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.     

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.     

Comparing the seasonal survival of resident and migratory oystercatchers: carry‐over effects of habitat quality and weather conditions

Events happening in one season can affect life‐history traits at (the) subsequent season(s) by carry‐over effects. Wintering conditions are known to affect breeding success, but few studies have investigated carry‐over effects on survival. The Eurasian oystercatcher Haematopus ostralegus is a coastal wader with sedentary populations at temperate sites and migratory populations in northern breeding grounds of Europe. We pooled continental European ringing‐recovery datasets from 1975 to 2000 to estimate winter and summer survival rates of migrant and resident populations and to investigate long‐term effects of winter habitat changes. During mild climatic periods, adults of both migratory and resident populations exhibited survival rates 2% lower in summer than in winter. Severe winters reduced survival rates (down to 25% reduction) and were often followed by a decline in survival during the following summer, via short‐term carry‐over effects. Habitat changes in the Dutch wintering grounds caused a reduction in food stocks, leading to reduced survival rates, particularly in young birds. Therefore, wintering habitat changes resulted in long‐term (>10 years) 8.7 and 9.4% decrease in adult annual survival of migrant and resident populations respectively. Studying the impact of carry‐over effects is crucial for understanding the life history of migratory birds and the development of conservation measures.     

Winter commingling of populations of migratory species can cause breeding range underpopulation

We build a model with large-scale demographic consequences for migratory species. The model operates where four elements co-occur, and we rely on empirical research using migratory birds to demonstrate them. First, breeding ranges have internal structure flowing from natal philopatry. Second, fecundity varies geographically. Third, populations of different breeding provenances commingle during winter. And fourth, a population-limiting carrying capacity operates during winter. In the absence of breeding season population-limitation, only the breeding population with maximum fecundity persists. Consequently, some potential breeding areas that offer suitable and productive habitat are bereft of breeding birds because of the interplay between the geographical fecundity gradient and the shared winter quarters. Where breeding season population-limitation also plays a role for at least one population, one (or more) breeding population becomes permanently depressed, resulting in a density well below the carrying capacity of the productive breeding habitat that is occupied. In either case, not all populations fare equally well, despite net positive breeding season productivity. Changes in winter carrying capacity, for example habitat degradation in winter quarters, can lead to uneven effects on geographically defined breeding populations, even though there has been no change in the circumstances of the breeding range.     

Recent population declines in Afro‐Palaearctic migratory birds: the influence of breeding and non‐breeding seasons

Aim

Recent, rapid population declines in many Afro‐Palaearctic migratory bird species have focussed attention on changing conditions within Africa. However, processes influencing population change can operate throughout the annual cycle and throughout migratory ranges. Here, we explore the evidence for impacts of breeding and non‐breeding conditions on population trends of British breeding birds of varying migratory status and wintering ecology.

Location

Great Britain (England & Scotland).

Methods

Within‐ and between‐species variation in population trends is quantified for 46 bird species with differing migration strategies.

Results

Between 1994 and 2007, rates of population change in Scotland and England differed significantly for 19 resident and 15 long‐distance migrant species, but were similar for 12 short‐distance migrant species. Of the six long‐distance migrant species that winter in the arid zone of Africa, five are increasing in abundance throughout Britain. In contrast, the seven species wintering in the humid zone of Africa are all declining in England, but five of these are increasing in Scotland. Consequently, populations of both arid and humid zone species are increasing significantly faster in Scotland than England, and only the English breeding populations of species wintering in the humid zone are declining.

Main conclusions

Population declines in long‐distance migrants, especially those wintering in the humid zone, but not residents or short‐distance migrants suggest an influence of non‐breeding season conditions on population trends. However, the consistently less favourable population trends in England than Scotland of long‐distance migrant and resident species strongly suggest that variation in the quality of breeding grounds is influencing recent population changes. The declines in humid zone species in England, but not Scotland, may result from poorer breeding conditions in England exacerbating the impacts of non‐breeding conditions or the costs associated with a longer migration, while better conditions in Scotland may be buffering these impacts.

     

The interacting effects of food,spring temperature,and global climate cycles on population dynamics of a migratory songbird

Although long‐distance migratory songbirds are widely believed to be at risk from warming temperature trends, species capable of attempting more than one brood in a breeding season could benefit from extended breeding seasons in warmer springs. To evaluate local and global factors affecting population dynamics of the black‐throated blue warbler (Setophaga caerulescens), a double‐brooded long‐distance migrant, we used Pradel models to analyze 25 years of mark–recapture data collected in New Hampshire, USA. We assessed the effects of spring temperature (local weather) and the El Niño Southern Oscillation index (a global climate cycle), as well as predator abundance, insect biomass, and local conspecific density on population growth in the subsequent year. Local and global climatic conditions affected warbler populations in different ways. We found that warbler population growth was lower following El Niño years (which have been linked to poor survival in the wintering grounds and low fledging weights in the breeding grounds) than La Niña years. At a local scale, populations increased following years with warm springs and abundant late‐season food, but were unaffected by spring temperature following years when food was scarce. These results indicate that the warming temperature trends might have a positive effect on recruitment and population growth of black‐throated blue warblers if food abundance is sustained in breeding areas. In contrast, potential intensification of future El Niño events could negatively impact vital rates and populations of this species.     

