Patterns of territory settlement and consequences for breeding success in the Northern Wheatear...

l b i ~ (2000) 142, 389-398

Patterns of territory settlement and consequences for breeding success in the Northern Wheatear

Oenanthe oenanthe

DAVE CURRIE1*, DES B.A. THOMPSONz & TERRY BURKE’ Department of Zoologx University of Leicester, Leicester LE 1 7RH, UK

*Scottish Natural Heritage, Advisory Services, Anderson Place, Edinburgh EH6 5NR UK

We examined the pattern of territory settlement and its consequences for breeding success in the Northern Wheatear Oenanthe oenanthe on Bardsey Island, Wales, during the breeding seasons of 1991-93. Males returned earlier than females, and older males returned earlier than first-year breeders. Although their boundaries shifted between years, the general location of territories was consistent during the three-year study. There was a high degree of fidelity to area and territory between years for both sexes. The order of territory settlement, from which a territory rank was calculated, was highly consistent for males between years irrespective of individual settlement patterns and territory fidelity. Patterns of territory settlement were less predictable for females, although there was a significant correlation between the mean territory ranks of paired males and females. There was a male-biased sex ratio in each year, and between 5% and 26% of males remained unpaired throughout the breeding season. Male mating status and breeding success were dependent on arrival date, territory rank and breeding density. Early-arriving, usually older, males were able to settle on territories first and were more likely to pair, while later-arriving individuals were more likely to remain unmated. These effects were consistent between years, and consequently territories could be classified as either preferred (accounting for proportionately more breeding attempts) or non-preferred. Territory quality as opposed to individual quality appeared to explain much of the variation in breeding success, and both sexes benefited by breeding on preferred territories through enhanced breeding success and an increased probability that their offspring would be recruited to the population.

Most terrestrial birds - 84% of passerines and 8 1 Yo of non-passerines - breed in pairs on territories (Lack 1968). Territory ownership and quality, especially for species that occupy an all-purpose territory, are likely to be major contributors to an inclvidual’s fitness and reproductive success. Competition for good territories is therefore likely to be intense as well as costly in terms of both time and energy (Goodburn 1991).

Within species, there is considerable variation in individual breeding success. This has been attributed to differences in territory quality (Hogstedt 1980, M d e r 1982), parent quality (Newton & Marquiss 1984, Goodburn 1991), experience of parents (Thompson

*Corresponding author. Present address: Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland. Email: [email protected]

et al. 1986) and in the timing of breeding, which typically shows a seasonal decline (Perrins 1970, Brooke 1979, Crick et al. 1993). Individuals are expected to develop territorial behaviours that allow them to maximize their reproductive success, such as settling preferentially on good territories (Brooke 1979, Merller 1983) and shifting to better sites when- ever possible (Beletsky & Orians 1987).

Individuals can use their experience of previous breeding seasons to assess the value of an area, although conditions on site may differ between years as a result of short- or long-term local changes in habitat or prey populations (O’Connor 1984). Birds resident throughout the year are more likely to be able to monitor such changes and modify their territory boundaries accordingly. However, migrants, which are absent for part of the year, and inclviduals breeding for

0 2000 British Ornithologists’ Union

390 D. Currie, D.B.A. Thompson & T: Burke

the first time, may need to assess a potential territory solrly on information gained at the time of their settlement. Furthermore, cues used in the selection process must also indicate the quality of the territory later during their breeding season and not just during the settlement period. I t will therefore be important for individuals to make the correct choice during settlement, as modification of boundaries at a later date may be difficult due to increased competition from additional settlers (Tye 1992).

In migrant species, the main benefits of arriving early are being able to settle on the best territories and having the opportunity to breed earlier, both of which may enhance reproductive success (M01ler 1994). The major cost of arriving early is the risk of mortality due to unsuitable environmental conditions (Mdler 1994). Variations in arrival times can therefore be considered to be phenotype-dependent and to have costs and benefits, and as such are likely to be a reliable indicator of an individual's quality (Meller 1994). As a consequence, territory and individual quality are likely to be correlated and early-arriving birds will also have the opportunity to pair with relatively higher quality mates.

Fidelity to a breeding area is common among many bird species (Greenwood 1980, Greenwood 13r Harvey 1982). The degree of site fidelity exhibited by a species is likely to depend on whether it is a resident or migrant, local population density and the degree of variation in territory quality (Bensch & Hasselquist 1 99 1 ) . Resident species typically exhibit high site fidelity, possibly due to the limited options of finding a better trrritory, but fidelity in migrants will in part be a consequence of differences in arrival times. Early- arriving individuals preferentially choosing good territories will force later arrivals to settle elsewhere (Harvey et (11. 1084, Beletsky & Orians 1987). Secondly, at higher breeding densities, an individual is more likely to lose its territory while seeking another. Individudls may benefit by being more site-faithful under such conditions (Searcy 1979, Weatherhead & Roag 1986). If' there i s large variation in territory quality, individuals may eshibit less fidelity to territories of relatively poor quality (Newton & Marquis 1984, Heletsky i;r Orians 1987, Thompson et al. 1986, Kollinpcr & Gavin 1989).

l n thc Northern Wheatear Oenaizthe oeiianthe, males defend an all-purposc territory from which the majority of food for the pair and dependent offspring is collected (Cramp 1988, Conder 1989, Tye 1992). Brooke ( 1979) found that wheatears shelved a consistent prtLference betwecn years for certain territories

which were associated with earlier breeding attempts and higher fledging success, and Tye (1992) showed that territory preference was based on vegetation characteristics which were a reliable indicator of food availability. We further examine patterns of territory settlement and consequences for breeding success in the Northern Wheatear. Specifically, we examine: (i) whether there is a predictable order of territory settlement controlling for individual settlement patterns and territory fidelity; and (ii) the consequences of territory settlement on breeding success by separating the effects of individual and territory quality on breeding performance.

