Population limitation in migrants

Unlike resident bird species, the population sizes of migratory species can be influenced by conditions in more than one part of the world. Changes in the numbers of migrant birds, either long‐term or year‐to‐year, may be caused by changes in conditions in the breeding or wintering areas or both. The strongest driver of numerical change is provided in whichever area the per capita effects of adverse factors on survival or fecundity are greatest. Examples are given of some species whose numbers have changed in association with conditions in breeding areas, and of others whose numbers have changed in association with conditions in wintering areas. In a few such species, the effects of potential limiting factors have been confirmed locally by experiment. In theory, population sizes might also be limited by severe competition at restricted stopover sites, where bird densities are often high and food supplies heavily depleted, but (with one striking exception) the evidence is as yet no more than suggestive. In some species, habitats occupied in wintering and migration areas, and their associated food supplies, can influence the body condition, migration dates and subsequent breeding success of migrants. Body reserves accumulated in spring by large waterfowl serve for migration and for subsequent breeding, and females with the largest reserves are most likely to produce young. Hence, the conditions experienced by individuals in winter in one region can affect their subsequent breeding success in another region. Such effects are apparent at the level of the individual and at the level of the population. Similarly, the numbers of young produced in one region could, through density‐dependent processes, affect subsequent overall mortality in another region. Events in breeding, migration and wintering areas are thus interlinked in their effects on bird numbers. Although in the last 30–40 years the numbers of some tropical wintering birds have declined in western Europe and others in eastern North America, the causes seem to differ. In Europe, declines have mainly involved species that winter in the arid savannas of tropical Africa, which have suffered from the effects of drought and increasing desertification. In several species, annual fluctuations in numbers and adult survival rates were correlated with annual fluctuations in rainfall, and by implication in winter food supplies. In North America, by contrast, numerical declines have affected many species that breed and winter in forest, especially those eastern species favouring the forest interior. Declines have been attributed ultimately to human‐induced changes in the breeding range, particularly forest fragmentation, which have led to increases in the densities of nest predators and parasitic cowbirds. These in turn are thought to have caused declines in the breeding success of some neotropical migrants, which is now too low to offset the usual adult mortality, but as yet convincing evidence is available for only a minority of species. The breeding rates and population changes of some migratory species have been influenced by natural changes in the availability of defoliating caterpillars. In other species, tropical deforestation is likely to have played the major role in population decline, and if recent rates of tropical deforestation continue, it is likely to affect an increasing range of migratory species in the future. Not all such species are likely to be affected adversely by deforestation, however, and some may benefit from the resulting habitat changes.     

千岛湖陆桥岛屿鸟类集团对栖息地片段化的敏感性及其季节变化

为探究千岛湖陆桥岛屿不同鸟类集团对栖息地片段化敏感性的差异和季节变化,于2009年4月—2012年1月鸟类繁殖季(4、5、6月)和冬季(11、12、1月)对千岛湖41个陆桥岛屿鸟类集团进行了研究。结果表明,冬季杂食鸟对片段化敏感性高于食虫鸟,繁殖季时二者无显著差异,繁殖季和冬季时下层鸟对片段化敏感性均高于林冠鸟,冬季留鸟对片段化敏感性高于候鸟,繁殖季则无显著差异。杂食鸟和留鸟对片段化敏感性存在季节差异,而食虫鸟、林冠鸟、下层鸟和候鸟对片段化敏感性均无季节差异。不同鸟类集团对栖息地片段化敏感性的差异和季节变化规律,有助于人们在栖息地管理和保护区设计时采取更有针对性的鸟类保护措施。     

Winter rainfall predicts phenology in widely separated populations of a migrant songbird