Carry‐over effects of winter habitat quality on en route timing and condition of a migratory passerine during spring migration

We examined how conditions prior to migration influenced migration performance of two breeding populations of black‐and‐white warblers Mniotilta varia by linking information on the migrant's winter habitat quality, measured via stable carbon isotopes, with information on their breeding destination, measured via stable hydrogen isotopes. The quality of winter habitat strongly influenced the timing of migration when we accounted for differential timing of migration between breeding populations. Among birds migrating to the same breeding destination, males and females arriving early to the stopover site originated from more mesic habitat than later arriving birds, suggesting that the benefits of occupying high‐quality mesic habitat during the winter positively influence the timing of migration. However, male warblers arriving early to the stopover site were not in better migratory condition than later arriving conspecifics that originated from poor‐quality xeric winter habitat, regardless of breeding destination. The two breeding populations stopover at the study site during different time periods, suggesting that the lower migratory condition of early birds is not a function of the time of season, but potentially a migrant's migration strategy. Strong selection pressures to arrive early on the breeding grounds to secure high‐quality breeding territories may drive males from high‐quality winter habitat to minimize time at the expense of energy. This migration strategy would result in a smaller margin of safety to buffer the effects of adverse weather or scarcity of food, increasing the risk of mortality. The migratory condition of females was the same regardless of the timing of migration or breeding destination, suggesting that females adopt a strategy that conserves energy during migration. This study fills an important gap in our understanding of the linkages between winter habitat quality and factors that influence the performance of migration, the phase of the annual cycle thought to be limiting most migratory bird populations.     

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.     

Seasonal distribution of a migratory bird: effects of local and regional resource tracking

Aim We studied how local and regional abundance of a migratory passerine (the blackcap Sylvia atricapilla) track resource availability in breeding and wintering grounds, in an attempt to understand the processes underlying the distribution and regulation of migratory bird populations in summer and in winter. Location Our study was conducted in Spain. In summer, we sampled five localities representing the diversity of environmental conditions met by breeding Spanish blackcaps. In winter, we sampled eight localities in the wintering range of the species including different habitat types (forests and shrublands). Methods Our approach was based on the matching rule, a model that predicts that any local variation in resource abundance between two adjacent habitat patches should be tracked by animals through a similar variation in population abundance. Eventually, this local process should conform to abundance distributions at regional scales. We sampled two habitat patches in each locality, each one including three to five line transects, 500‐m long and 50‐m wide, where we counted blackcaps and measured vegetation structure and fruit abundance. Results During the breeding season, the abundance of blackcaps was strongly correlated with the ground cover of brambles (Rubus spp.), a bush which grows in moist sectors in Mediterranean forests and is the commonest nesting substrate of Spanish blackcaps. Both local and regional changes in bramble cover were tracked by variations in blackcap abundance. However, the rate of increase in blackcap abundance with increasing bramble cover along the Spanish gradient was lower than the one predicted under resource matching. In winter, abundance of fruiting shrubs was the best predictor of blackcap abundance, although local abundance of blackcaps not always fitted local abundance of fruits. Notwithstanding this effect, the regional pattern of abundance tracked changes in fruit availability according to the matching rule. Main conclusions Our results support the strong effect of habitat quality on the abundance distribution of blackcaps and the tracking of different key resources along the year. Together with the different degrees of resource tracking by blackcaps at local and regional scales, these results also support the view that both breeding and wintering processes have to be studied, and studies have to be conducted at the appropriate spatial scales, if we are to understand the processes underlying the abundance distribution of migratory birds.     

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.     