METHODS

The Northern Wheatear is a medium-sized (approxi- mately 25 g), migratory, socially monogamous, sexually dimorphic passerine (Cramp 1988). This study was conducted on Bardsey Island (52"46'N, 4"47'W), located off the northwest coast of Wales, UK (1991-93). For more details of the study area see Currie et al. (1998) and Jones (1988).

The study site was monitored three times daily from 06:00-08:00 h, 12:OO-14:OO h, and 16:OO-18:OO h, from the middle of March until late April in each year, to obtain the dates of arrival, settlement and pairing for occupying individuals. An individual was considered to have settled only when it stayed in an area where it subsequently bred or, if unpaired, remained for most of the breeding season. Few birds were colour-ringed a t the start of the study and, in 1991, territorial males were initially identified by their behaviour, e.g. singing and song flight displays (Cramp 1988, Conder 1989), before they were trapped very soon after their arrival, using spring and Potter traps, and colour-ringed for individual identification. Individuals of both sexes returned to breed in subsequent years; in 1992 and 1993 the following proportions were ringed when they arrived: in 1992, 47% 18/17) of females and 58Yo (14/24) of males; in 1993, 39% (7/18) of females and 53O/0 (10/19) of males. Unringed adults in 1992 and 1993 were initially identified by their behaviour and plumage characteristics. The settlement dates of seven males (out of 27) in 199 1 were not obtained. Similarly, female settlement and pairing data were very incomplete for 1991 and are excluded from the analysis. Complete settlement and pairing data were obtained for all individuals in I992 and 1993.

Territory boundaries were plotted during the breeding season (prior to hatching) on a scale map (1 :10 560) by noting the position of boundary disputes

((J 2000 British Ornithoioglsts Union Ibis, 142, 389-398

Territory settlement in the Northern Wheatear 391

and the outermost locations for each pair. Territories were assumed to be unchanged between years if the same nest-site was used or if the defended area contained the nest-site from previous or subsequent years. If territories were consistently unused for breeding then there had to be an overlap of a t least 50% in the defended area between years for them to be regarded as the same. The mean proportion of overlap of territories between years was 0.67 0.24 sd (n = 26) and only five territories had an average overlap of less than 50Yo between years. However, we considered these territories to be the same between years because all were consistently used for breeding and the defended area contained the nest-site from the previous or subsequent years. There was no difference in the proportion of overlap for territories used for breeding and those consistently occupied by unpaired males (mean proportion of overlap for territories used for breeding: 0.67 t 0.26 sd, n = 22; mean proportion of overlap of territories associated with unpaired males: 0.68 f 0.13 sd, n = 4; Mann-Whitney U-test, z = -0.07, ns).

Territories were ranked in relation to settlement of the first individual of each sex to compensate for differences in arrival and settlement dates between years; high-ranked territories were occupied earlier than low-ranked ones. Thirty territories were identi-

fied, 28 of which were used a t least twice in the three- year study. Although boundaries varied slightly between years, territories remained relatively constant (Fig. 1). Given this consistency between years, a mean settlement rank for males and females was calculated for a total of 26 territories. No rank was obtained for two territories used in 1991 (territories 5 and 23, Fig. 1 a) and two territories were used only once (territories 3 and 16; Fig. la). Territories were categorized as preferred or non-preferred on the basis of timing of Settlement; those that were occupied first were considered the most preferred. There was a significant correlation between mean male and mean female settlement ranks (Spearman rank correlation, r, = 0.75, n = 21, P < 0.0023, so we ranked territory preference on the basis of mean male ranks. Preferred territories were ranked 1-7 and non-preferred territories were ranked 8-14. Mating success of males was defined according to the presence or absence of a female during the breeding season.

For trapped birds we measured the wing-length to the nearest mm (maximum chord, flattened), mass to the nearest 0.1 g, and tarsus-length to the nearest 0.1 mm using vernier callipers (Henderson 1991). To calculate a male body condition index (BCI) we used the residuals derived from the regression of body mass (standardized within each year) on log tarsus-length

Figure 1. Territory maps for (a) 1991, (b) 1992 and (c) 1993. The maps show preferred territories, non-preferred territories and territories occupied by unpaired males. Territories with the same numbers between years are considered to be the same. Territories A-D are not accurately drawn and are not considered in this study.

@ 2000 British Ornithologists' Union, Ibis, 142, 389-398

392 D. Currie, D.B.A. Thompson & T Burke

(Packard & Boardman 1987, Currie et al. 1998). Males were aged as either first-year or older on the basis of plumage characteristics (Svensson 1984). Females could only be accurately aged if their ringing history was known.

