size) was up slightly on 2015 and was the highest since 1998 when the annual census began. The total number of burrows was also up on the previous year. Therefore the overall upward trend since 1998 continues. The census was again undertaken entirely by WTSWW staff and volunteers, including a dedicated Seabird Monitoring Volunteer.

Figure 3Number of responses in breeding study plots 1998-2016

Figure 4Number of burrows in breeding study plots 1998-2016

Table 6Manx Shearwater responses to playback in census plots 1998-2016

Site/Year

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

A

12

15

17

12

20

15

16

12

28

10

23

20

9

13

17

24

14

21

B

19

35

18

19

32

28

32

15

21

30

12

15

9

18

19

20

15

13

C

56

45

27

35

36

45

52

41

53

66

69

82

30

66

41

102

72

70

D

81

65

61

51

71

55

52

64

64

73

61

57

31

80

97

112

70

89

E

17

14

17

15

14

7

9

9

10

5

8

3

5

5

5

5

6

4

F

3

3

2

5

5

6

4

7

8

6

6

3

4

3

9

9

4

13

G

2

6

4

3

9

7

5

8

9

2

9

12

6

7

9

16

10

10

H

23

17

10

15

16

10

14

16

13

17

14

22

12

18

32

12

16

28

I

72

88

74

117

75

67

102

134

111

116

83

169

110

135

144

134

143

176

J

77

75

107

67

54

66

81

73

42

70

72

80

46

95

93

118

63

88

L

147

132

186

131

142

164

185

244

150

157

156

222

123

159

179

215

200

221

M

85

80

67

62

79

94

71

75

66

73

65

81

33

95

89

85

73

81

N

51

67

39

49

52

44

40

63

75

23

37

70

41

82

62

77

64

74

O

27

29

38

34

30

36

84

34

40

29

25

38

30

51

45

47

55

56

P

30

60

57

67

78

77

32

67

95

72

117

93

80

107

127

98

99

96

Q

34

26

17

17

29

26

32

32

32

31

20

65

20

25

28

27

33

34

R

48

44

65

39

56

83

91

92

72

65

62

53

65

79

65

77

69

97

S

37

67

45

51

63

75

63

65

55

73

69

96

87

75

56

80

115

102

Total

821

868

851

789

861

905

965

1051

944

918

908

1181

741

1113

1117

1258

1121

1273

Table 7Manx Shearwater burrows in census plots 1998-2016

Site/Year

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

A

51

70

87

94

98

145

87

35

105

62

91

61

87

69

100

97

62

99

B

75

102

193

240

98

91

78

81

74

108

49

91

53

74

176

79

79

61

C

299

255

259

202

193

332

287

262

309

387

346

236

246

385

358

429

301

278

D

200

235

296

244

320

313

98

210

253

303

204

206

201

238

316

428

286

256

E

63

65

66

67

61

58

48

37

49

38

48

32

46

40

42

39

63

33

F

14

17

12

11

17

20

15

18

15

13

13

12

17

17

15

40

26

24

G

11

16

15

14

22

21

14

22

29

19

34

25

19

28

21

53

34

27

H

98

97

120

120

140

126

88

118

85

167

84

87

89

141

110

143

106

109

I

271

293

199

321

260

309

236

389

230

331

246

465

278

437

442

395

367

431

J

339

311

455

401

360

359

305

224

219

337

407

315

275

351

438

439

322

395

L

473

506

596

560

593

661

527

693

445

709

472

604

422

560

716

749

641

711

M

234

231

240

188

175

218

167

141

168

154

152

191

157

213

212

214

178

172

N

207

249

261

288

248

261

221

252

282

214

235

215

221

222

226

223

265

250

O

93

99

140

152

110

142

278

119

125

156

139

84

185

148

246

182

163

171

P

151

205

234

204

228

270

124

283

264

257

254

303

256

329

319

301

262

317

Q

84

82

77

95

85

71

112

132

108

119

85

111

77

106

104

125

123

100

R

190

235

329

236

214

314

278

276

279

197

158

167

189

287

214

237

213

231

S

97

187

127

237

213

274

241

244

286

344

260

311

248

209

260

268

282

345

Total

2950

3255

3706

3674

3435

3985

3204

3536

3325

3915

3277

3516

3066

3854

4315

4441

3773

4010

Skomer Seabird Report 2016: WTSWW & UoG

5.2Breeding Success

Breeding success in 2016 was 0.63, just 0.03 higher than 2015. This is slightly higher than the five year average of 0.62 (2012-2016) and the 1995-2016 average of 0.62.

Manx Shearwater breeding success in The Isthmus study plot in 2016 is detailed in Table 8; Figure 5 shows annual variation in breeding success since 1995.

Table 8Manx Shearwater breeding success at The Isthmus study plot in 2016

Total Number of eggs laid

83

Number of eggs known or assumed to have failed1

15

Number of eggs known or assumed to have hatched2

68

Number of chicks known or assumed to have died3

16

Number of chicks surviving to ringing age

52

Hatching success4

82%

Fledging success5

76%

Number of fledged young per egg laid

0.63

Notes:

1. Twelve eggs are known to have failed, having been found abandoned or broken, or having disappeared before they could possibly have hatched. Three were assumed to have failed at the egg stage, the burrow being completely empty when checked on 4th or 12th July.

2. Sixty-eight chicks were found between 3rd July and 27th July. By this latter date all monitored burrows were known to have either successfully hatched, or failed at the egg or young chick stage.