Climate change is affecting behaviour and phenology in many animals. In migratory birds, weather patterns both at breeding and at non-breeding sites can influence the timing of spring migration and breeding. However, variation in responses to weather across a species range has rarely been studied, particularly among populations that may winter in different locations. We used prior knowledge of migratory connectivity to test the influence of weather from predicted non-breeding sites on bird phenology in two breeding populations of a long-distance migratory bird species separated by 3,000 km. We found that winter rainfall showed similar associations with arrival and egg-laying dates in separate breeding populations on an east–west axis: greater rainfall in Jamaica and eastern Mexico was generally associated with advanced American redstart (Setophaga ruticilla) phenology in Ontario and Alberta, respectively. In Ontario, these patterns of response could largely be explained by changes in the behaviour of individual birds, i.e., phenotypic plasticity. By explicitly incorporating migratory connectivity into responses to climate, our data suggest that widely separated breeding populations can show independent and geographically specific associations with changing weather conditions. The tendency of individuals to delay migration and breeding following dry winters could result in population declines due to predicted drying trends in tropical areas and the tight linkage between early arrival/breeding and reproductive success in long-distance migrants.     

Testing agri‐environment delivery for farmland birds at the farm scale: the Hillesden experiment

The Hillesden experiment, established in 2005/2006 to test the delivery of biodiversity benefits under Environmental Stewardship, covers c. 1000 ha of arable farmland in central lowland England. It is a randomized block experiment with five replicates of three treatments: (1) CC: cross compliance, the control; (2) ELS: 1% of land removed from production for wildlife habitat provision; and (3) ELS‐X: 5% of land used for wildlife habitat, each treatment being applied to contiguous areas of 70–80 ha. Bird usage of winter food patches, comprising three different seed mixes, was monitored through the winter and was also related to seed yield. Winter and breeding season bird/territory abundance was recorded before and after the provision of the winter food patches. Bird use of the patches differed between seed mixes. There was large variation between individual patches in both seed yield and bird numbers and between individual bird species in their use of different seed mixes, suggesting that the availability of a range of patch types would be beneficial. Use of all patch types declined sharply in late January to February, indicating depletion and/or inability of birds to access shed seed. Winter bird abundance at a farm scale for all species combined, granivorous species and nine individual species increased for all monitored species when seed patches were available. At a treatment level, the increases tended to be greater in ELS‐X, where most of the patches were located. In the breeding season at a farm scale, the numbers of territories for all species combined and granivorous species increased significantly when seed patches had been available in the previous winter. There was little evidence of a treatment‐scale response. The provision of winter food appeared to increase winter bird abundance and to follow on into an overall increase in the breeding population, but if the latter effect is to be reflected elsewhere, this requires that sufficient breeding habitat is available to accommodate an increase.     

Investigating connectivity and seasonal differences in wind assistance in the migration of Common Sandpipers

Many migratory bird species have undergone recent population declines, but there is considerable variation in trends between species and between populations employing different migratory routes. Understanding species-specific migratory behaviours is therefore of critical importance for their conservation. The Common Sandpiper Actitis hypoleucos is an Afro-Palaearctic migratory bird species whose European populations are in decline. We fitted geolocators to individuals breeding in England or wintering in Senegal to determine their migration routes and breeding or non-breeding locations. We used these geolocator data in combination with previously published data from Scottish breeding birds to determine the distributions and migratory connectivity of breeding (English and Scottish) and wintering (Senegalese) populations of the Common Sandpiper, and used simulated random migrations to investigate wind assistance during autumn and spring migration. We revealed that the Common Sandpipers tagged in England spent the winter in West Africa, and that at least some birds wintering in Senegal bred in Scandinavia; this provides insights into the links between European breeding populations and their wintering grounds. Furthermore, birds tagged in England, Scotland and Senegal overlapped considerably in their migration routes and wintering locations, meaning that local breeding populations could be buffered against habitat change, but susceptible to large-scale environmental changes. These findings also suggest that contrasting population trends in England and Scotland are unlikely to be the result of population-specific migration routes and wintering regions. Finally, we found that birds used wind to facilitate their migration in autumn, but less so in spring, when the wind costs associated with their migrations were higher than expected at random. This was despite the wind costs of simulated migrations being significantly lower in spring than in autumn. Indeed, theory suggests that individuals are under greater time pressures in spring than in autumn because of the time constraints associated with reproduction.     

