Population-specific migratory strategies of Twite Linaria flavirostris in Western Europe

Migratory connectivity describes the extent to which different individuals from specific breeding populations use similar non-breeding sites, with important consequences for the demography and population structure of birds. We used mark-recapture data from the EUring data bank to describe the non-breeding range of Twite Linaria flavirostris within Western Europe. We quantified the strength of migratory connectivity across all European breeding populations, and for the British and continental breeding populations separately. Twite from different parts of the British breeding range used different non-breeding areas from one another, whereas Twite from different parts of the continental breeding range used similar non-breeding areas. There was essentially no overlap during the non-breeding season between British and continental Twite.     

Patterns in departure phenology and mass gain on African non‐breeding territories prior to the Sahara crossing in a long‐distance migrant

Afro‐Palaearctic migrants are declining to a greater degree than other European species, suggesting that processes occurring in Africa or on migration may be driving these trends. Constraints on food availability on the wintering grounds may contribute to these declines but little is known about when and where these resource constraints may occur. Sufficient resources are particularly important prior to spring migration, when migrants must cross the Sahara Desert. We examined mass gain and departure phenology in a long‐distance Palaearctic passerine migrant to determine the degree to which pre‐migratory fattening occurs in their long‐term non‐breeding territories in the Guinea Savannah region of Africa. We monitored 75 Whinchats Saxicola rubetra for departure from their non‐breeding territories in one spring, and analysed mass data of 377 Whinchats collected over three non‐breeding seasons plus 141 migrating Whinchats caught in April over 8 years, all within the same few square kilometres of human‐modified Guinea Savannah in central Nigeria. Whinchats left their winter territories throughout April, with males departing on average 8 days earlier than females. However, there was no evidence that time of departure from territory was linked to age, body size or mass at capture. Whinchats departed their territories with a predicted mass of 16.8 ± 0.3 g, considerably less than the c. 24 g required for the average Whinchat to cross the Sahara directly. Comparing departure dates with arrival dates in southern Europe showed a discrepancy of at least 2 weeks, suggesting that many Whinchats spend considerable time on pre‐migratory fuelling outside their winter territory prior to crossing the Sahara. Overwintering birds gained mass slowly during February and March (0.03 g/day), and non‐territorial or migrating birds at a much higher rate in April (at least 0.23 g/day), with up to 20% of migrating Whinchats in April potentially having sufficient fuel loads to cross the Sahara directly from central Nigeria. Our results suggest that most Whinchats leave their winter territories to fatten up locally or, possibly, by staging further north, closer to the southern limit of the Sahara. Resource constraints are therefore likely to be particularly focused in West Africa during mid‐April and possibly at staging areas before the crossing of the Sahara Desert.     

Low and annually variable migratory connectivity in a long‐distance migrant: Whinchats Saxicola rubetra may show a bet‐hedging strategy

The spatial scale of non‐breeding areas used by long‐distance migrant animals can vary from specific, relatively small non‐breeding areas for each independent breeding population (high connectivity) to a distribution over a large non‐breeding area with mixing of breeding populations (low connectivity). Measuring variation in the degree of connectivity and how it arises is crucial to predict how migratory animals can respond to global habitat and climate change because low connectivity is likely to be an adaptation to environmental uncertainty. Here, we assess whether use of non‐breeding areas in a long‐distance migrant may be stochastic by measuring the degree of connectivity, and whether it is annually variable. Twenty‐nine wintering Whinchats tagged with geolocators over 2 years within 40 km² in central Nigeria were found to be breeding over 2.55 million km² (26% of the land area of Europe), without an asymptote being approached in the relationship between area and sample size. Ranges differed in size between years by 1.51 million km² and only 15% of the total breeding range across both years overlapped (8% overlap between years when only first‐year birds were considered), well above the range size difference and below the proportion of overlap that would be predicted from two equivalent groups breeding at random locations within the observed range. Mean distance between breeding locations (i.e. migratory spread) differed significantly between years (604 ± 18 km in 2013 and 869 ± 33 km in 2014). The results showed very low and variable connectivity that was reasonably robust to the errors and assumptions inherent in the use of geolocators, but with the caveat of having only ranges of 2 years to compare, and the sensitivity of range to the breeding locations of a small number of individuals. However, if representative, the results suggest the scope for between‐year variation (cohort effects) to determine migrant distribution on a large scale. Furthermore, for species with similarly low connectivity, we would predict breeding population trends to reflect average conditions across large non‐breeding areas: thus, as large areas of Africa become subject to habitat loss, migrant populations throughout Europe will decline.     