3. Ten chicks were found dead inside the burrows and six were assumed to have failed. These six burrows did not have a chick in them when checked in late July/early August. Evidence of a hatched egg shell, or chick fluff was found inside all of them so they were assumed to have failed after hatching, rather than at the egg stage.

4. Hatching success = % of eggs known or assumed to have hatched.

5. Fledging success = % of chicks surviving to a large size.

In Figure 5, a clear parallel can be seen between the two datasets. The weather in 2012 meant that many burrows were flooded and so very few of the hatchlings survived to fledging age.

Figure 5Annual variation in Manx Shearwater breeding success 1995-2016

Productivity varies markedly between years, but has a stable long term average. The potential effects of temporal variation in productivity and survival (Section 1.1.2) on the demography of shearwaters warrants further study, in relation to the annual breeding census undertaken at sites across Skomer Island since 1998 – the only annual index of population available.

5.3Adult survival

The Shearwater survival estimates are based on birds that are marked in burrows on The Isthmus. All but a few of the nests are reached every year and the majority of the birds breeding in them are caught. In recent seasons, night searches for adults in the vicinity have turned up a few “missing” birds – birds that had survived, but were not breeding in the study burrows; presumably they were living nearby.

Figure 6 shows annual variation in breeding survival estimates for Manx Shearwaters. Recent analyses indicate that the data set is most robust for the analysis of trends in survival since 1992 (M.J. Wood et al. in prep). Although there is no significant time-associated variation in adult breeding survival since 1992, there is a clear decline in breeding adult survival since 1994, which may be a potential concern for Manx Shearwaters on Skomer Island. As reported previously, these survival estimates remain low, both in comparison with more detailed studies carried out in the 1960s and 70s on Skokholm and with what might be expected for a bird with such a low reproductive rate. The effects of this recent decline require further analysis.

Survival rate for adult breeding Manx Shearwaters in 2014-15 was 0.72, well below the study average (1978-2014: 0.86) and the average since the study became more robust in 1992 (1992-2014: 0.88).

Figure 6Survival rates of adult breeding Manx Shearwaters 1978-2015

Notes:

1.Fitted line shows the five-year moving average, error bars ± 1 standard error

2.Survival was non-estimable in 1981-2, 1988-9, 1991-2 (the last transition in such analyses is non-estimable, requiring at least one further year’s data. See Section 1.1)

3.Appendix 2 gives the estimated survival rates for 1977-8 to 2014-2015.

6Great Cormorant Phalacrocorax carbo

6.1Breeding numbers

The Great Cormorant (here after Cormorant) colony on the Mew Stone unexpectedly moved in 2016, from the southern face to the western corner. This meant that the nesting birds were much more visible and could be viewed from land. However, only four pairs made nests which is a drop of almost 50% on the previous year. Cormorants therefore remain at a low level since the highs of the 1980’s.

Figure 7Great Cormorant breeding numbers 1960-2016

6.2Breeding success

Checks were made regularly from land between late May and late July. Six chicks were seen on the 12th June and all six were seen as fledged on the 22nd July. This gives a productivity figure of 1.5 chicks fledged per nesting pair.

7European Shag Phalacrocorax aristotelis

7.1Breeding numbers

In the 1960s/70s the European Shag (hereafter Shag) colony on Skomer was concentrated on Shag Hole Bay, on the north side of The Neck. Over a period of years these moved to Middleholm and by the early nineties Shags had more or less ceased to breed on the main part of Skomer (Figure 8). A few pairs, however, have continued to breed on the north coast of Skomer, at the base of Double Cliff, and on the Garland Stone. This information is useful in explaining the trend in Figure 8. In 2016 there were two nests on the southern face of the Mew Stone, close to nesting Cormorants and in a place not used by Shags in recent times.

Several visits were made by boat to the colony near Double Cliff in June and July and 4 AONs were identified. This may be an underestimate as nests are well hidden and difficult to observe. With the two nests on the Mew Stone, six nests in total were found in 2016 which is an increase of 50% on the four found last year. This perceived increase should be viewed with caution due to the small sample size and the aforementioned difficulty of finding nests at the Double Cliff colony.

Figure 8European Shag breeding numbers 1960-2016

7.2Breeding success

The two nests on the Mew Stone fledged two chicks and the four nests identified at the base of Double Cliff fledged at least six chicks. This gives a minimum likely productivity figure for Skomer of 1.33 chicks fledged per AON.

The Shag colony on Middleholm has been monitored and all accessible chicks ringed to obtain breeding data since the early 1980’s. This work has been undertaken by the South Pembrokeshire Ringing Group with permission from the National Trust, who own the island. In 2016 a visit was made on the 18th of June to monitor nests and ring chicks. Fourteen nests were visited (found) containing a minimum of 33 chicks (21 ringed, six ready to fledge and six not reachable), which gives a minimum likely productivity of 2.36 chicks per AON. Although probably not all nests were found (due to the fact that the field workers available on the day had less experience of this particular colony) the productivity figure (for the fourteen nests found) should still be reliable and it seems that the population on Middleholm has had some fairly good breeding success over the last few years.

8Lesser Black-backed Gull Larus fuscus

8.1Methods for estimating breeding numbers

The Lesser Black-backed Gull colonies were counted by eye (Eye Counts) from established vantage points between the 28th of May and 2nd of June. Mike Wallen, a volunteer, has been doing these eye counts for many years and, to keep the counts consistent, this was continued in 2016. In addition to Eye Counts, Mike also made an assessment of vegetation height and burrow density which he recorded for each sub-colony to build up a picture of the detectability of nests, suitability for systematic walk-through counts and a choice of sub-colonies that reflects vegetation across the island.