Impacts of climate and land‐use change on wintering bird populations in Finland

Few studies have covered both the effects of climate and land‐use change on animal populations under a single framework. Besides, the scarce multi‐species studies conducted have focused on breeding data, and there is little information published on changes in wintering populations. Here, we relate the pattern of long‐term temporal trends of wintering bird populations in Finland, north Europe, to covariates associated with climate and land‐use change. Finnish wintering populations have been monitored using ca 10 km winter bird census routes (> 420 routes counted annually) during 1959–2012. Population trends of 63 species were related to migratory strategy, urbanity, and thermal niche measured as species‐specific centre of gravity of the wintering distribution. Waterbird trends have shown a marked increase compared to landbirds. Among landbirds, forest species have suffered severe declines, whereas urban species have considerably increased in their wintering numbers. To follow up these results, we produced three multi‐species indices (for waterbirds, forest and urban species, respectively), which can improve our ability to detect and monitor the specific consequences of climate change and changes in land‐use, but at the same time act as a feedback to track the conservation status of the species. Our results suggest that waterbirds are responding to climate change, given their dependence on open water and the correlation with early‐winter temperature over the last decades. On the other hand, we believe trends of landbirds have been mainly driven by human‐induced land‐use changes. While urban species have likely benefited from the increase of supplementary feeding, forest species have probably suffered from the loss of native habitats.     

Can forest management have season-dependent effects on bird diversity?

Forest management modified the original structure of most European forests, and in the most extreme cases, genuinely natural and semi-natural forests were turned into plantations through clear felling and replanting, often using non-native species. We compared the bird community structure of native oak woods of northern Italy with that of their anthropogenic counter-parts: black locust and sweet chestnut woods. The three stand types were compared in terms of vegetation structure, bird species richness, diversity and abundance of foraging guilds. We analysed both the overwintering and the breeding community, to assess whether management had specific seasonal effects on bird diversity. Forestry-imposed disturbances affected bird diversity more consistently in winter than in breeding time: bird species richness and diversity were significantly greater in oak and chestnut stands, which were the preferred habitat for bark foragers and foliage gleaners. In the breeding period, bird diversity of black locust woodlands increased, and inter-stand differences were not significant. At this time of year, understorey gleaners were more abundant in black locust stands (where shrubs were denser). In winter, species richness, diversity and the abundance of several guilds were positively correlated with stand age, whereas in the breeding period canopy gleaners preferred younger woodlots. Despite the lack of inter-stand differences in breeding bird diversity, young-managed woods benefited generalist birds that need no particular conservation efforts. Conversely, priority species for forest conservation such as specialised bark foragers positively selected native and mature stands throughout the year. We suggest that detailed year-round studies on diversity and community composition could sharpen the precision with which it is possible to prescribe conservation measures in forested areas.     

Annual and seasonal trends in the use of garden feeders by birds in winter

Provision of bird food in gardens is a common activity that may provide an alternative food source to birds in winter. Long-term survey data recording the weekly presence of all bird species using garden feeders in the winter were analysed to see if there was any evidence of trends in feeder use between 1970 and 2000 and whether these trends were correlated with breeding bird population trends. Of 41 species analysed, 21 showed significant increases in occurrence at garden feeders between 1970 and 2000. Many of these increases were evident only in the last 10 years. Several species showed significant positive correlations between trends in winter garden use and trends in relative population size in the previous breeding season. This was especially the case for species with population change (either increase or decrease) of the greatest magnitude. There was no evidence that seasonal shifts in garden feeder use were associated with population change in any species. Temperature was an important predictor of garden use but could not explain the year-to-year trends. However, as the number of feeding stations had increased over time the response of birds to the greater food availability in gardens may have been responsible for the widespread increases in occurrence of several species.     

Does migration promote or restrict circumpolar breeding ranges of arctic birds?

