Avian mobility CHAPTER 4. Avian dispersal Definition of dispersal ‘dispersal’ refers to...

Avian mobilityAvian mobility

CHAPTER 4CHAPTER 4

Avian dispersalAvian dispersal

Definition of dispersalDefinition of dispersal

• ‘dispersal’ refers to movements that, at the population level, have no fixed direction or distance; result in a mixing of individuals from different localities, but do not necessarily bring about any change in overall distribution

• ‘migration’ refers to movements in restricted directions for more or less fixed distances that produce regular seasonal shifts in the centre of gravity of a population

• dispersal can lead to range extension and colonization and can have important genetic consequences (i.e. reducing inbreeding, promoting gene exchange, affecting rates at which populations can differentiate)



• ‘natal dispersal’: measured by the linear distances between natal and first breeding sites

• ‘breeding dispersal’: measured by the distances between the breeding sites of successive years

• ‘non-breeding dispersal’: measured by the distances between the wintering sites of successive years

• natal dispersal generally involves much larger distances than the other types



• species generally form pairs on breeding areas, hence natal and breeding dispersal affect the genetic structure of populations

• some species of migratory waterfowl form pairs on winter quarters with males accompanying females back to their chosen breeding area, thereby adding additional component to gene flow

• degree of site fidelity (philopatry) differs between species and sexes; female birds generally disperse further than males

• studies on dispersal distances are heavily biased by sizes of study areas or anthropogenic effects

• rare and mostly undetected long-distance migrants contribute most to gene flow and colonization events



• species-specific dispersal patterns are outcome of evolutionary cost-benefit balances

• benefits of side fidelity: local knowledge, competitive advantage due to prior ownership, local adaptation

• benefits of dispersal: movement to better quality habitat, occupation of successional or patchy/ephemeral habitat, reduction of inbreeding

• costs of dispersal: failing to find suitable habitat, risk of mortality, maladaptation


Natal dispersalNatal dispersal

• in most species, natal dispersal does not show directional preference, but recoveries tend to decline progressively with increasing distance (at different scales), both in resident and migrant species


Location of Eurasian sparrowhawks ringed as chicks and recovered in a later breeding season, in relation to hatch site (centre)


Natal dispersal patterns of several species ringed as chicks and recovered in a later breeding season



• to solve scale problem in comparisons, dispersal distances are often expressed in number of territories transversed rather than in metric distances.

• larger species tend to disperse further than smaller species, and female birds further than males (passerines, owls, raptors, waders, colonial seabirds)

• within any one population, dispersal distances of both sexes may overlap greatly



• in group-living birds (e.g. co-operative breeders), young remain with their parents on the natal territory for up to several years before dispersing (Florida Scrub Jay, Acorn Woodpecker, Arabian Babbler, Siberian Jay)



• delayed dispersal may develop when all suitable habitat is occupied by territorial groups (experimental evidence in Seychelles Warbler)

• young males may inherit paternal territory or take over neighbouring one, while young females mostly move to another territory



• in northern waterfowl males move furthest; since migrant duck often change mate every year, they may also change their breeding sites, ensuring genetic mixing

• Geese and Swan keep the same mates for several years and show higher site fidelity, causing well-marked subspecific differentiation in these species



• in shorebirds that show “sex-role reversal” (e.g. Phalaropes), males also tend to disperse furthest



• several types of evidence for effects of competition (for territories, food, nest sites, mates …) on dispersal distances

• (i) fledglings tend to disperse further from their natal sites in years of higher population density (e.g. European greenfinch)

• (ii) fledglings tend to disperse further in years of low food-supply (e.g. Coal tit); especially evident in owl species that feed on voles (Tengmalm’s owl, Tawny owl, Ural owl) where natal dispersal distances roughly vary in three-year periodicity

• (iii) experimental provision of food reduced dispersal distances in Song sparrows



• (iv) late-fledging young disperse further than early fledgers (e.g. Tree sparrows)