We checked nests during the period of nest building to determine first-egg dates (FEDs) and once during incubation to determine clutch size. FEDs were numbered as days after the earliest first-egg date in the population in each year to compensate for differences in the onset of breeding between years. Chicks were weighed nine days after hatching to give an indication of fledging mass (chicks fledge about two weeks after hatching; Conder 1989, Moreno 1989). Breeding success is expressed as the number of fledglings per nest.

Repeatability (r) , which describes the proportion of variance in a character between rather than within individuals, was used to quantify the separate effects of male, female and territory quality on patterns of territory settlement and several measures of breeding performance (lay date, clutch size and hatching success). Repeatability was calculated as the intra-class correlation coefficient derived from a one-way analysis of variance (ANOVA; Sokal & Rohlf 1981, see also Lessells & Boag 1987). Comparisons were made for: (i) same male or female on the same territory ( n = 9 and 5, respectively); (ii) same male or female on different territory (n = 11 and 5, respectively); (iii) same territory with different male or female ( n = 17 and 16, respectively). To minimize replication of individuals between years due to a high degree of area fidelity for both males and females, many analyses of breeding success and pattern settlement were performed on a yearly basis. Data on breeding success only refer to first clutches. Statistical tests are two-tailed and corrected

for ties, and follow Siege1 and Castellan (1988) and Sokal and Rohlf (1981). Data were analysed using Statview 5 12 (Abacus Concepts 1988).

RESULTS

Patterns of arrival and territory settlement

Males returned to the study area earlier than females (Table l), and settled on a territory soon after their arrival. On arrival, early-settling unpaired males typically defended areas greater than four breeding territories (see also Tye 1992). However, territory size decreased when individuals paired and other males arrived, and as a result there was no effect of male settlement date on final territory size (Spearman rank correlation, settlement date versus territory size: 1991-93, all tests ns; mean territory size k se = 2.13 + 0.12 hectares, n = 69).

Although male body condition (BCI) was consistent between years ( r = 0.73, calculated from one-way ANOVA, male vs BCI, F,, 3o = 7.38, P < O.OOl) , there was no indication that male BCI correlated with date of arrival (Spearman rank correlation, 1991-93, all tests ns). However, there was an age component in male arrival dates as older individuals returned significantly earlier than first-year breeders (Table 1).

On average, 56% (24/43) of males and 43% (15/35) of females were observed to return between years (1991-93), and 5070 (12) and 53% (8) of the same males and females, respectively, bred on the territory they had used in the previous year (although only four pairs remained faithful in subsequent years). Individual male settlement dates were not consistent between years ( r = 0.04; Table 2). However, the pattern of male

Table 1. Mean settlement dates expressed as days afler 20 March (i se) for all males and females, paired males and unpaired males, and first-year and older (1 year +) males. Sample sizes are given in parentheses. Settlement data for females in 1991 were incomplete and are excluded from the analyses.

Year All males Females Paired males

1991 13 58 i 1 63' - 1 1 8 8 r 1 5 6

1992 2061 2 2 3 4 35 39 i 1 42 1 7 5 6 k 1 9 9

1993 1311k192 20 29 k 1 99 12 71 t 1 99

~ -~ ~~~ ~ ~

(20) (1 7)

(23) (17) (17)

(19) (1 8 ) (18)

Unpaired males

22.67 i 2.33 (3)

31.60 i 6.32 (6) 20 (1)

First-year males

19.29 r 1.85

32.00 * 4.45

18.00 i 2.15

_.._________

(8)

(8)

(7)

Older males

10.25 i 1.76

15.63 i 1.64

10.36 * 2.50

_ _ _ _ ~

(1 2)

(1 5)

(1 2)

*Settlement data are only available for 20/27 resident males. Males arrived earlier than females (Mann-Whitney U-test, males versus females; 1992, z = 4.92, P < 0.001; 1993, z = 3.01, P < 0.01), paired males arrived earlier than unpaired males (Mann-Whitney U-test, male mating status versus arrival date; 1991, z = 2.20, P < 0.05; 1992, z = 2.28, P < 0.01; there were an insufficient number of unpaired males to perform similar tests for 1993), and old males returned earlier than first year breeders (Mann-Whitney U-test, male age versus arrival date; 1991, z = 2.71, P < 0.01; 1992, z = 3.28, P < 0.01 ; 1993, z = 2.09, P < 0.05).

0 2000 British Orni!hologists Union, Ihs, 142, 389-398


Table 2. Repeatability (r) of settlement dates for mates and females (1991-93). rcalculated from one-way ANOVAS (after Lessells & Boag 1987). Degrees of freedom (in parentheses) and Fratio are shown for one-way ANOVAS.