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.     

High within‐winter and annual survival rates in a declining Afro‐Palaearctic migratory bird suggest that wintering conditions do not limit populations

For migratory birds, it is necessary to estimate annual and overwinter survival rates, identify factors that influence survival, and assess whether survival varies with age and sex if we are to understand population dynamics and thus inform conservation. This study is one of the first to document overwinter and annual survival from the wintering grounds of a declining Afro‐Palaearctic migrant bird, the Whinchat Saxicola rubetra. We monitored a population of marked individuals for which dispersal was low and detectability was high, allowing accurate estimates of survival. Annual survival was at least 52% and did not differ significantly across demographic groups or with habitat characteristics or residency time in the previous winter. Overwinter survival was very high and monthly survival at least 98% at some sites. Although winter residency varied spatially and with age, lower residency did not correlate with reduced annual survival, suggesting occupancy of multiple wintering sites rather than higher winter mortality of individuals with shorter residency. Our results suggest that mortality occurs primarily outside the wintering period, probably during migration, and that wintering conditions have minimal influence on survival. The similarity between survival rates for all age and sex classes when measured on the wintering grounds implies that any difference in survival with age or sex occurs only during the first migration or during the post‐fledging stage, and that selection of wintering habitat, or territory quality, makes little difference to survival in Whinchats. Our findings suggest that the wintering grounds do not limit populations as much as the migratory and breeding stages, with implications for the conservation of declining Afro‐Palaearctic migrants more widely.     

A quantitative measure of migratory connectivity

The study of the extent of the connection between areas where populations of birds breed and areas where they winter has flourished in recent years mainly thanks to the development of new techniques, but also due to traditional ringing and recovery schemes, which allow tracking of individuals or populations linking wintering and breeding distributions. Currently, studies on migratory connectivity focus on retention of breeding population spatial structure on the non-breeding grounds and vice versa.Here we propose a method to quantify migratory connectivity based on Mantel correlation coefficients and to statistically test for deviations of the observed connectivity from a random mix of individuals. In addition, we propose a procedure, based on clustering algorithms, to identify whether observed connectivity depends on aggregation of individuals or on rigid transference of distribution patterns between areas.We applied this method to a large dataset of ringing recoveries of barn swallows (Hirundo rustica L) migrating from their Western Palearctic breeding areas to sub-Saharan winter quarters. We show that migration of barn swallow populations connects specific breeding and wintering areas, and that the “sub-populations” quantitatively identified by our method are consistent with qualitative patterns of migratory connectivity identified by studies of individual geographical populations based on other methods. Finally, we tested the performance of the method by running simulations under different scenarios. Such simulations showed that the method is robust and able to correctly detect migratory connectivity even with smaller datasets and when a strong geographical pattern is not present in the population. Our method provides a quantitative measure of migratory connectivity and allows for the identification of populations showing high connectivity between the breeding and wintering areas. This method is suitable for a generalized application to diverse animal taxa as well as to large scale analyses of connectivity for conservation purposes.     

Density and behaviour of Whinchats Saxicola rubetra on African farmland suggest that winter habitat conditions do not limit European breeding populations

The Whinchat Saxicola rubetra is an Afro‐Palaearctic migrant undergoing widespread population decline. Whinchats winter in West Africa but there are almost no data on their habitat use and behaviour there that may help to explain the cause of this decline. We measured the density of Whinchats, the habitat characteristics associated with their occurrence on farmland, and the relationships between behavioural and habitat variation on farmland around Jos, central Nigeria, over three winters. Whinchats occurred in many fields harvested in the dry season, the density at three sites varying from 0.03 to 0.43 birds/ha, but they were absent at a fourth site. Whinchats were less likely to be found in farmland without particular crops (e.g. structural stem crops such as maize and millet), with more trees, lower amounts of short vegetation (grass, weeds, crops and crop stubble less than 10 cm in height), and higher amounts of medium vegetation (coverage of vegetation 10–100 cm in height) and litter (dead, unburned, vegetation on the ground). Whinchat abundance in areas of farmland where they were present was independent of most variables considered, but density was higher where there was more short vegetation cover. Foraging behaviour did not vary significantly between farmland habitats. All predictors were consistent between season, years and across sites. The presence/absence model was very poor at predicting presence and there were no strong predictors of abundance or foraging variation. This is consistent with a species well below carrying capacity within its environment so that many suitable areas do not have birds present and there is little aggregation at better sites. Overall, Whinchats were abundant and appeared to have plentiful habitat; densities have probably increased alongside the intensification of agriculture (presence of fallow farmland, short vegetation and structural crops). The results suggest that West African farmland in the dry season can support large numbers of Whinchats and that recent population declines in Europe are unlikely to be caused primarily by lack of suitable wintering habitat.     