Aim Migration has been suggested to promote large breeding ranges among birds because of the greater mobility of migratory compared with non‐migratory species, but migration has also been suggested to restrict breeding ranges because of evolutionary constraints imposed by the genetically based migration control programme. We aim to investigate the association between migration and the breeding ranges of both land birds and pelagic birds breeding in the Arctic region. Location The Arctic region. Methods Information on breeding and wintering ranges and migratory status of bird species breeding in the arctic tundra biome was compiled from the literature. The association between breeding range, migration distance and primary winter habitat was tested using multivariate generalized linear models and pair‐wise Mann–Whitney U‐tests. Phylogenetic effects were tested for using Mantel’s permutation tests. Results We found different relationships depending on the species’ major winter habitat. Among birds that are pelagic during winter, long‐distance migrants have the largest breeding ranges, while among terrestrial birds, residents and short‐distance migrants have the largest breeding ranges. Breeding ranges of coastal birds of all migratory distance classes are comparatively restricted. Main conclusions As a new explanation for this pattern we suggest that the possibility of colonizing large winter ranges is a key factor for the subsequent expansion of breeding ranges in arctic bird communities and possibly also in bird communities of other regions of the world. Because of the reversal in the relative extent of continents and oceans between the hemispheres, longitudinally wide winter ranges are more likely for long‐distance than short‐distance migrants among pelagic birds, while the reverse holds true for birds that use terrestrial winter habitats. For coastal birds both continents and oceans form barriers restricting colonization of extensive winter quarters and consequently also of extensive breeding ranges, regardless of the distance to the winter quarters.     

Woodland small mammal population ecology in Britain: a preliminary review of the Mammal Society survey of Wood Mice Apodemus sylvaticus and Bank Voles Clethrionomys glareolus, 1982–87

The Mammal Society has co-ordinated a population survey of Wood Mice Apodemus sylvaticus and Bank Voles Clethrionomys glareolus in 13 0.81 -hectare sites in Britain. Numbers of mice and voles live-trapped using standard methods were collated every May/June and November/December from 1982 to 1987. The data were analysed with results from four independent studies in England and the corresponding assessments of tree seed crop size. Wood Mouse numbers are usually higher in winter than in summer but Bank Vole fluctuations are less regular. In deciduous woodland, Wood Mouse mean relative densities are significantly greater in the winter and the following summer after a good seed crop than after a poor one; rates of population change from summer to winter are significantly higher when a good seed crop falls. Bank Vole relative densities are significantly greater in the summer following a good seed crop than after a poor one, and rates of change from winter to summer are significantly higher. In Wood Mouse populations, tests for density dependence suggest that it is strong from summer to winter but absent from winter to summer; in Bank Voles weaker density dependence is present in both halves of the year. Thus, Wood Mouse numbers are regulated in autumn but are also influenced by seed crop size in winter and the following summer; Bank Vole numbers are less strongly regulated during both autumn and spring and are influenced by seed crop size in the following summer. Evidence is presented suggesting that populations of each species in deciduous woodlands are synchronized over the country in summer and that Wood Mice are also synchronized in winter; highs and lows tend to coincide between different sites. The yield of tree seed is shown to vary significantly from year to year and may be the cause of the synchrony, but weather effects may also be involved.     