Systematic Counts of a subsample of colonies (Table 9) were then made between the 27th and 31st of May. Nests, including empty nests, in selected sub-colonies were systematically searched for and counted by fieldworkers. The method assumes that each pair builds one nest. Systematic counts usually detect more nests than eye counts, so a correction factor (ratio of systematic counts to eye counts) was used to scale up whole-island eye counts to the total number of AONs on the island.

Since 2011, at the request of JNCC, the sub-colonies selected for systematic counts have been rotated each year to avoid subjecting the same areas to the inevitable disturbance of census work that may have an adverse impact on the accuracy of survey results. The aim is to build up a rolling picture of the correction factors for specific sections over the course of several years. The rationale is as follows:

· Four sub-colonies were checked by doing walk-through counts in 2016 (1,8, 21 (now includes X) and Y)

· Where correction factors have been obtained in 2016, and other sub-colonies since 2011, these are used to calculate the number of AONs per sub-colony from eye counts (Table 10)

· Where more than one correction factor exists for a sub-colony, the average for that sub-colony is used

· Where no correction factor exists, the average correction factor over all sub-colonies is used (2011-2016 average = 2.57)

There are limitations to this approach: (i) it is assumed that the detectability of nests remains constant between years when vegetation height that may obscure both eye and systematic counts is known to vary, and (ii) it is assumed that correction factors remain constant in space when local features such as habitat type and breeding density are known to vary. Applying mean correction factors to sub-colonies not systematically surveyed, and carrying over correction factors between years is unlikely to be entirely accurate, but not using a correction factor would greatly under-estimate the number of AONs. It is hoped that the accuracy of this method will improve as systematic counts are rotated through more sub-colonies on the island.

8.2Breeding numbers – results

From Eye Counts, a total of 2,728 AONs and AOTs were identified from standardised viewpoints around the island (Table 10). The number of AONs, including empty (but active) nests, in selected sub-colonies were systematically counted by walking through selected colonies soon afterwards (also Table 10, see Section 8.1 for methods).

Of 43 sub-colonies, 24 have been counted systematically since 2011, and these correction factors (ratios of systematic counts to eye counts) are used to calculate the number of AONs for these sub-colonies. The average correction factor (see above) is used for sub-colonies not yet systematically counted.

This gives a population estimate of 6,936 breeding pairs, which is 9.10% lower than 2015 and a 19.49% decline compared to the five year average (2011-2015). The population remains at a historically low level in 2016, a 65.66% decrease since 1993 when the population was at its peak (Figure 9).

Poor adult survival is implicated as one of the drivers of this long term decline (see Section 8.4) but it is not known how many of these ‘missing’ birds die over the winter and how many simply move to another colony.

National status and trends are poorly known. However, after an increase in the number of coastal nesting Lesser Black-backed Gulls in the 1970s and 80s, a result of increased food availability from fishery discards and from landfill sites, many colonies began to decline thereafter. This may essentially be a reversal of the factors responsible for earlier population increases, namely a decrease in the availability of domestic refuse and reduced discards from fisheries. The number of inland roof-nesting birds has increased dramatically in recent years. The causes of this increase in urban areas may have been facilitated by an abundance of locally available food (e.g. from fast-food street litter and domestic/commercial rubbish bins), and safe (predator-free) nesting sites in the form of flat roofed buildings.

Figure 9Lesser Black-backed Gull breeding numbers 1961-2016

Table 9Record of Lesser Black-backed Gull systematic counts in sub-colonies

2011 & previous years

2012

2013

2014

2015

2016

1 South Old Wall

2 Marble Rocks

3 Abyssinia

4 Anvil Rock

5 Bull Hole

6 Pyramid Rock

7 North Plain

8 Sheer Face West

9 Sheer Face East

10 Sheer Face (The Hill)

11 Double Cliff

12 North slopes

13 North Valley Rise

14 Green Plain

15 South Neck – Thorn Rock

16 W/S Field +24

17 Saunders Fist

18 Harold Stone

19 Wick Cliff

20 Tom’s House-Skomer Head

21 colony now joined with X

22 Garland Stone

23 North West Neck

24 East of West Pond – see 16

25 Toms House to Wick

26 Mew Stone

A Lantern

B Neck East

C Neck main ridge

D South Castle

E Neck South West Coast

F South Haven

G South Stream Cliff

H Welsh Way

I High Cliff

J South Wick Ridge

K Wick

L Welsh Way Ridge

M Wick Ridge North

N Wick Ridge North

O Moorey Meadow

P South Stream

Q Bramble

R Lower Shearing Hays

S New Park

T Shearing Hays

U Captain Kites

V Wick Basin

W The Basin

X / 21 (see 21)