Energetic consequences of contrasting winter migratory strategies in a sympatric Arctic seabird duet

At the onset of winter, warm‐blooded animals inhabiting seasonal environments may remain resident and face poorer climatic conditions, or migrate towards more favourable habitats. While the origins and evolution of migratory choices have been extensively studied, their consequences on avian energy balance and winter survival are poorly understood, especially in species difficult to observe such as seabirds. Using miniaturized geolocators, time‐depth recorders and a mechanistic model, we investigated the migratory strategies, the activity levels and the energy expenditure of the closely‐related, sympatrically breeding Brünnich's guillemots Uria lomvia and common guillemots Uria aalge from Bjørnøya, Svalbard. The two guillemot species from this region present contrasting migratory strategies and wintering quarters: Brünnich's guillemots migrate across the North Atlantic to overwinter off southeast Greenland and Faroe Islands, while common guillemots remain resident in the Barents, the Norwegian and the White Seas. Results show that both species display a marked behavioural plasticity to respond to environmental constraint, notably modulating their foraging effort and diving behaviour. Nevertheless, we provide evidence that the migratory strategy adopted by guillemots can have important consequences for their energy balance. Overall energy expenditure estimated for the non‐breeding season is relatively similar between both species, suggesting that both southward migration and high‐arctic winter residency are energetically equivalent and suitable strategies. However, we also demonstrate that the migratory strategy adopted by Brünnich's guillemots allows them to have reduced daily energy expenditures during the challenging winter period. We therefore speculate that ‘resident’ common guillemots are more vulnerable than ‘migrating’ Brünnich's guillemots to harsh winter environmental conditions.     

Short‐distance partial migration of Neotropical birds: a community‐level test of the foraging limitation hypothesis

Partial migration of tropical birds was long believed to be driven by variation in food abundance. Recent evidence from a partially‐migratory species suggests that in contrast, limited foraging opportunities at high elevations during severe wet season storms drives the most metabolically‐challenged individuals down to elevations where rainfall is lighter. Here, I test community‐level predictions of this hypothesis by examining the relationship between high‐elevation rainfall in the second half of the year and counts of migrant birds in lowland forest during late December each year from 1990–2009. I contrast results derived from analysis of all migrant species with both analyses of only the frugivorous migrants, and analyses of resident species. Counts of migrant species were on average positively associated with montane rainfall with differences of up to 72% in the numbers of birds counted in drier or wetter years. Frugivores and smaller birds responded more strongly to variation in rainfall compared to the broader migrant species pool. Interestingly, counts of resident species were also higher following wetter montane wet seasons. Results of analyses exploring the cause of resident responses were not consistent with climatic effects on breeding productivity or short‐term weather effects on detectability. Results were, however, consistent with cryptic down‐slope migration of individuals breeding at higher elevations augmenting lowland resident populations in wet years. These results suggest that changes in rainfall amount, storm intensity, and timing of severe weather events would lead to large increases in or losses of an important behaviour.     

Do migratory pathways affect the regional abundance of wintering birds? A test in northern Spain

Aim The abundance distribution of organisms at regional scales is commonly interpreted as the result of spatial variation in habitat suitability. However, the possibility that geography itself may affect patterns of distribution has received less attention. For example, the abundance of wintering bird populations might be influenced by the cost of reaching areas located far away from the main migratory pathways. We studied the abundance distribution of three common migratory passerines (meadow pipits, Anthus pratensis; common chaffinches, Fringilla coelebs; and European robins, Erithacus rubecula) wintering in farmlands located in the 600‐km long Cantabrian coastal sector of northern Spain, roughly perpendicular to the west Pyrenean migratory pathway that drives European migrant birds into the Iberian Peninsula. Location The study area occupies a belt located between the Atlantic coast and the Cantabrian Mountains in northern Spain. Methods We counted wintering and breeding birds and measured the structure of vegetation and environmental variables (altitude, rainfall, temperature) in 68 farmlands distributed at different distances from the west Pyrenean migratory flyway. We also studied the distribution of birds ringed in central and northern Europe and recovered in the study area between October and February. Analyses were based on single univariate statistics (chi‐square tests), ordination by principal components analysis and multiple regression. Results Controlling for the effects of climate, vegetation structure and local abundance of breeding conspecifics, the winter abundance of all three species decreased with the distance from their main migratory route in the western Pyrenees. Such patterns fitted well to the observed distribution of ringing recoveries. Main conclusions Our results support a link between the movements of birds along the Pyrenean migratory pathway and their winter abundance in northern Spain. According to this view, the sectors located near the migratory pathway seem to be more easily occupied by migrants, supporting the idea that proximity to passage areas may explain the fine‐grain regional patterning of species abundance in wintering grounds.     

Migrants in tropical bird communities: the balanced breeding limitation hypothesis

Explanations for the ecological integration of migratory and non-migratory (resident) insectivorous birds in the tropics have been complicated by the paradox that arthropod abundances are low when bird abundances reach their annual peak. The breeding currency hypothesis and the nest predation hypothesis both account for this paradox by postulating that residents are held below the non-breeding season carrying capacity, which frees resources available for migratory insectivores. The breeding currency hypothesis suggests residents are limited by food suitable for nestlings, whereas the nest predation hypothesis emphasizes the primacy of high rates of nest predation. However, theoretical arguments suggest that food availability and predation risk interact strongly to limit breeding birds. We use graphical analyses to extend the breeding currency hypothesis to incorporate effects of nest predation. This yields a more synthetic and realistic model for the integration of migrant and resident insectivores in the tropics – the balanced breeding limitation hypothesis.     