• conclusion: dispersal distances are density-dependent and range expansions most likely occur with the build-up of population pressure within existing ranges


Breeding dispersalBreeding dispersal

• birds that have previously bred in an area tend to stay there, or return to breed year after year

• in many species of seabirds and raptors, individuals often return to exactly the same sites


Percentage of adults in migratory bird populations that returned to the same breeding (upper) or wintering

(lower) sites in successive years


Proportion of adult Peregrine falcons that were present at nests in the same study areas in successive years



• within species, mean adult breeding dispersal distances are typically about one-half of natal distances, but often much smaller

Relationships between the geometric means of the natal and breeding dispersal distances of various bird species



• (i) higher site fidelity in males

• (ii) higher site fidelity at later age

• (iii) greater tendency to change territorities after breeding failure

• (iv) tendency to move to better territories through early life

• (v) change in territory often associated with change of mate



• patterns may be partly interdependent, as young birds are more likely to obtain poor territories and fail in their breeding

• territory shifts can either be voluntarily or forced by dominant individuals; may result in mixing of individuals hatched in different localities and, hence, in gene flow

• multiple broods may involve different partners at close locations (e.g. House sparrow); other species may nest in widely separated localities within single breeding season, i.e. due to sporadic food-supplies (e.g. Eurasian bullfinch, Common redpoll, Eurasian siskin) or because they stop and breed at two or more point on migration route (e.g. +400 km for Eurasian dotterel)


Long-distance dispersalLong-distance dispersal

• when species depend on unpredictable habitat or food supplies, year-to-year homing behaviour is not expected (e.g. spring snow conditions in tundra-nesting species, fluctuating water levels in shorebirds, irregular rainfall patterns in desert species, tree-seed masts in boreal finches, rodent densities in predatory species)


Annual variation in breeding distribution of Great grey owl in Finland



• speed with which local numbers respond suggest largely nomadic ranging behaviour (e.g. Northern Pintail occupying shallow and emphemeral wetlands)


Nomadic ranging behaviour of

Northern pintails



• opportunistic behaviour of dabbling ducks reflected in long natal and breeding dispersal and low site (and mate) fidelity

Sex and age differences in the percentage return rates of waterfowl to particular study areas in successive breeding seasons



• among rodent-eaters, most information is available on Tengmalm’s owls breeding in nestboxes

• long movements mainly observed in females, not in males

• greater residency has been attributed to need to guard cavity nest sites which are scarce in coniferous habitat while smaller size allows them to survive on small birds


Movements of individual adult Tengmalm’s owls between nesting sites in different years



• extraordinarily long natal and breeding dispersal distances recorded from Common Crossbills in western Eurasia, where main food plant is Norway Spruce (strong spatial variation in seed mast)

• yearly make one major movement in June-August (sometimes > 2000 km)

• in Pine areas, individuals show much higher site fidelity


Movements of individual Common crossbills between presumed natal and breeding sites (dashed lines) and

between presumed breeding sites of different years (continuous lines)


Non-breeding dispersalNon-breeding dispersal

• in migrating to wintering areas, young birds do not have the benefits of prior experience as adults do

• young of Swans, Geese and Cranes accompany parents on first autumn migration; may maintain integrity of particular breeding populations



• in most species, young may migrate earlier or later, producing much more scatter in the wintering sites (but not in breeding sites)

• although less substantiated with data, site fidelity in wintering sites shows same pattern as in breeding season (e.g. European greenfinch, Common bullfinch, Ruddy turnstone)


Annual return rates of migrant birds to specific study areas in successive winters



• some individuals of migratory species may hold territories in wintering quarters (e.g. Black redstarts in Spain)

• significant higher proportion of territorial than non-territorial birds observed during consecutive winters in same study area

• non-territorial individuals may suffer greater mortality or may move elsewhere

• apart from differential survival and site fidelity, variation in duration of stay in particular area may affect chance of resighting