Males

F ratio r

All males (16,23) 1.10 0.04 All territories (18,28) 2.57* 0.46 Same male, same territory (8,lO) 0.61 -0.22 Same male, different territory (1 1,14) 1.05 0.02 Same territory, different male (1 6,22) 3.69’* 0.54

Females

F ratio r -

All females (6,7) 0.80 -0.11

Same female, same territorya - - Same female, different territory (4,5) 1.46 0.19

All territories (1 2,13) 1.65 0.24

Same territory, different female (9,lO) 2.01 0.27

ahsufficient data. *P < 0.05. **P < 0.01

territory settlement was repeatable between years (all territories, r = 0.46; Table 2) independent of individual settlement patterns and territory fidelity (same territory, different male, r = 0.54; Table 2 ) . Further- more, there were seven instances between 1992 and 1993 when a returning male could have occupied his territory from the previous year but moved to another. We examined territory changes by males between 1991and 1992 using the male settlement ranks from 199 1, and for territory changes between 1992 and 1993 we used mean male settlement ranks from 1991 to 1992, thus avoiding the effect of an individual’s settlement within that year. Males that changed territories between years showed a significant tendency to move to a higher-ranlung territory (Wilcoxon paired-sign rank test, T. = 28, n = 7 , P < 0.05).

Females were rarely observed visiting males or territories prior to settling, and their settlement on territory was rapid. Individual settlement dates were not consistent between years (r = 4 . 1 1 ; Table 2), and there was no evidence of a repeatable pattern of territory settlement ( r = 0.24; Table 2). However, there was a significant correlation between mean male and mean female territory ranks (see Methods), and four females that could have settled on the territory on which they bred in the previous year settled on one with a higher mean (female) rank. This indicates that there was at least some consistency in the pattern of female territory settlement. Sample sizes were small but, like males, older females also showed a tendency to return earlier than young females (Mann-Whitney U-test, return date versus female age [old, n = 14, versus young, n = 9, data combined for all years], z = -1.87, P = 0.06).

All switches to lower-ranked territories by both sexes resulted from their previous territory already being occupied by the time they returned. Territory and mate infidelity both seemed to be consequences of individual arrival times. Late-arriving individuals were unable to settle on the same territory or pair with

the same mate due to the territory being occupied or the other member of the pair already being mated; mate fidelity was never observed on territories other than the one on which the pair had previously bred together.

Mating status

There were no unpaired females, but 5-26% of males were unpaired every year (Table 3). A male’s mating status depended largely on his time of arrival at the study area and individuals which arrived early (and consequently settled on higher-ranked territories) were more likely to be paired (Table 1). Because of age differences in arrival dates (Table I), unpaired males were usually individuals returning to breed in their first year; ten of the 11 unpaired males, excluding a male who was unpaired in successive years, were all first-year individuals, which occurred on just six territories. The fact that these males were unpaired was not a consequence of them returning later than the females. For example, in 1992 only 17% (3/17) of females had returned by the time all resident males had acquired a territory. In all but one case, territories associated with unpaired males were either not used, or they were incorporated into other territories or occupied by different individuals in subsequent years. There were proportionately more unpaired males a t higher densities and when the sex ratio was more male biased (Table 3).

Preferred territories were occupied more frequently and, excluding territories that were used only once, there was a turnover of about two males and two females per territory during the study (number of males per territory f se = 2.04 f 0.11; number of females per territory = 2.00 * 0.19). However, preferred territories had a lower turnover of individuals than non-preferred territories (number of maleshumber of years territory occupied f se: pre-

0 2000 British Ornithologists’ Union, Ibis, 142, 389-398

394 D. Currie, D.B.A. Thompson & T Burke

Table 3. The number of resident females and males, sex ratio (females:males), number of first-year and older males (1 year +), and percentage of males unpaired in each of the study years Numbers of unpaired males in each age class are given in parentheses.

No of females No of males Sex ratio 1st-year males Old males Unpaired (%) ~ ~~~~~~~ - - - ~~ ~~ ~~

1991 20 27 0.75 14 (7) 13 (0) 26 1992 17 23 0 71 8 (5) 15 (1) 26 1993 18 19 0 95 7 (0) 12 (1) 5

Old males were more likely to be paired than young males (male age vs mating status, 1991, chi-squared test, x2 = 9 58, df = 1, P = 0 0023, 1992, Fishers exact test P = 0 0147 there were insufficient unpaired males to perform a similar test in 1993)

fcrred territories = 0.71 f 0.05, n = 13; non-preferred territories = 0.90 0.08, n = 12; Mann-Whitney U-test, 2 = 2.01, P < 0.05; number of femaleshumber of years occupied 2 se: preferred territories = 0.82 f O.O6, ti = 13; non-preferred territories = 1 .OO * 0.00, n = 9; Mann-Whitney U-test, z = 2.3 1, P < 0.05).

Breeding success

Females layed about three weeks after settlement (mean delay se = 25.25 2 1.49, n = 34, range 14 -46 days), with a shorter delay between arrival and the onset of laying for latcr settling females (Spearman rank correlation, female settlement date versus [FED minus female settlement date]: 1992, r, = -0.49, n = 17, P < 0.05; 1993, r, = -0.50, n = 17, P < 0.05). There was a seasonal decline in clutch size in each year (Spearman rank correlation, FED versus clutch size: 1991, r, = -0.68, n = 19, P < 0.01; 1992, r, = -0.67, 17 = 17, P < 0.02; 1993, r, = -0.50, n = 17, P < 0.05) but no similar dccline in breeding success (Spearman rank correlation; 1991, r, = -0.38, n = 20, P < 0.1; 1992,r,=-0.35,n=17,ns; 1993,rt=-0.34,n=17,ns).