Y Field 11

Z Basin-South Pond

Table 10Lesser Black-backed Gull counts of AONs in 2016

Sub-colony

Mean eye count

Number of correction factors 2011-2016

Correction factor

AON

1 South Old Wall

24

2

6.30

151

2 Marble Rocks

44

1

1.87

82

3 Abyssinia + 24

57

1

1.74

99

4 Anvil Rock

58

1

2.85

165

5 Bull Hole

107

3

2.98

319

6 Pyramid Rock

39

0

0

7 North Plain

195

1

2.24

437

8 Sheer Face W

47

3

2.63

124

9 Sheer Face E

44

1

4.06

179

10 Sheer Face

0

0

0

11 Double Cliff

29

1

2.37

69

12 North slopes

38

0

0

13 N Valley Rise

159

0

0

14 Green Plain

363

0

0

16 W/S Field +24

35

2

1.77

62

19 Wick Cliff

0

0

0

20 Tom's House-Sk Head

9

0

0

21 colony now joined with X

99

1

2.83

280

22 Garland Stone

26

0

0

23 NW Neck

21

0

0

25 Toms House to Wick

17

0

0

26 Mew stone

9

0

0

B Neck E

79

1

2.67

211

C Neck main ridge

43

0

0

D South Castle

84

0

0

E Neck SW coast

8

0

0

F South Haven

35

0

0

G S Stream Cliff

46

0

0

H Welsh Way

38

0

0

I High Cliff

37

1

1.63

60

J S Wick Ridge

48

1

1.55

74

L Welsh Way Ridge

51

1

2.20

112

M Wick Ridges

155

0

0

N Wick Ridges

80

1

3.03

242

O Moory Meadow

47

0

0

P South Stream

60

1

2.19

131

Q Bramble

13

0

0

R Lower Shearing Hays

108

0

0

S New Park

53

0

0

T Shearing Hays

51

0

0

U Captain Kites

92

1

1.97

181

V Wick Basin

1

0

0

W The Basin

8

1

1.71

14

X (see 21)

0

0

Y Field 11

63

3

3.44

217

Z Basin-South Pond

108

1

2.02

218

Tables 11 & 12 and Figure 10 indicate that after an increase in the percentage of empty nests for five years in a row (2010-2014), the percentage has fallen over the last two years.

Table 11Percentage of empty Lesser Black-backed Gull nests counted in May 2016

Sub-colony

Total nests

Empty nests

% empty

1

South Old Wall

216

40

19

8

Sheer Face West

186

42

23

21

Colony now joined with X

290

46

16

Y

Field 11

323

82

25

Totals and mean %

1015

210

21

Table 12Lesser Black-backed Gull empty nests 1998–2016

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

% Empty nests

19

26

39

28

40

49

23

14

20

24

16

31

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

36

19

19

19

28

19

11

23

25

28

41

44

30

21

Figure 10Percentage of empty Lesser Black-backed Gull nests 1991–2016

8.3Breeding success

The estimated number of Lesser Black-Backed Gull fledglings in 2016 is 2,254, less than half the number estimated in 2015 (4,709). Productivity was thus down to the low levels seen in the last 20 years on Skomer, at just 0.36 chicks per breeding pair. The number of fledglings are calculated using a simple capture:recapture technique (Lincoln-Petersen estimate). As many large chicks as possible are ringed, and then the ringed:unringed ratio observed in the field when most of the chicks have fledged. This ratio is used to ‘scale up’ from the number of fledglings ringed to an estimate of the total number on the island. The standard target is to ring at least 300 large chicks, although in recent years it has been difficult to find this number, due to successive poor breeding seasons.

In 2016, 250 chicks were ringed, a challenging task as chicks were hard to find in vegetation but volunteers were on hand to assist. The ringed/resighting estimates based on these are shown in Table 13 and the productivity in Table 14.

Table 13Estimated number of Lesser Black-backed Gull fledglings in 2016

Date

No. ringed fledglings seen

No. unringed fledglings seen

Total no. fledglings seen

Est. No. of fledglings

04/08/2016

41

377

418

2549

05/08/2016

40

331

371

2319

08/08/2016

35

277

312

2229

12/08/2016

43

287

330

1919

Mean

2254

Note: Estimated number of fledglings = (total fledglings seen x number of fledglings ringed) / number of ringed fledglings seen.

Table 14Estimated productivity of Lesser Black-back Backed Gulls in 2016

 

Estimated number of fledglings

Productivity

Maximum

2549

0.40

Minimum

1919

0.30

Mean

2254

0.36

Note: Productivity is calculated as the number of fledglings per AON. Number of fledglings is estimated by mark:recapture on Skomer Island excluding The Neck (6324 in 2016), and the number of AON from corrected eye-counts). See Section 8.1 for methods.

Figure 11 shows the estimated productivity of Lesser Black-backed Gulls on Skomer since 1981. After a sharp decline in the 1980s, average productivity has since remained low with frequent years of very low productivity. Productivity in 2016 was 0.36 chicks per breeding pair.

Figure 11Productivity of Lesser Black-backed Gulls per AON 1981-2016

8.4Adult survival

These birds are all from the study area in Lower Shearing Hays. Previously, it has been noted that there has been a decline in the breeding population. Overall survival 1978-2015 has averaged 0.88, but there has been considerable variation over time (Figure 12). The steady decline in survival from the late 1970s to the early 2000s appears to have recovered somewhat in recent years, but not to the levels of the 1970s and 80s, and likely to contribute to the continued population decline. Survival of breeding adult birds in 2014-2015 was 0.82.

Figure 12Survival rates of adult breeding Lesser Black Backed Gulls 1978-2015

Notes:

1.Fitted line shows the five-year moving average, error bars ± 1 standard error

2.The final transition in the series in such analyses cannot be estimated reliably without at least one further year’s data (see Section 1.1)

3.Appendix 2 gives the estimated survival rates for 1978-2015

9Herring Gull Larus argentatus

9.1Breeding numbers

A total of 321 AONs were counted in 2016. This represents a 14.85% decline on the previous year and a 19.75% decline over a five year period. Approximately 265 of the AONs counted in 2016 were coastal with the remainder (17) nesting inland. This means that 83% of Herring Gulls were coastal nesting in 2016.