• some long-lived species (Grey heron, Black kite, Herring gull, Eurasian oystercatcher) do not breed until several years old

• often show progressive changes to shorter moves or shorter periods away from the breeding areas with increasing age

• while sex differences in winter site fidelity are apparent in some species, they are absent in others


Genetic research and dispersalGenetic research and dispersal

• genetic studies may give additional insight into past dispersal events that influenced present population structure

• usually compare relatedness within and among populations , using measures of DNA structure

• mitochondrial DNA often been used for this purpose, but nuclear microsatellites and minisatellites evolve (mutate) more rapidly, so generally show more variation between subpopulations

• comparisons can yield estimates of degree of genetic differentiation between populations that can be used to assess rates of gene flow



NG

MBCH



Nr

167



• effects of dispersal distances on genetic structure of populations also evident in North American jays of genus Aphelocoma



• pair-breeding species (A. californica): dispersal over long distances (> 50 km)

• co-operative group-breeding species (A. ultramarine, A. coerulescens): dispersal over very short distances (mostly < 2 km)

• in both types, genetic differences between pops increase with distance, but rate of genetic change per unit distance 3 times larger in group-breeding species

• faster rate of molecular differentiation may translate to higher speciation rates

Callens et al. 2011, Mol. Ecol.




• difference in polytypy versus monotypy of Chaffinch – Bramling and European goldfinch - European siskin

Avian migrationAvian migration

Definition of migrationDefinition of migration

• defined as yearly, large-scale return movement of population between regular breeding and wintering (or non-breeding) areas

• produces massive redistribution of birds over the earth’ surface twice each year

• most apparent in seasonal environments, i.e. with contrasting summer-winter or wet-dry conditions

• in shorebirds, conditions in summer (arctic tundra) and winter (i.e. tidal areas) may not allow local wintering and breeding, respectively


Examples of long-distance migrants of birds: (1) Pacific golden plover; (2) Arctic tern; (3) Swainson’s hawk; (4) Snow goose; (5) North American breeding species; (6) Ruff;

(7) European breeding species; (8) Northern wheatear; (9) Amur falcon; (10) Arctic warbler; (11) Short-tailed shearwater


Main migration patterns found in northern hemisphere birds, based on the degree of separation between breeding and wintering ranges



• most marked at high latitudes but also occurs in tropics, i.e. savannahs and grasslands (mainly related to rainfall patterns)

• insectivorous birds confined to (stable) equatorial rain forest least migratory (but see nectar- and fruit-eaters and altitudinal migrants)

• less mobile organisms may cope with seasonal shortages in other ways (e.g. hibernation)

• year-round resident species exploit year-round available food supplies, e.g. species in northern coniferous forests feeding on bark-dwelling arthropods, fruits, seeds, buds, small mammals or birds (Tits, Treecreepers, Corvids, Finches, Grouse, Raptors, Owls)



• migratory species feed on active, leaf-dwelling or aerial insects, or foods that become inaccessible under snow or ice (Warblers, Hirundines, Ground-feeding finches, Thrushes, Raptors, Waterfowl, Waders).

• towards equator, wider ranges of food-types remain available year-round

• various examples of relationship between migration and diet


Migration in relation to diet in west Palaearctic songbirds: % species breeding at different latitudes that migrate south for the

winter (upper); distances moved by migrants (lower)


Migration in relation to diet in west Palaearctic raptors: % species breeding at different latitudes that migrate south for the winter

(left); distances moved (right)


Evolution of migrationEvolution of migration

• can be regarded as product of natural selection and is expected to evolve whenever it provides a fitness advantage (e.g. due to fluctuations in food availability)

• autumn migration mainly driven by improved winter survival

• birds may return in spring because temporary abundance of food, longer days and declining rates of predation on eggs and chicks with latitude allows raising more young

• spring migration mainly driven by improved breeding success

• compared to survival, reproduction often has more stringent requirements in terms of specific food needs and predation avoidance