Few individu~l measures of breeding performance were consistent between years. For females there was only a significant degree of repeatability in clutch size ( r = 0.44; Table 4) and a weak pattern of repeatability for clutch size for individuals breeding on the same ter-

ritory ( r = 0.55; Table 4) and different territories ( r = 0.47; Table 4), while for males there was only a significant repeatability for FED (r = 0.37; Table 4) and for individuals breeding on the same territory between years ( r = 0.58; Table 4). This indicates that female quality as opposed to territory quality was the determining factor in accounting for clutch size, while FED was subject to the influence of either male or territory quality or some combination of the two.

If territories were grouped as preferred or non- preferred on the basis of mean male settlement ranks (see Methods), preferred territories had earlier FEDs, were more frequently used for breeding (number of years used for breeding/number of years occupied), and had higher breeding success than less preferred territories (Table 5). FEDs were independent of male settlement date (Spearman rank correlation, male settlement date versus FED: 199 1, r, = 0.11, n = 17, ns; 1992, r, = 0.27, n = 17, ns; 1993, r, = 0.07, n = 17, ns), and therefore earlier FEDs on preferred territories were not an artefact of basing territory preference on male settlement patterns. Furthermore, since there was no difference in mean clutch size between preferred and non-preferred territories, the lower breeding performance observed on less preferred territories was not a consequence of smaller clutches being laid later in the season (see above).

Recruits were more likely to fledge from preferred

Table 4. Repeatabilities of male and female breeding parameters (1 991-93). Repeatabilities ( r ) calculated from one-way ANOVAS (after Lessells & Boag 1987). Degrees of freedom for one-way ANOVAS are shown in parentheses.

Males Same male same territory Same male, different territory Same territory, different male All males ~ ~~~~~~~ ~~~~~~~~ - - - _ _ _ ~ _ _ _ _ _ - - - ~~~~

First-egg date 0.58’(8,10) Clutch size 0.00 (8,lO) Breeding success -0.01 (8,lO)

-0.11 (8,lO) -0.26 (8,lO) -0.02 (8,lO)

0.04 (13,19) 0.37‘(13,19)

0.01 (13,19) 0.06 (13,19) -0.17 (13,19) -0.17 (13,19)

Females Same female, same territory Same female, different territory Same territory, different female All females

First-egg date 0.45 (4,6) Clutch size 0.55”(4,6) Breeding success -0.01 (4,6)

0.26 (6,8) 0.47”(6,8)

-0.28 (6.8)

0.22 (15,25) 0.32 (9,14) -0.08 (1 5,255) 0.44’(9,14) -0.01 (15,25) -0.18 (9,14)

“ ‘ P < 0 1 ‘ P < 0 0 5

L) 2000 British Ornithologists Union, / b ~ , 142, 389-398


Table 5. Breeding data (means * se) for preferred and non-preferred Wheatear territories (1991-93). Number of territories are given in parentheses; z statistics refer to Mann-Whitney U-tests.

Non-preferred Z Significance Preferred ____. _.__

** Breeding score 0.93 rt 0.70 (14) 0.51 * 0.11 (14) 3.51 First-egg date 8.36 * 1.46 (14) 13.09 * 2.03 (1 0) 2.01 Clutch size 6.00 i 0.10 (14) 5.48 0.83 (10) 1.53 ns Breeding success 5.19 f 0.87 (14) 2.69 * 0.83 (10) 2.42 Mean no. of recruits 0.86 * 0.18 (14) 0.20 * 0.20 (10) 2.55 Mean no. of recruitsho. of years bred 0.31 0.06 (14) 0.07 k 0.07 (10) 2.53 Chick weight (9) 24.63 ~t 0.48 (1 3) 24.18 ~t 0.91 (6) 0.37 ns

Breeding score = number of years used for breeding/number of years territory was occupied. *P < 0.05, **P < 0.01.

**

** **

territories, even when we controlled for the number of years a territory was used for breeding (number of recruitshumber of years bred; Table 5). Increased breeding success and recruitment from preferred territories was not a consequence of either higher chick weights (one-way ANOVA, recruitsho recruits versus mean brood weight, F,,,, = 0.06, ns) or time of breeding (one-way ANOVA, recruitsho recruits versus FED, Fl,34 = 0.86, ns). Furthermore, there was no correlation between breeding success or chick recruitment and male age or BCI (both tests, ns). Fledgling recruitment was not biased to certain males and the 17 fledglings (nine females and eight males) that were either recruited to the population or observed to visit the study area temporarily (nine from 1991 and eight from 1992) originated from 11 different territories (14 putative fathers). Only one male was known to have produced recruits in successive years.

Inter-year territory movements had no effect on female reproductive success (RS, number of fledglings produced; Wilcoxon paired-sign rank, RS prior to territory change vs RS after territory change, ns). However, inter-year territory movements did have a significant effect on male RS (Wilcoxon paired-sign rank; number of fledglings prior to change vs number of fledglings after change, T+ = 55.5, n = 11, P < 0.05). This was due to unpaired males that had previously settled on less preferred territories returning the following breeding season to settle on more preferred, high ranlung territories, where they successfully paired and bred; males which settled on high ranlung territories were more likely to be paired.