Winter range compression of migrants in Central America

Where there is seasonal disparity among opportunities, the season with those in shortest supply is most likely to limit populations. Among migrant birds that travel between different breeding and winter ranges, any of breeding, migratory or winter conditions could exclusively constitute such population-limiting factors. In both the New and Old Worlds, landmass is disproportionately concentrated in temperate latitudes. In the Americas, most passerine bird species that breed in the USA and Canada spend the winter further south, commonly in parts of the tropics where landmass is significantly less. Using a sample of 89 migratory species (eight passerine families) that breed in eastern North America, I considered patterns of geographic breeding range size, winter range size and winter distribution. Winter range size is usually smaller than breeding range size (84 of 89 species), often substantially so (minimum 8%, mean 52%). Wintering latitude explains significant variation in both breeding range size and winter range size, as well as in winter range size relative to breeding range size. In particular, all three measures vary latitudinally in patterns similar to latitudinal variation in landmass. These patterns collectively suggest that the reduction in landmass in the latitudes of Central America and the Caribbean is a limiting factor for migrant bird populations, adding to other research concluding that winter conditions sometimes prevail over breeding conditions in the limitation of populations. Hectare for hectare, habitat destruction in the tropics is likely to have the greater impact on the welfare of passerine populations breeding in North America.     

Winter range compression of migrants in Central America

Where there is seasonal disparity among opportunities, the season with those in shortest supply is most likely to limit populations. Among migrant birds that travel between different breeding and winter ranges, any of breeding, migratory or winter conditions could exclusively constitute such population‐limiting factors. In both the New and Old Worlds, landmass is disproportionately concentrated in temperate latitudes. In the Americas, most passerine bird species that breed in the USA and Canada spend the winter further south, commonly in parts of the tropics where landmass is significantly less. Using a sample of 89 migratory species (eight passerine families) that breed in eastern North America, I considered patterns of geographic breeding range size, winter range size and winter distribution. Winter range size is usually smaller than breeding range size (84 of 89 species), often substantially so (minimum 8%, mean 52%). Wintering latitude explains significant variation in both breeding range size and winter range size, as well as in winter range size relative to breeding range size. In particular, all three measures vary latitudinally in patterns similar to latitudinal variation in landmass. These patterns collectively suggest that the reduction in landmass in the latitudes of Central America and the Caribbean is a limiting factor for migrant bird populations, adding to other research concluding that winter conditions sometimes prevail over breeding conditions in the limitation of populations. Hectare for hectare, habitat destruction in the tropics is likely to have the greater impact on the welfare of passerine populations breeding in North America.     

Seasonal difference in the effects of fragmentation on seed dispersal by birds in Japanese temperate forests

Many studies have demonstrated that forest fragmentation reduces populations of animal species and causes local extinction, triggering many cascading effects. The effect of fragmentation on animals can be exerted through various processes, but such effects have been understudied. In this study, we posed the possibility of differences in the seasonal effects of fragmentation on frugivorous birds and their dispersal of seeds belonging to five tree species. We hypothesized that these effects may be caused by birds and their habitat selection for suitable breeding forests. We compared the abundance and species richness of frugivorous birds and the number of bird-removed fruits between a well-preserved and a fragmented temperate forest for two consecutive years. The abundance of birds was lower in the fragmented compared to the well-preserved forest during the breeding season, although no clear differences in species richness were observed. In contrast, similar decreases in bird abundance were not observed during the migratory season. After controlling for variation in crop size, the number of bird-removed fruits was lower in the fragmented forest compared to the well-preserved forest during the breeding season, but there was no such tendency during the migratory season. These results indicate that evaluations regarding the effects of fragmentation on seed dispersal that do not consider seasonal factors may lead to erroneous conclusions. This study suggests that the effects of fragmentation can be exerted though various processes, many of which remain poorly studied and warrant further examination.     

Migratory connectivity in a declining bird species: using feather isotopes to inform demographic modelling