Skomer’s Herring Gulls fell into heavy decline in the 1980s but have stabilised at a lower level since then (Figure 13). The national trend is also one of stabilisation after a decline since monitoring began in 1969-70. Botulism may have been an important factor in this decline as well as changes in refuse management and fisheries discards.

Figure 13Herring Gull: Number of AONs 1961-2016

9.2 Breeding success

The average breeding success for all years monitored between 1962 and 2016 is 0.70 large chicks per AON. Productivity for 2016 was 0.52 (Table 15), this being lower than the overall island average and a reduction on the 0.69 achieved in 2015.

The study site at Tom’s House was visited on the 2nd and 3rd of May to identify and map AONs, with a further nine visits made between the 13th of May and the 18th of July to monitor chick development and record large chicks/fledglings.

Herring Gull productivity monitoring in 2016 was conducted by the Assistant Warden, Jason Moss.

Table 15Productivity of Herring Gulls on Skomer in 2016

AON

Large Chicks

Productivity

Tom’s House

23

12

0.52

Figure 14Breeding success of coast-nesting Herring Gulls, 1962-2016

9.3Adult survival

This study was originally based on birds nesting along the north coast, but the breeding population at that colony dropped so markedly that a second study plot in the area from Tom’s House to Skomer Head is now used instead. However, the population size here is still smaller than desirable, and it may is necessary to start an additional study plot.

Adult breeding survival has declined steadily in recent years, mirroring the sharp declines seen in the years up to 1980-1 and 1997-8. This may be cause for concern and warrants further analysis. The most recent annual survival estimate (2014-15: 0.80) is close to the average for the study (1978-2015: 0.82); poor adult survival in recent years is likely to be contributing to the decline of the population on Skomer.

Figure 15Survival rates of adult breeding Herring Gulls 1978-2015

Notes:

1.Fitted line shows the five-year moving average, error bars ± 1 standard error

2.Survival was non-estimable in 1980-81, 1993-4, 1997-8 (the final transition in the series in such analyses is not estimable, Section 1.1)

3.Appendix 2 gives the estimated survival rates for 1978-2015

10Great Black-backed Gull Larus marinus

10.1Breeding numbers

A whole island total of 108 Great Black-back Gull nests were counted in 2016. This is a drop of 13.8% on the previous year but is 6.93% higher than the mean of the previous ten years (2006-2015) of 101 AON.

The decline since the 1960s has been attributed largely to control measures in the 1960s and 1970s that were implemented as a result of the species perceived predatory impact on other seabirds. An outbreak of botulism in the early 1980s also contributed to the decline (Sutcliffe 1997).

The national trend has shown a slow decline since 1999. Although recent data from Skomer Island suggests that the population may be recovering from earlier setbacks between the 1960s and 80s (Figure 16), a trend even more marked on the nearby Skokholm Island (Brown, R. and Eagle, G. 2015).

Figure 16Great Black-backed Gull breeding numbers 1960-2016

10.2Breeding success

Monitoring of the breeding success of Great Black-backed Gulls has been included in the JNCC contract since 1999.

Twenty-five Great Black-backed Gull nests (AON) were identified during May across the island. These were visited between the 22nd June and 19th July resulting in a total of 36 large chicks being recorded. Of these, nine nests had no chicks, two had one chick, eight had two chicks, and six nests had three chicks. This gives a productivity of 1.44 chicks per AON, lower than the last couple of years productivity but higher than the overall historical mean (1996 – 2016) of 1.29 (Figure 17).

Figure 17Great Black-backed Gull productivity on Skomer Island 1996-2016

10.3Diet study

A trial study to monitor the diet of GBBGUs was initiated in 2008 and has continued up to 2016. The prey remains around a sample of 25 nests were recorded. The sample represented nests from differing habitats and shearwater densities. The survey was carried out after chicks fledged (from late July to early August).

Prey items within a five metre radius, cross-shaped transect centred on each nest were recorded. Additionally the number of Manx Shearwater and Rabbit carcasses within a 10m radius search area around the nests was recorded for comparison with historic records of Manx Shearwater predation levels.

Figure 18Great Blacked-backed Gull diet remains within a five metre radius cross-shaped transect around 25 Great Black-backed Gull nests 2016

Figure 19Frequency of occurrence of food items within a five metre radius cross-shaped transect around 25 Great Black-backed Gull nests in 2016

The five metre radius cross-shaped transect around 25 Great Black-backed Gull nests in 2016 shows that 8% of the prey items recorded were Manx Shearwaters (Figure 18), the same percentage as last year but less than the 14% recorded in 2014 and 20% recorded in 2013. Manx Shearwater remains were recorded at 84% of the nests studied (Figure 19).

Feather pellets and fur were the most prevalent prey items, being found at 96% and 92% of the nests respectively, representing 29% and 14% of totals items respectively. Refuse was found in 84% of the nests studied (Figure 19, a similar percentage to 2015 and 2014) and represent 15% of the prey items (Figure 18), a higher percentage than the 9.36% in 2015. Other birds were found at 76% of the nests (Figure 19) and included Puffin, Guillemot, Razorbill, Lesser Black-backed Gull, Kittiwake and Magpie.