DISCUSSION

Although the Northern Wheatear is a migratory species, territories remained relatively constant between years (Fig. 1; see also Brooke 1979, Conder 1989). Since territories are usually selected by the

males (females rarely settled in the absence of a male; Tye 1992, pers. obs.), the constancy of territory use observed in this study is likely to be a consequence of male settlement behaviour, the availability of suitable habitat and previous experience on territories (Brooke 1979). There was good evidence that the pattern of male territory settlement was non-random, predictable between years and independent of individual settlement patterns. There was less evidence that the pattern of territory settlement was as predictable in females, although there was a significant correlation between mean male and female territory ranks.

There was a strong effect of age on arrival dates in both sexes, with older individuals usually arriving earlier than those returning to breed in their first year (see also Brooke 1979, Conder 1989). This had an important effect on the pattern of male territory settlement, and early-arriving older males were usually able to settle on preferred territories (see also Dhondt & Huble 1968, Brooke 1979, Yasukawa 1980, Jarvi 1983). Arrival dates also had an important effect on male mating status; early-arriving individuals being more likely to be paired (see also Alatalo et al. 1984, Arvidsson 81 Neergaard 199 1).

All unpaired males were late arrivals (usually first- year breeders, although late-arriving old males also remained unpaired) and were concentrated on six low- ranking territories. These males did not remain unpaired because they returned to the study area after the females, and furthermore several of these males returned in subsequent years to breed successfully on different (higher ranked) territories. The additive effect of arrival times and territory preference on a male's mating status was more acute in years with a high population density and a male-skewed sex ratio. Less-preferred territories occupied in high density years were either not utilized or incorporated into other territories in years with a smaller breeding population, indicating that less-preferred areas were only

0 2000 British Ornithologists' Union, Ibis, 142, 389-398

396 0. Currie, D.B.A. Thompson & T Burke

used in higher density years (see M d e r 1982 and references therein).

Arrival and settlement dates in migrant species are likely to correlate with individual quality (Mdler 1994). For males there was no correlation between body condition and settlement date, age, or mating status. However, female settlement patterns could also be explained, in part, by individuals preferentially pairing with older males; age can be considered to be a reliable measure of male 'quality' (Brooke 1979, Askenmo 1984, Weatherhead 1984) because older males have proved their ability to survive between years. Females may therefore have been choosing good quality males as well as good quality territories. The relative importance of male and/or territory quality in determining patterns of female settlement is likely to vary depend- ing on species biology and the amount of variation between territories in habitat quality. For example, M~lller (1994) observed that in the Barn Swallow Hirundo rustica, a species in which most food is collected off territory, there was no consistency in territory quality between years, and male quality accounted for female settlement patterns. However, variation in territory quality is probably a more important factor affecting female settlement patterns in species which utilize an all-purpose territory (Slagsvold 1986, Alatalo et al. 1986), although in some instances male quality may also be important in determining femalc settlement patterns if variation in habitat quality between territories is small (Lifjeld & Slagsvold 1988). Furthermore, individuals of both sexes are also able to modify mate choice at a later date via the pursuit of extra-pair copulations (Smith 1988, Kempenaers et d., 1992, Wagner 1991, Graves et a/ . 1993). However, there was limited evidence that this was the case in this study and natural levels of extra- pair paternity (EPP) were comparatively low (1 1 %; Currie et al. 19983, and there was no correlation between mean male or female territory ranks and the presence of EPP (Currie 1996).

The benefits to individuals of settling on high-ranking territories appeared to be primarily associated with mating and reproductive success. Female mating status was unaffected by pattern of territory settlement as there was a male-skewed sex ratio in each of the study years, and more than one female was unable to settle on the same territory due to inter-female aggression (pcrs. 013s.). Male mating status depended upon arrival times and their settlement on preferred territories; individuals that had settled on high-ranhng territories were more likely to be paired. The importance to males of acquiring a preferred territory is highlighted

by the finding that 70% of males (1991-93) that failed to produce any offspring were unpaired (Currie 1996). Movement to other territories increased male reproductive success but not that of females, and movements between years always resulted in males moving to higher-ranked territories. Extra-pair fertil- izations contribute little to annual male reproductive success in this population and the number of chicks produced is an accurate indicator of breeding success (Currie et al. 1998). Therefore, it is important for males to settle on a gooapreferred territory to be able to breed successfully.