Strong migratory connectivity and seasonally shifting isotopic niches in geographically separated populations of a long-distance migrating songbird

Whether migratory animals use similar resources during continental-scale movements that characterize their annual cycles is highly relevant to both individual performances and population dynamics. Direct knowledge of the locations and resources used by migrants during non-breeding is generally scarce. Our goal was to estimate migratory connectivity of a small Palaearctic long-distance migrant, the common nightingale Luscinia megarhynchos, and to compare resources used in non-breeding areas with resources used at the breeding grounds. We tracked individuals of three geographically separated populations and characterised their stable isotope niches during breeding and non-breeding over 2 years. Individuals spent the non-breeding period in population-specific clusters from west to central Africa, indicating strong migratory connectivity at the population level. Irrespective of origin, their isotopic niches were surprisingly similar within a particular period, although sites of residence were distant. However, niche characteristics differed markedly between breeding and non-breeding periods, indicating a consistent seasonal isotopic niche shift in the sampled populations. Although nightingales of distinct breeding populations migrated to different non-breeding areas, they chose similar foraging conditions within specific periods. However, nightingales clearly changed resource use between breeding and non-breeding periods, indicating adaptations to changes in food availability.     

Accipiter hawks of the Laurentian Upland and the Interior Plains undertake the longest migrations: insights from birds banded or recovered in Veracruz,Mexico

Bird banding has allowed us to understand diverse aspects of the life histories of migratory raptors. However, most banding stations are located at northern latitudes so what we know about the movements of these raptors is biased toward higher latitudes, primarily from Canada and the United States, leaving important gaps in our knowledge of their movements at lower latitudes. Our objective was to describe the migratory movements of Sharp-shinned (Accipiter striatus) and Cooper’s (A. cooperii) hawks based on banding and recapture records of birds that migrate through Veracruz, Mexico. More specifically, we sought to determine their breeding, migration, and non-breeding locations, estimate their migration distances, and contribute to a better understanding of their migration patterns. With a total of 80 records, we calculated migration distances and used Kernel Density Estimation analyses to identify where these hawks were recaptured or recovered by season. The distribution of recaptures and recoveries largely coincided with breeding locations in the Laurentian Upland and the Interior Plains physiographic regions. All migration records follow a trajectory that extends from the midwestern United States to the Gulf coastal plain of Mexico. The mean breeding season migration distance to Veracruz was 3374 km (a difference of 27 degrees of latitude) for Sharp-shinned Hawks and 2926 km (a difference of 25 degrees of latitude) for Cooper’s Hawks. Our non-breeding records indicate that populations of Accipiter hawks from these North American populations migrate the longest distances to reach Central America, the southernmost distribution of their migratory populations. Distances covered by both species represent round-trip migrations that may be as long as 10,000 km. Our results support those of previous studies and illustrate how continental physiography influences the migration routes, migratory behavior, and migratory connectivity of these hawks.     

The pull of the Central Flyway? Veeries breeding in western Canada migrate using an ancestral eastern route

Understanding non‐breeding season movements and identifying wintering areas of different populations of migratory birds is important for establishing patterns of migratory connectivity over the annual cycle. We analyzed archival solar geolocation (N = 5) and global positioning data (N = 1) to investigate migration routes, stopover sites, and wintering areas of a western‐most breeding population of Veeries (Catharus fuscescens) in the Pemberton Valley, British Columbia, Canada. Geolocation data were analyzed using a Bayesian state‐space model to improve likely position estimates. We compared our results with those from a Veery population located ~250 km east across a mountain chain in the Okanagan Valley, British Columbia, and with an eastern population in Delaware, U.S.A. Migrating Veeries from the Pemberton Valley used an eastern trajectory through the Rocky Mountains to the Great Plains to join a central flyway during fall and spring migration, a route similar to that used by Veeries breeding in the Okanagan Valley. However, wintering destinations of Pemberton Valley birds were more varied, with inter‐individual wintering distances ~1000 km greater than birds from the Okanagan Valley population and ~500 km from the previously known winter range of Veeries. The observed eastern migration path likely follows an ancestral route that evolved following the most recent glacial retreat. Consistent with patterns observed from the Okanagan and Delaware populations, Veeries from the Pemberton Valley undertook an intra‐tropical migration on the wintering grounds, but this winter movement differed from those of previously studied populations. Such winter movements may thus be idiosyncratic or show coarse population associations. Intra‐wintering‐ground movements likely occur either in response to seasonal changes in habitat suitability or as a means of optimizing pre‐migratory fueling prior to long‐distance spring movements to North America.     