In 2016 a total of 188 Shearwater carcasses were found at the sample of 25 nest sites within a 10m radius, giving a mean of 7.52 carcasses per nest (Figure 20). The total number is slightly lower than in 2015 (8.68 carcasses per nest), but very similar to the historical records overall mean of 7.60 (1959, 1965, 1973, 1992, 2008-2015).The number of rabbit carcasses discovered this year was 1.32 rabbit carcasses per nest compared to 2.04 in 2015.

Figure 20Mean number of Shearwater carcasses found within 10m radius of 25 Great Black-backed Gull nests 1959, 1965, 1973, 1992, 2008-2016

11Black-legged Kittiwake Rissa tridactyla

11.1Breeding numbers

Black-legged Kittiwakes (hereafter Kittiwake) had undergone several years of slow decline on Skomer but the last two (2014 and 2015) had shown signs of recovery. This year’s count of 1,477 nests is once again a drop in numbers (4.5% less than the previous year) and is cause for concern given the dramatic declines of this species in the north of the UK. Two whole island counts were completed with a mean of 1,477 AON and a Range of 1,556 – 1,398. This represents a drop of 7.34% over a five year period and is 16.88% below the mean of the previous ten years (1,777 AONs).

Nationally (and especially in Scotland) the Kittiwake population has undergone a steep and well-documented decline since the mid-1980s. This has been most dramatic in Scotland with a 77% decline since 1986. Wales’ and Skomer’s population has shown more stability followed by a slower decline over this period, and Kittiwake numbers on Skomer have fallen by only 31% since 1986. This decline has likely been caused by low productivity coupled with low survival, and looks likely to continue.

Figure 21Black-legged Kittiwake breeding numbers 1960-2016

Table 16Black-legged Kittiwake whole island count details 2007-2016

Total

% change from 2015

5 year % change

2007

1942

-6

-16.1

2008

2282

+17.5

+45.4

2009

2046

-10.3

-10.3

2010

1992

-6.06

-7.01

2011

1837

-4.02

-5.41

2012

1594

-13.23

-30.15

2013

1045

-34.44

-48.93

2014

1488

+42.40

-22.58

2015

1546

+3.90

-20.74

2016

1477

-4.46

-7.34

11.2Breeding success

11.2.1Methods

The breeding success of 399 kittiwake AONs was monitored at the same three sub-colonies studied since 1989: South Stream (Plot 1 and 2), High Cliff (plot 2) and the Wick (plot 3, 4, 5 and 6). However, in 2015 and 2016 the plot High Cliff 1 and Wick 7 were dropped in order to decrease workload and also allow for increased visit frequency to the other existing plots. Plots 8A and B were also dropped due to them containing too few nests.

Photographs of the cliffs were used and each nest marked on a transparent overlay. Visits were made to each sub-colony to monitor progress from nest construction to fledging. In 2015 and 2016 the number of visits during the chick rearing period was increased in order to obtain a higher level of accuracy. All chicks that were large (class 'e' in Walsh et al (1995)) were assumed to have fledged. On the last visit any chicks of medium/large size (class 'd') were also assumed to have fledged. Standard recording sheets from the Seabird Monitoring Handbook Walsh et al. (1995) were used for data collection.

The final productivity for the island is given as the total number of large/fledged chicks divided by the number of AONs across all plots.

The monitoring was conducted by the WTSWW Field Worker with help from the Visitor Officer, Leighton Newman, who monitored plot Wick 3.

11.2.2Results

This year 424 nests were started in the study areas, resulting in 399 AONs (447 nests were started in 2015 resulting in 416 AONs). The 399 AONs produced a minimum of 424 chicks (because of the difficulty of recording small chicks in some of the plots this is likely to be an underestimate). A total of 260 reached a 'large' size and were considered to have fledged successfully (319 in 2015).

The final productivity was 0.65 (Table 17), lower than last year’s productivity of 0.77 but higher than the 5 year overall mean (2011 – 2015) of 0.56 (Table 18). Comparing this year’s productivity with the mean of historical records (1989 - 2016) of 0.70 it is slightly lower (Figure 22).

The High Cliff plot showed the greatest drop in productivity, from 0.91 in 2015 to 0.56 in 2016. However, no particular event (such as predation) was detected as the reason for this. The Wick and South Stream also showed lower productivity than the previous year, 7% lower at South Stream and 4% lower at The Wick.

In 2016, 93% of AONs went on to apparently incubate eggs (97% in 2015), with 78% of these producing chicks (77% in 2015). 6% of pairs did not complete a nests (‘trace’ nests only) compared to 8% in 2015.

Table 17Black-legged Kittiwake productivity per AON on Skomer Island – 2016

Site

Nests started

AON'S

Incubating Pairs

Nest with chicks

Total chicks

Large chicks

Productivity per site

S. Stream 1

42

41

40

33

49

32

0.78

S. Stream 2

33

30

30

22

32

20

0.67

High Cliff

97

88

80

62

89

49

0.56

The Wick 3

38

36

36

24

43

21

0.58

The Wick 4

69

66

58

46

67

50

0.76

The Wick 5

108

101

91

72

107

77

0.76

The Wick 6

37

37

36

30

37

11

0.30

Totals

424

399

371

289

424

260

Productivity (SMP method)

0.65

 

 

 

 

 

 

 

 

Table 18Black-legged Kittiwake productivity on Skomer Island 2011-2015

Year

Nest started

AON’s

Large chicks

Productivity

2011

718

702

414

0.59

2012

632

591

194

0.33

2013

500

394

160

0.41

2014

544

491

345

0.70

2015

447

416

319

0.77

Mean

0.56

SD

0.19

SE

0.08

Note: The historical productivity data in this table is given as the total number of fledged chicks divided by the number of AOS’ across all plots.