Differential breeding success may reflect differences in habitat quality (Hogstedt 1980) or parental quality (Goodburn 199 1). Fledglings produced on preferred territories were more likely to survive, irrespective of chick weight and lay date (but see Perrins 1965, 1979, Magrath 199 1, M d e r 1994, 1995). The lack of difference in chick weights between preferred and non-preferred territories, albeit considering small sample sizes, indicates that either some aspect of territory quality (e.g. chick diet and nest hole quality) or an aspect of parental quality (e.g. good genes or older and more experienced parents) were important in determining breeding success. Data are limited, but there were no significant differences in aspects of breeding success (FED, clutch size or fledging success) between old and first-year breeders (excluding unpaired individuals) within years. Although we were unable to identify factors associated with territory quality (Currie 1996), the predictable settlement on territories by males - and higher rates of occupancy, territory shifts and increased fidelity to preferred areas by both sexes - indicates that individuals recognized variations in territory 'quality' and (when possible) adjusted their pattern of settlement accordingly (Brooke 1979, M d e r 1983). I t is possible that prior knowledge of given territories by residents outweighs actual benefits for any specific territory featurels) or characteristic(s) and the mere fact that individuals show fidelity to par- ticular areas may be sufficient to influence further preference for these areas in subsequent years by other individuals. However, the benefits accrued by individuals from breeding on 'preferred' territories indicates that the pattern of settlement in this study, at least to some extent, was dictated by some aspect of territory quality.

We thank Ian Hartley, Jari Valkama, Vesa Ruusila and an anonomyous referee for comments on the manuscript. We are also grateful to the Bardsey Island Trust for allowing access to their land, and to the Bardsey Island residents for their help and support during the study. D.C. was supported

'9 2000 British Ornithologists' Union. Ibis. 142, 389-398


by a NERC CASE studentship in association with the Countryside Council for Wales. T.B. was supported by a Royal Society University Research Fellowship and grants from the BBSRC and NERC.

REFERENCES

Abacus Concepts 1988. StatviewTM SE + Graphics. The Solution for Data Analysis and Presentation Graphics. Ver. 1.03. Berkley, CA: Abacus Concepts Inc.

Alatalo, R.V., Lundberg, A. & Stahlbrandt, K. 1984. Female mate choice in the pied flycatcher Ficedula hypoleuca. Behav. Ecol. Sociobiol. 14: 253-261.

Alatalo, R.V., Lundberg, A. & Glynn, C. 1986. Female pied flycatchers choose territory quality and not male characteristics. Nature 323: 152-1 53.

Arvidsson, B.L. & Neergaard, R. 1991. Mate choice in the willow warbler: a field experiment. Behav. Ecol. Sociobiol. 29: 225-229.

Askenmo, C.E.H. 1984. Polygyny and nest-site selection in the pied flycatcher. Anim. Behav. 32: 972-980.

Beletsky, L. & Orians, G.H. 1987. Territoriality among red-winged blackbirds. 1. Site fidelity and movement patterns. Behav. Ecol. Sociobiol. 20: 21-34.

Bensch, S. & Hasselquist, D. 1991. Territory infidelity in the poly- gynous great reed warbler Acrocephalus arundinaceus: the effect of variation in territory attractiveness. J. Anim. Ecol. 60:

Bollinger, E.K. & Gavin, T.A. 1989. The effect of site quality on breeding-site fidelity to bobolinks. Auk 106: 584-594.

Brooke, M. de L. 1979. Differences in the quality of territories held by wheatears (Oenanfhe oenanthe). J. Anim. Ecol. 48: 21-32.

Conder, P. 1989. The Wheatear. Christopher Helm, London. Cramp, S. 1988. The Birds of the Western Palearctic, Vol. 5. Oxford:

Oxford University Press. Crick, H.O.P., Gibbons, D.W. & Magrath, R.D. 1993. Seasonal

changes in clutch size in British birds. J. Anim. Ecol. 62:

Currie, D.R. 1996. Male reproductive strategies and parental invest- ment in the wheatear, Oenanthe oenanfhe. PhD thesis, University of Leicester.

Currie, D.R., Burke, T.A., Whitney, R. & Thompson, D.B.A. 1998. Male and female behaviour and extra-pair paternity in the wheatear. Anim. Behav. 55: 689-703.

Dhondt, A.A. & Huble, J. 1968. Age and territory in the great tit (Parus major L.). Angew. Ornithol. 3: 20-24.

Goodburn, S.F. 1991. Territory quality or bird quality? Factors determining breeding success in the Magpie Pica pica. lbis 133:

Graves, J., Ortega-Ruano, J. & Slater, P.J.B. 1993. Extra-pair copulations and paternity in shags: do females choose better males? Proc. R. SOC. Lond. B253: 3-7.

Greenwood, P.J. 1980. Mating systems, philopatry and dispersal in birds and mammals. Anim. Behav. 28: 1140-1 162.

Greenwood, P.J. & Harvey, P.H. 1982. The natal and breeding dispersal of birds. Ann. Rev. Ecol. Sys. 13: 1-21.

Harvey, P.H., Greenwood, P.J., Campbell, B. & Stenning, M.J. 1984. Breeding dispersal of the pied flycatcher (Ficedula hypoleuca). J. Anim. Ecol. 53: 727-736.

Henderson, I.G. 1991. Sexing a population of Jackdaws on the basis of biometric characteristics. Ringing Migr. 12: 23-27.

Hogstedt, G. 1980. Evolution of clutch size in birds: adaptive varia-

857-871.

263-273.

85-90.

tion in relation to territory quality. Science 210: 1 1 48-1 150. Jarvi, T. 1983. The evolution of song versatility in the Willow Warbler

Phylloscopus trochilus: a case of evolution by intersexual selection explained by ‘female choice of the best mate’. Ornis Scand. 14: 123-1 28.