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.     

Heterospecific attraction and food resources in migrants' breeding patch selection in northern boreal forest

We studied experimentally how heterospecific attraction may affect habitat selection of migrant passerine birds in Finnish Lapland. We manipulated the densities of resident tit species (Parus spp.). In four study plots residents were removed before the arrival of the migrants in the first study year, and in four other plots their densities were increased by releasing caught individuals. In the second year the treatments of the areas were reversed, allowing paired comparisons within each plot. We also investigated the relative abundance of arthropods in the study plots by the sweep-net method. This allowed us to estimate the effect of food resources on the abundance of birds. The heterospecific attraction hypothesis predicts that densities of migrant species (especially habitat generalists) would be higher during increased resident density. Results supported this prediction. Densities and number of the most abundant migrant species were significantly higher when resident density was increased than when they were removed. On the species level the redwing (Turdus iliacus) showed the strongest positive response to the increased abundance of tits. Migrant bird abundances seemed not to vary in parallel with relative arthropod abundance, with the exception of the pied flycatcher (Ficedula hypoleuca) which showed a strongly positive correlation with many arthropod groups. The results of the experiment indicate that migrants can use resident tit species as a cue to a profitable breeding patch. The relationship between the abundance of the birds and arthropods suggests that annual changes in food resources during the breeding season probably do not have a very important effect on bird populations in these areas. The results stress the importance of positive interspecific interactions in structuring northern breeding bird communities. Received: 1 September 1997 / Accepted: 22 January 1998     

Seasonal matching of habitat quality and fitness in a migratory bird

When species occupy habitats that vary in quality, choice of habitat can be critical in determining individual fitness. In most migratory species, juveniles migrate independently of their parents and must therefore choose both breeding and winter habitats. Using a unique dataset of marked black-tailed godwits (Limosa limosa islandica) tracked throughout their migratory range, combined with analyses of stable carbon isotope ratios, we show that those individuals that occupy higher quality breeding sites also use higher quality winter sites. This seasonal matching can severely inflate inequalities in individual fitness. This population has expanded over the last century into poorer quality breeding and winter habitats and, across the whole population; individual birds tend to occupy either novel or traditional sites in both seasons. Winter and breeding season habitat selection are thus strongly linked throughout this population; these links have profound implications for a wide range of population and evolutionary processes. As adult godwits are highly philopatric, the initial choice of winter habitat by juveniles will be critical in determining future survival, timing of migration and breeding success.     

Climatic patterns in the establishment of wintering areas by North American migratory birds

Long‐distance migration in birds is relatively well studied in nature; however, one aspect of this phenomenon that remains poorly understood is the pattern of distribution presented by species during arrival to and establishment of wintering areas. Some studies suggest that the selection of areas in winter is somehow determined by climate, given its influence on both the distribution of bird species and their resources. We analyzed whether different migrant passerine species of North America present climatic preferences during arrival to and departure from their wintering areas. We used ecological niche modeling to generate monthly potential climatic distributions for 13 migratory bird species during the winter season by combining the locations recorded per month with four environmental layers. We calculated monthly coefficients of climate variation and then compared two GLM (generalized linear models), evaluated with the AIC (Akaike information criterion), to describe how these coefficients varied over the course of the season, as a measure of the patterns of establishment in the wintering areas. For 11 species, the sites show nonlinear patterns of variation in climatic preferences, with low coefficients of variation at the beginning and end of the season and higher values found in the intermediate months. The remaining two species analyzed showed a different climatic pattern of selective establishment of wintering areas, probably due to taxonomic discrepancy, which would affect their modeled winter distribution. Patterns of establishment of wintering areas in the species showed a climatic preference at the macroscale, suggesting that individuals of several species actively select wintering areas that meet specific climatic conditions. This probably gives them an advantage over the winter and during the return to breeding areas. As these areas become full of migrants, alternative suboptimal sites are occupied. Nonrandom winter area selection may also have consequences for the conservation of migratory bird species, particularly under a scenario of climate change.     

Behaviour and Ecology of the Chough and the Alpine Chough

Analysis of Nest Record Cards reveals altitudinal and habitat differencesbetween these two species. The resident or partially migratory Stonechat has a longer breeding season (with a potential for rearing three broods) than the Whinchat which is a trans-Saharan migrant and arrives later. It is suggested that the greater productivity of the Stonechat might counterbalance a higher winter mortality.