Aim Conservation programmes for endangered migratory species or populations require locating and evaluating breeding, stopover and wintering areas. We used multiple stable isotopes in two endangered European populations of wrynecks, Jynx torquilla L., to locate wintering regions and assess the degree of migratory connectivity between breeding and wintering populations. Location Switzerland and Germany. Methods We analysed stable nitrogen (δ¹⁵N), carbon (δ¹³C) and hydrogen (δD) isotopes from wing feathers from two populations of wrynecks to infer their wintering origins and to assess the strength of migratory connectivity. We tested whether variation in feather isotopic values within the Swiss population was affected by bird age and collection year and then considered differences in isotopic values between the two breeding populations. We used isotopic values of summer‐ and winter‐grown feathers to estimate seasonal distributions. Finally, we calculated a species‐specific δD discrimination factor between feathers and mean annual δD values to assign winter‐grown feathers to origin. Results Bird age and collection year caused substantial isotopic variation in winter‐grown feathers, which may be because of annually variable weather conditions, movements of birds among wintering sites and/or reflect asynchronous moulting or selection pressure. The large isotopic variance in winter‐grown feathers nevertheless suggested low migratory connectivity for each breeding population, with partially overlapping wintering regions for the two populations. Main conclusions Isotopic variance in winter‐grown feathers of two breeding populations of wrynecks and their geographical assignment point to defined, albeit overlapping, wintering areas, suggesting both leapfrog migration and low migratory connectivity. On this basis, integrative demographic models can be built looking at seasonal survival patterns with links to local environmental conditions on both breeding and wintering grounds, which may elucidate causes of declines in migratory bird species.     

Comparing population trend estimates of migratory birds from breeding censuses and capture data at a spring migration bottleneck

Europe has a well‐established network of breeding bird monitoring that is used to produce supranational indices of population trends for many species. However, a comparison of breeding bird censuses with other methods may be beneficial to confirm the validity of such indices. The aim of this study was to assess the value of standardized capture data of migratory birds at migration bottlenecks as an indicator of the effective breeding populations. One limitation to this method is that several populations are co‐occurring at these bottlenecks and their catchment areas need to be clearly identified to allow extrapolation of population indices. Here, we used standardized trends in capture numbers of 30 species on the island of Ponza, a migration bottleneck in the central Mediterranean, and compared them to population trends estimated in the putative catchment breeding areas between 2005 and 2016. The catchment areas were identified through the analysis of ring recoveries during the breeding season of birds passing through Ponza. Our results show an agreement between the population trends observed on Ponza and those in the breeding areas in 15 out of 30 species. The correlations were strongest in species with a more robust definition of the catchment areas, that is, species with more than 10 recoveries, and for which the recoveries were most likely of breeding birds. The main reason for disagreement between the two indices in the remaining species might be related to different intensity of sampling in different areas. This issue can be solved by further developing monitoring projects in underrepresented countries, as well as by intensifying monitoring through ringing, both in the breeding grounds and at migration bottlenecks. These results show that spring migration monitoring at bottlenecks has the potential to provide a valuable complement and an independent control of breeding bird surveys, allowing raising early warnings of population declines and contributing to their conservation.     

Winter availability of cereal stubbles attracts declining farmland birds and positively influences breeding population trends

Many studies have demonstrated the selection of stubble fields by farmland birds in winter, but none have shown whether provisioning of this key habitat positively influences national population trends for widespread farmland birds. We use two complementary extensive bird surveys undertaken at the same localities in summer and winter and show that the area of stubble in winter attracts increased numbers of several bird species of conservation concern. Moreover, for several farmland specialists, the availability of stubble fields in winter positively influenced the 10 year breeding population trend (1994-2003) whereas hedgerow bird species were less affected. For skylarks and yellowhammers, initially negative trends showed recovery with 10-20 ha of stubble per 1 km square. Thus, agri-environment schemes that promote retention of over-winter stubbles will attract birds locally and are capable of reversing current population declines if stubbles are available in sufficient quantity.     

Understanding population change in migratory songbirds: long‐term and experimental studies of Neotropical migrants in breeding and wintering areas

Effective conservation and management of migratory bird species requires an understanding of when and how their populations are limited and regulated. Since 1969, my colleagues and I have been studying migratory songbird populations in their breeding quarters at the Hubbard Brook Experimental Forest in north‐central New Hampshire, USA, and since 1986, in their winter quarters in the Greater Antilles (Jamaica). Long‐term data on the abundance and demography of these populations, coupled with experimental tests of mechanisms, indicate that processes operating in the breeding area (e.g. density‐dependent fecundity, food limitation) are sufficient to limit and regulate the local abundance of these species. At the same time, limiting factors operating in the non‐breeding season (e.g. climate‐induced food limitation in winter quarters and especially mortality during migration) also have important impacts on migrant populations. Furthermore, recent studies have shown that limiting processes during the winter period can carry over into the breeding season and affect reproductive output. These findings clearly demonstrate that to understand changes in abundance of long‐distance migrant species requires knowledge of events operating throughout the annual cycle, which presents a challenge to researchers, managers and others concerned with the welfare of these species.     