Jones, P.J. 1988. The Natural History of Bardsey Cardiff: Natural Museum of Wales.

Kempenaers, B., Verheyen, G.R., Van den Broeck, M., Burke, T., Van Broeckhoven, C. & Dhondt, A.A. 1992. Extra-pair paternity results from female preference for high-quality males in the blue tit. Nature 357: 494-497.

Lack, D. 1968. Ecological Adaptations for Breeding in Birds. London: Chapman and Hall.

Lessells, C.M. & Boag, P.T. 1987. Unrepeatable repeatabilities: a common mistake. Auk 104: 116-121.

Lifjeld, J.T. & Slagsvold, T. 1988. Female pied flycatchers ficedula hypoleuca choose male characteristics in homogeneous habitats. Behav. Ecol. Sociobiol. 22: 27-36.

Magrath, R.D. 1991. Nestling weight and juvenile survival in the blackbird, Turdus merula. J. Anim. Ecol. 60: 335-351.

Moreno, J. 1989. The breeding biology of the Wheatear Oenanfhe oenanthe in southern Sweden during three contrasting years. J. Ornithol. 130: 321 -334.

Meller, A.P. 1982. Characteristics of Magpie Pica pica territories of varying duration. Ornis Scand. 13: 94-100.

Msller, A.P. 1983. Song activity and territory quality in the Corn Bunting Miliaria calandra: with comments on mate selection. Ornis Scand. 14: 81-89.

Meller, A.P. 1994. Phenotype-dependent arrival time and its consequences in a migratory bird. Behav. Ecol. Sociobiol. 35: 115-122.

Meller, A.P. 1995. Developmental stability and ideal despotic distribution of blackbirds in a patchy environment. Oikos 72:

Newton, 1. & Marquiss, M. 1984. Seasonal trends in the breeding performance of sparrowhawks. J. Anim. Ecol. 53: 809-830.

O’Connor, R.J. 1984. Behavioural regulation of bird populations: a review of habitat use in relation to migration and residency. In Sibly, R.M. & Smith, R.H. (eds) Behavioural Ecology: Ecological Consequences of Adaptive Behaviour. Oxford: Blackwell Scien- tific Publications.

Packard, G.C. & Boardman, T.J. 1987. The misuse of ratios to scale physiological data that vary allometrically with body size. In Feder, E.F., Bennett, A.F., Burggren, W.W. & Huey R.B. (eds) New Directions in Ecological Physiology: 21 6-239. Cambridge: Cambridge University Press.

Perrins, C.M. 1965. Population fluctuations and clutch size in great tits Parus major L. J. Anim. Ecol. 34: 601-647.

Perrins, C.M. 1970. The timing of birds’ breeding seasons. lbis 112:

Perrins, C.M. 1979. British Tits. London: Collins. Searcy, W.A. 1979. Female choice of mates: a general model for

birds and its application to red-winged blackbirds. Am. Nat 114:

Siegel, S. & Castellan, N.J. 1988. Non-parametric Statistics for the Behavioral Sciences. Singapore: McGraw-Hill Book Company.

Slagsvold, T. 1986. Nest-site settlement by the Pied Flycatcher: does the female choose her mate for the quality of his house or himself? Ornis Scand. 17: 210-220.

Smith, S.M. 1988. Extra-pair copulations in black-capped chick- adees: the role of the female. Behaviour 107: 15-23.

Sokal, R.R. & Rohlf, F.J. 1981. Biometry; 2nd edn. San Francisco:

228-234.

242-255.

77-1 00.

0 2000 British Ornithologists’ Union, Ibis, 142, 389-398

398 D. Currie, D.B.A. Thompson & T: Burke

W.H. Freeman and Company. Svensson, L. 1984. ldentification Guide to European Passefines,

3rd edn. Stockholm: published by the author, distributed by the British Trust for Ornithology, Thetford.

Thompson, D.B.A., Thompson, P.S. & Nethersole-Thompson, D. 1986. Timing of breeding and breeding performance in a population of Greenshanks (Tringa nebularia). J. h i m . Ecol. 55:

Tye, A. 1992. Assessment of territory quality and its effects on breeding success in a migrant passerine, the Wheatear Oenan- the oenanfhe. lbis 134: 273-285.

181 -1 99.

Wagner, R.H. 1991. Evidence that female razorbills control extra-

Weatherhead, P. 1984. Mate choice in polygyny: why do females

Weatherhead, P.J. & Boag, K.A. 1986. Site infidelity in song spar-

Yasukawa, K. 1980. Song repertoires and sexual selection in the

pair copulations. Behaviour 118: 157-1 69.

prefer older males? Am. Nat. 123: 873-875.

rows. Anim. Behav. 34: 129-1 31,

red-winged blackbird. Behav. fcol. Sociobiol. 7: 233-238.

Received 29 September 1998, revision accepted 29 June 1999

(cl 2000 British Ornithologists Union Ibis, 142, 389-398

Patterns of territory settlement and consequences for breeding success in the Northern Wheatear...

Documents

Transcript of Patterns of territory settlement and consequences for breeding success in the Northern Wheatear...