Figure 22Black-legged Kittiwake productivity per AON on Skomer Island 1989-2016

Note: The historical productivity data in this table is given as the total number of fledged chicks divided by the number of AOS’ across all plots.

In order to complete the productivity monitoring study 12 visits were conducted. The first two visits (late May and beginning of June) were to identify completed nests, separating them from trace nests. Following this, the monitoring took place every 5 to 7 days once there were chicks present.

11.3Timing of breeding

Nest building was first noted on the 2nd May, with the first egg seen on the 20th May at High Cliff, and the first chick on the 14th June at both High Cliff and South Stream sites. First fledgling was seen on the 25th of July at South Stream.

Table 19Black-legged Kittiwake phenology records 2010 - 2016

2010

2011

2012

2013

2014

2015

2016

Nest building start

30th April

7th May

7th May

10th May

23rd April

4th May

2nd May

First egg

21st May

13th May

20th May

28th May

22nd May

19th May

20th May

First chick

8th June

10th June

11th June

23rd June

24th June

20th June

14th June

First fledgling

27th July

24th July

25th July

11.4Breeding adult survival

These analyses are based on colour-ringed birds nesting at the South Stream Cliff study plot, as well as any others found around the island that have moved from that site.

Over the period 1978-2015, survival of breeding adults averages 0.84 (Figure 23, Appendix 2). There continues to be wide fluctuation in adult breeding survival between years, despite a high probability of re-sighting live birds (>90% encounter probability in the last ten years). There appears to be a long-term decline in survival rate, but this requires further analysis as part of a demographic study that draws together the population parameters measured on Skomer. The survival rate of breeding adult Kittiwakes in 2014-15 was 0.80, a continued decline in recent years that may be due to the apparently decadal cycles of adult survival since 1978, or the arrival of difficulties faced by Kittiwake populations further north in the UK.

Figure 23Survival rates of breeding adult Kittiwakes 1978-2015

Notes:

1.Fitted line shows the five-year moving average, error bars ±1 standard error

2.Survival was non-estimable in 1991-2

3.The final transition in the series is not estimable, requiring one further year’s data (see Section 1.1)

4.Appendix 2 gives the estimated survival rates for 1978-2015

12Common Guillemot Uria aalge

12.1Breeding numbers - whole island counts

Common Guillemots (heareafter Guillemot) were not censused (whole island) in 2016 as a decision had been made the previous winter to rotate counts of Guillemots with those of Razorbills and Fulmars, meaning that in 2017 Guillemots will be counted but Razorbills and Fulmars will not (see 2.1 Whole island counts). Study plot counts of Guillemots will still be done each year and are thought to be representative of the whole island population. Data on whole island counts up to 2015 is retained for completeness.

Table 20Common Guillemot whole-island counts 2004-2015

Year

Land count

% change

Sea count

% change

Total count

% change

5-year

% change

2008

11579

-23.6

5509

+56.5

17088

-2.60

+20.45

2009

14339

+23.8

5173

-6.10

19512

+14.19

-1.01

2010

15643

+9.09

4319

-16.51

19962

+2.31

+17.58

2011

15064

-3.70

6624

+53.37

21688

+8.65

+23.62

2012

16557

+3.78

5951

-10.17

22508

+3.78

+31.72

2013

15025

-9.25

5837

-1.92

20862

-7.31

+6.92

2014

12437

-17.22

11056

+89.41

23493

+12.61

+17.69

2015

12822

+3.10

10924

-1.06

23746

+1.08

+9.49

Figure 24Common Guillemot numbers (whole island) 1963-2015

12.2Breeding numbers - study plot counts

The study plots are thought to be representative of the whole island population (Wilson 1992) and may reflect any population change more accurately than the whole island counts, as repeated counts take account of variations in attendance that is thought to occur within colonies. For details of counts refer to Appendix 4.

A total of ten counts were made at each study plot this year by the seabird Field Worker and a Long Term Volunteer (Hannah Andrews) during the first three weeks of June. All study plot counts were completed on the same day and each individual plot was done by the same observer to reduce variation between repeat counts. The Long Term Volunteer conducted counts at the Bull Hole site and the Field Worker at the other sites.

The number of Common Guillemots within the study plots as a whole has changed little over the last five years. However, there does still seem to be an increase, with this year’s population being 5.2% higher than the 2011-2015 five year mean (Table 21). If the 2016 totals are compared to those of last year the plot counts show an increase of 3.7% (6846.0 individuals in 2015 to 7097.8 individuals in 2016). High Cliff and South Stream showed a significant increase in comparison to last year’s counts; 10.4% and 13.2% respectively. Bull Hole, however, suffered an (albeit non-significant) decrease in numbers.