Non-migratory stonechats show seasonal changes in the hormonal regulation of non-seasonal territorial aggression

In many birds and mammals, male territorial aggression is modulated by elevated circulating concentrations of the steroid hormone testosterone (T) during the breeding season. However, many species are territorial also during the non-breeding season, when plasma T levels are basal. The endocrine control of non-breeding territorial aggression differs considerably between species, and previous studies on wintering birds suggest differences between migratory and resident species. We investigated the endocrine modulation of territorial aggression during the breeding and non-breeding season in a resident population of European stonechats (Saxicola torquata rubicola). We recorded the aggressive response to a simulated territorial intrusion in spring and winter. Then, we compared the territorial aggression between seasons and in an experiment in which we blocked the androgenic and estrogenic action of T. We found no difference in the aggressive response between the breeding and the non-breeding season. However, similarly to what is found in migratory stonechats, the hormonal treatment decreased aggressive behaviors in resident males in the breeding season, whereas no effects were recorded in the non-breeding season. When we compared the aggressive responses of untreated birds with those obtained from migratory populations in a previous study, we found that territorial aggression of resident males was lower than that of migratory males during the breeding season. Our results show that in a resident population of stonechats T and/or its metabolites control territorial aggression in the breeding but not in the non-breeding season. In addition, our study supports the hypothesis that migratory status does modulate the intensity of aggressive behavior.     

Light‐level geolocators reveal migratory connectivity in European populations of pied flycatchers Ficedula hypoleuca

Understanding what drives or prevents long‐distance migrants to respond to environmental change requires basic knowledge about the wintering and breeding grounds, and the timing of movements between them. Both strong and weak migratory connectivity have been reported for Palearctic passerines wintering in Africa, but this remains unknown for most species. We investigated whether pied flycatchers Ficedula hypoleuca from different breeding populations also differ in wintering locations in west‐Africa. Light‐level geolocator data revealed that flycatchers from different breeding populations travelled to different wintering sites, despite similarity in routes during most of the autumn migration. We found support for strong migratory connectivity showing an unexpected pattern: individuals breeding in Fennoscandia (S‐Finland and S‐Norway) wintered further west compared to individuals breeding at more southern latitudes in the Netherlands and SW‐United Kingdom. The same pattern was found in ring recovery data from sub‐Saharan Africa of individuals with confirmed breeding origin. Furthermore, population‐specific migratory connectivity was associated with geographical variation in breeding and migration phenology: birds from populations which breed and migrate earlier wintered further east than birds from ‘late’ populations. There was no indication that wintering locations were affected by geolocation deployment, as we found high repeatability and consistency in δ¹³C and δ¹⁵N stable isotope ratios of winter grown feathers of individuals with and without a geolocator. We discuss the potential ecological factors causing such an unexpected pattern of migratory connectivity. We hypothesise that population differences in wintering longitudes of pied flycatchers result from geographical variation in breeding phenology and the timing of fuelling for spring migration at the wintering grounds. Future research should aim at describing how temporal dynamics in food availability across the wintering range affects migration, wintering distribution and populations’ capacity to respond to environmental changes.     

Disentangling Migratory Routes and Wintering Grounds of Iberian Near-Threatened European Rollers Coracias garrulus