黑尾蜡嘴雀冬季对树木果实的取食作用

2009年11月至2011年2月,在江苏省南京市观察到黑尾蜡嘴雀冬季取食12种树木的果实及种子。该鸟通常咬破果皮或种皮,取食种子的胚和胚乳等营养物质,为种子捕食者。由于其他一些食果鸟类也同期取食有关树木的果实,并传播其种子,黑尾蜡嘴雀的取食对这些树木的种子传播的影响是有限的。在初冬季节,黑尾蜡嘴雀常在树冠层取食果实,而在深冬季节,则主要在地表取食落果。黑尾蜡嘴雀能够咬碎枫杨等坚果的坚硬果皮,可为一起在地表觅食的麻雀取食种子碎屑物提供便利条件。黑尾蜡嘴雀与麻雀之间的偏利取食关系属于首次报道。文中对于城市环境管理中合理利用树木落果,为黑尾蜡嘴雀等鸟类提供越冬食物等方面提出了相关建议。     

Habitat associations of British breeding farmland birds

Capsule Territory distribution for ten species was most strongly positively influenced by the presence of hedges and woodland edge.

Aims To describe and rank the importance of different habitat predictors on the distribution of bird territories.

Methods We derived territory maps for ten bird species across 25 sites on English lowland farmland in 2002. We related habitat predictors to the distribution of these species using information theoretic methods.

Results Habitat predictors were ranked as follows (numbers in parentheses indicate the number of species with a strong effect): hedge presence (8), boundary height (7), woodland edge (6), tree presence in boundary (4), brassica (mainly oil seed rape) (3), within‐field vegetation height (3), boundary strip (3), boundary width (3), tilled fields (3), winter set‐aside (2), ditch (1), winter stubble fields (1).

Conclusions Non‐cropped habitats had the most consistent positive effects across all ten species, with crop types and their margins exerting smaller effects.     

The role of migration for spatial turnover of arctic bird species in a circumpolar perspective

Several different factors may determine where species range limits are located within regions of otherwise continuously available habitat and suitable climate. Within the Arctic tundra biome many bird species are migratory and their breeding distributions are affected by migration routes that are in turn limited by factors such as suitable winter habitat, migratory stopover sites, geographical barriers and historical routes of colonization. We identified longitudinal zones in the circumpolar Arctic of pronounced changes in the avian species composition (high species spatial turnover; ‘species divides’). We tested for the association between migratory status and the geographical location and numbers of such species divides for species with non‐breeding habitats mainly within terrestrial, pelagic and coastal ecosystems. Our results demonstrate that migration is of profound importance for both the number and locations of species divides in the Arctic. Long‐distance migration is associated with a large number of divides among terrestrial and coastal arctic birds but with a reduced number of divides among pelagic birds. We suggest that long‐distance migration permits pelagic but not terrestrial and coastal birds to colonize large winter ranges, which in turn causes expansion of breeding ranges, with more homogenous communities and reduction of species divides as consequences, among the long‐distance migrants of pelagic but not of terrestrial and coastal birds. Furthermore, the divides among long‐distance migrants are situated in two main regions, the Beringia and Greenland zones, while divides among short‐distance migrants are more evenly spaced throughout the circumpolar Arctic. The Beringia and Greenland divides result largely from inter‐continental colonization of new breeding ranges but retainment of original winter quarters in a process of evolution through extension of migration routes, leading to aggregated divides in the meeting zones of major global flyways.