Table 21Common Guillemot study plot totals 2012-2016

Study Plot

Year

Mean

SD

SE

%change

5yr %change

Bull Hole

2011

3569.1

348.1

110.1

+2.2

+21.4

2012

4201.3

214.1

80.9

+17.7

+43.8

2013

3553.9

144.6

54.6

-15.4

+0.5

2014

3607.9

136.5

43.2

+1.5

-1.5

2015

3757.2

111.4

35.2

+4.1

+2.0

2016

3662.3

141.4

44.7

-2.5

-2.0

High Cliff

2011

2006.5

124.1

39.2

-0.9

+ 31.6

2012

1801.7

346.6

131.0

-10.2

+19.3

2013

2161.4

106.0

40.5

+20.0

+15.0

2014

2290.4

79.3

25.1

+6.0

+13.9

2015

2175.9

160.2

50.7

-5.0

+5.79

2016

2402.0

50.43

15.95

+10.4

+15.1

S. Stream

2011

804.1

47.3

15.0

-8.8

+19.3

2012

908.1

100.6

38.0

+12.9

+40.6

2013

1021.3

41.5

15.7

+12.5

+23.4

2014

972.4

71.4

22.6

-4.8

+7.7

2015

912.9

64.4

20.4

-6.1

-0.5

2016

1033.5

61.9

19.6

+13.2

+11.9

All Plots

2011

6379.8

419.2

148.2

-0.6

+23.7

2012

6911.1

416.5

157.4

+8.66

+36.1

2013

6736.6

282.2

106.7

-2.5

+7.9

2014

6870.7

213.5

67.5

+2.0

+4.5

2015

6846.0

263.1

83.2

-0.4

+2.8

2016

7097.8

206.3

65.3

+3.7

+5.2

Figure 25Common Guillemot numbers (study plot counts) 1973-2016

12.3Breeding success

12.3.1Methods

The number of active and regularly occupied sites was established at study plots and their histories were followed, using the methodology outlined in Walsh et al. (1995). Visits were made from mid-April to begin mapping the location of pairs. Full monitoring began on the 29th April and the last visit was made on the 23rd July (from 1st May to 19th July in 2015).

Sites were visited every one or two days, with the greatest effort made during the egg laying, hatching and fledging periods. The number of visits was 70, a similar number to the last two years, but a significantly higher effort than 2013 (Range: 26 to 44) and 2012 (Range: 45-51).

This is the second year where the number of plots to monitor Guillemot breeding success under the JNCC contract has been reduced to one (this was Wick 1G in 2015 and Wick Corner in 2016) until further notice. Sheffield University has been undertaking Guillemot productivity monitoring at the Amos for many years, and there has been an agreement to share the data instead of conducting the productivity monitoring independently, so that two sites are effectively being monitored. For results from the Amos plot see Appendix 1.

The final productivity is given as the total number of fledged or apparently fledged chicks (last seen at 15 or more days old) divided by the number of active and regularly occupied site and active site only for the plot Wick Corner. The historical data in Table 22 has also been calculated in this way, dividing the total number of large chicks by the number of active and regular and active sites only across all plots.

12.3.2Results

One hundred and twenty three nest sites were monitored at the plot Wick Corner in 2016. The mean productivity was 0.63 fledged birds per active and regularly occupied site (and active only) which is lower than last year’s productivity of 0.76. It is important to highlight the differences in structure between last year’s plot (Wick 1G) and this years (Wick Corner), which may in part explain the difference in breeding success. For example, the Wick Corner plot contains several isolated sites above the main ledge (these being much more vulnerable to predators than the ones on the main ledge), whereas last year’s plot did not have any of these type of sites. If we compare this year’s productivity with the overall mean (1989-2015) of 0.67 per active and regular site and 0.71 per active only (table 22), it is slightly lower.

Table 22Common Guillemot productivity on Skomer Island 1989-2016

Year

Number of sites, active + regular.

Number of sites, active only.

Number of chicks fledged

Productivity per active + regular site

Productivity per active site only

1989

120

-

96

0.80

-

1990

112

-

80

0.71

-

1991

140

-

107

0.76

-

1992

171

-

121

0.71

-

1993

198

-

141

0.71

-

1994

187

-

135

0.72

-

1995

198

190

151

0.76

0.79

1996

210

195

161

0.77

0.83

1997

211

205

174

0.82

0.85

1998

199

191

154

0.77

0.81

1999

242

190

147

0.61

0.77

2000

227

212

143

0.63

0.67

2001

259

232

160

0.62

0.69

2002

259

236

170

0.66

0.72

2003

262

234

179

0.68

0.76

2004

292

257

184

0.63

0.72

2005

297

267

200

0.67

0.75

2006

287

264

142

0.49

0.54

2007

258

243

164

0.64

0.67

2008

283

249

164

0.58

0.66

2009

254

241

185

0.73

0.77

2010

315

285

211

0.67

0.74

2011

292

260

153

0.52

0.59

2012

318

283

185

0.58

0.65

2013

328

283

212

0.63

0.75

2014

300

293

183

0.61

0.62

2015*

63

63

48

0.76

0.76

2016**

123

122

77

0.63

0.63

Mean

0.67

0.72

SD

0.08

0.08

SE

0.02

0.02

Notes: *In 2015 only plot Wick 1G was monitored (Wick 2G, Wick Corner and Bull Hole were dropped in 2015).

**In 2016 only plot Wick Corner was monitored (Wick 1G, Wick 2G and Bull Hole have been dropped in 2016 until further notice).

The historical productivity data in this table is given as the total number of fledged chicks divided by the number of AOS’ across all plots.

If we compare the 2016 results to previous years for the same study plot (Wick Corner), the productivity figure for the current year is also slightly lower than the overall mean (1995-2014,2016) for the plot Wick Corner (0.68). Figure 26 shows the productivity picture of the Wick Corner site since 1995