• in some species, full range of variation in migration behaviour (fully migrant, partial migrant, resident) can be found among different populations

• main adaptations for long-distance migration (fat reserves, timing mechanisms and navigation skills/homing behaviour) all found with somewhat different functions in resident species

• over generations, seasonal timing, directional preferences and distances for movement can become fixed by natural selection, leading to fixed migration routes and destinations



• role of genetic factors in control of migration has been shown experimentally (e.g. on Blackcaps by Peter Berthold and colleagues) by measuring ‘migration restlessness’ (fluttering and wing-whirring in caged birds during migration seasons) in ‘orientation cages’



• when cross-breeding birds from different populations, offspring showed intermediate migration features (timing, duration, direction, propensity to migrate) within 3-6 generations

• all migration properties can be changed rapidly by natural selection; e.g. new migration route among Blackcaps



• other examples of changes in migratory habits confirming that migration is a dynamic phenomenon subject to change in prevailing conditions:

migratory to sedentary

• Lesser black-backed gull: large numbers overwinter in Britain, not prior to 1940

• Eurasian blackbird: British and mid European populations progressively more sedentary during last century




sedentary to migratory

• European serin: spreading north in early 20th century and becoming migratory; previously restricted to southern Europe and resident; recently, migratory population partially resident

• Cattle egret: developing new migrations in newly colonized parts of Old and New world

• more general: species that extended their breeding ranges into areas where overwintering is not possible




shortening of migration routes

• North-American Canada geese and European Cranes responded to increased food due to agricultural changes or creation of refuges to display short-stopping behaviour

• Barnacle geese established nesting colonies well south of historical range, shortening their migration by 1300 km




lengthening of migration routes

• Red-breasted geese winter much further from breeding areas than in 1950s due to land-use changes in former winter sites




timing of migration

• under presumed influence of long-term global warming, many species arrive earlier and departing later from their breeding areas, spending more time on their breeding areas

• still, arrival dates fluctuate from year to year in line with local temperatures


Latitudinal trends and shiftsLatitudinal trends and shifts

• key question: what effects do seasonal migrations have on wide-scale distributions of birds ?

• proportion of migrating species within avifauna’s of different regions closely correlated with climatic seasonality (e.g. temperature extremes between hottest and coldest months) and temporal variation in food availability

• proportion of wholly migratory breeding species increases from 29% (35°N, North Africa) to 83% (80°N, Svalbard), on average 1.3% increase for every degree of latitude


Proportion of western European breeding species at different latitudes that migrate south for the winter



• similar pattern in eastern North America (1.4% per degree, but 17% more at any given latitude in North America); i.e. similar slopes but different intercepts


Proportion of eastern North American breeding species at different latitudes that migrate south for the winter



• seasonal differential in carrying capacity also varies from W to E according to changes in climate (warmer and drier summers, colder winters); e.g. Common starlings on Shetland islands (residential) and at same latitude in Russia (wholly migratory)

• proportion of Australian migratory species declines with increase in amount and eveness of annual rainfall (rather than temperature)



• due to migration, overall species numbers highest during northern summer (N hemisphere) and austral summer (S hemisphere); huge latitudinal shift in avifaunal distribution(e.g. 177 migrant landbirds in Afrotropics)


Latitudinal shift between summer and winter distributions of birds that breed in Europe. Includes wintering areas in Europe, Asia and Africa


Breeding versus wintering areasBreeding versus wintering areas

• implies that most species live at greater densities in African wintering areas than in breeding areas, possibly reflecting differences in per-unit-area productivity

• individuals may also need less space in Africa when only feeding themselves; alternatively, warmer weather may decrease individual daily energy (e.g. Passerine in Africa ca 60% of breeding season needs); similar pattern in New World warblers



• landbirds breeding in S hemisphere do not extend N of tropics on migration, while reverse is true

• Africa: no migration N of Sahara; South-America: nearly no migration N of Panama; Australasia: no migration beyond Indonesia

• mainly short-distance (and/or partial, altitudinal, nomadic) migration by S hemisphere species possibly because encountering progressively widening land areas/settlement opportunities towards N (vice versa for N hemisphere species)



• few migratory bird species have separate breeding populations N and S of tropics, e.g. Little tern, Whiskered tern (Asia-Australië), Black stork, Booted eagle (Europe-Africa), Turkey vulture, Black vulture (N-S America)

• in all species, northern birds winter in S when southern birds are breeding (in Terns, southern birds winter in N when northern birds are breeding)

• raises question why same individuals do not breed twice a year (once in each area)

• possibly because birds moult in winter quarters (mutually exclusive with breeding in most non-migratory species) and migratory birds often move within winter quarters in response to changing food-supplies (avoiding competition with local breeding residents which are tied to fixed nesting areas)



• same trends within species, i.e. when comparing proportion of completely versus partial migrants and distances travelled.

• some species (Fox sparrow, Common ringed plover) show leapfrog migration (i.e. most northerly breeding individuals wintering furthest south)

• possibly following last glaciation when breeding populations spreading north and had to migrate to progressively more southern latitudes to find sufficient vacant habitat

• persisted until now because of continuing competition


Leapfrog migration in Fox sparrows (left) and Common ringed plovers (right)


Breeding versus wintering areasBreeding versus wintering areas

• some European migrants show west-east patterns in distribution (both in breeding/wintering areas); western Lesser whitethroats and Red-backed shrikes migrate via eastern Europe to eastern Africa where joined by eastern breeders; opposite pattern in eastern Pied flycatchers


Loop migration of European Pied flycatchers based on ringing recoveries


Course of first-year migration in White storks from SW (westerly route) and NW (easterly route) Germany


Parallel migrations within Europe, as shown from recoveries of Chaffinches ringed in Russia (filled) and Switzerland (open)


Altitudinal migrationAltitudinal migration

• by moving few hundred meters down the sides of a mountain, birds can achieve as much climatic benefit as by moving several hundred km to lower latitude, but without extra winter daylength; e.g. Citril finch, Rosy finch, Three-wattled bellbird, African Olive pigeon, Olive whistler

• nectar feeders move in reponse to progressively later flowering at higher elevations

• altitudinal/latitudinal movements not mutually exclusive, while some birds move upslope in winter (e.g. Blue grouse)


Movements within breeding seasonMovements within breeding season

• itinerant breeding species breed in more than one area each year; e.g. Common redpolls, European siskins, Spanish sparrow, Common quail (young can mature and breed three months old); similar examples in North America and Africa (Red-billed quelea)

• may be common in semi-arid regions where birds breed during particular stage in dry-wet seasonal cycle

• raising of successive broods at different sites on migration route could occur in a much wider ranges of birds


Movements within non-breeding seasonMovements within non-breeding season

• in some northern species, proportion of birds leaving breeding ranges (and distances traveled) varies greatly from year to year (Common redpoll, Bohemian waxwing, Rough-legged buzzard, Snowy owl); may take form of invasions or irruptions


North American winter ranges of Snowy owl and

Common redpoll



• hard weather movements especially known from species that obtain food from water or soft ground (Northern lapwing, Common starling)

• may return after few days already, possibly to avoid competition in overcrowded hard weather refuges

• some waterfowl (Common shelduck, Eared grebe) perform ‘moult migrations’ after breeding (moving to traditional sites that offer food and safety); may shed all flight feathers at once, remaining flightless for several weeks



• nomadic species exploit sporadic habitats or food resources, i.e. showing little or no year-to-year consistency in their movement patterns

• examples include fruit-eating birds of tropical forests (small scale), rodent-eating owls and raptors of tundra, boreal and arid regions, boreal seed-eaters, and desert species; e.g. Pallas’ sandgrouse (seeds), Rosy starling (grasshoppers)

• remains unknown how these birds find suitable localities within such vast areas; dramatic example: Banded stilts (Australia)

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