Long-distance migrants are suffering drastic declines in the last decades. Causes beneath this problem are complex due to the wide spatial and temporal scale involved. We aim to reveal migratory routes, stopover areas, wintering grounds, and migratory strategies for the most southwestern populations of the near-threatened European Roller Coracias garrulus in order to identify conservation key areas for the non-breeding stage of this species. To this end, we used tracking data from seven satellite transmitters fitted to birds breeding in different populations throughout the Iberian Peninsula and four geolocators fitted to individuals in a southeastern Iberian population. Precise satellite data were used to describe daily activity patterns and speed in relation to the main regions crossed during the migration. Individuals from the most southwestern Iberian populations made a detour towards the Atlantic African coast whereas those from northeastern populations followed a straight north-to-south route. We identified important stopover areas in the Sahel belt, mainly in the surroundings of the Lake Chad, and wintering grounds on southwestern Africa farther west than previously reported for the species. Concerning the migratory strategy, satellite data revealed: 1) a mainly nocturnal flying activity, 2) that migration speed depended on the type of crossed habitat, with higher average speed while crossing the desert; and 3) that the migration was slower and lasted longer in autumn than in spring. The studied populations showed weak migratory connectivity, suggesting the confluence of birds from a wide range of breeding grounds in a restricted wintering area. Therefore, we suggest to target on defining precisely key areas for this species and identifying specific threats in them in order to develop an appropriate global conservation programme for the European Roller.     

Birds of a feather winter together: migratory connectivity in the Reed Warbler Acrocephalus scirpaceus

To investigate migratory connectivity in the Reed Warbler Acrocephalus scirpaceus, we analysed (1) all available sub-Saharan ringing recoveries and (2) stable isotopes in feathers grown in Africa sampled at 17 European breeding sites across a migratory divide. A cluster analysis of ringing recoveries showed remarkable connectivity between breeding and non-breeding grounds. Two main clusters represented populations taking the two main migratory routes [southwesterly (SW) and southeasterly (SE)]. Stable isotope analysis confirmed the separation of wintering areas of SW- and SE-migrating populations. Higher δ¹⁵N values in feathers of SE-migrating birds indicated that they occupied more xeric biome types. Values of δ¹³C that did not differ significantly among populations were higher than those from feathers of known European origin and indicated a C4 biome. Three populations with an unknown migratory direction were assigned to the SE-migrating populations on the basis of δ¹⁵N values.     

Dichotomous strategies? The migration of Whimbrels breeding in the eastern Canadian sub‐Arctic

The conservation of migratory birds requires internationally coordinated efforts that, in turn, demand an understanding of population dynamics and connectivity throughout a species' range. Whimbrels (Numenius phaeopus) are a widespread long‐distance migratory shorebird with two disparate North American breeding populations. Monitoring efforts suggest that at least one of these populations is declining, but the level of migratory connectivity linking the two populations to specific non‐breeding sites or identifiable conservation threats remains unclear. We deployed light‐level geolocators in 2012 to track the migration of Whimbrels breeding near Churchill, Manitoba, Canada. In 2013, we recovered 11 of these geolocators, yielding complete migration tracks for nine individuals. During southbound migration, six of the nine Whimbrels stopped at two staging sites on the mid‐Atlantic seaboard of the United States for an average of 22 days, whereas three individuals made nonstop flights of ~8000 km from Churchill to South America. All individuals subsequently spent the entire non‐breeding season along the northern coasts of Brazil and Suriname. On their way north, all birds stopped at the same two staging sites used during southbound migration. Individuals staged at these sites for an average of 34 days, significantly longer than during southbound migration, and all departed within a 5‐day period to undertake nonstop flights ranging from 2600 to 3100 km to the breeding grounds. These extended spring stopovers suggest that female Whimbrels likely employ a mixed breeding strategy, drawing on both endogenous and exogenous reserves to produce their eggs. Our results also demonstrate that this breeding population exhibits a high degree of connectivity among breeding, staging, and wintering sites. As with other long‐distance migratory shorebirds, conservation efforts for this population of Whimbrels must therefore focus on a small, but widely spaced, suite of sites that support a large proportion of the population.     

Consistent annual schedules in a migratory shorebird

Many migratory birds start prebreeding moult and premigratory fuelling some months before the breeding season and face severe time constraints, while travelling up to 15,000 km between non-breeding and breeding grounds. Shorebirds typically leave Southern Hemisphere non-breeding areas over a 3-4 week period, but whether they benefit from interannually consistent timing of departure is unknown. Here, I show that individual bar-tailed godwits (Limosa limosa baueri) from New Zealand are highly consistent in their migratory scheduling. Most birds left within the same week each year (between-year repeatability, r, of 0.83) and adult males, which moult into a bright breeding plumage, were also highly repeatable in the extent of their prebreeding moult (r=0.86). This is consistent with the hypothesis that birds have individually optimized migration schedules. Within adult males, but not females, smaller birds tended to migrate earlier than large birds. Whether this reflects differences in size-related migration speed, optimal breeding time at different sites or size-related natural or sexual selection pressures, remains unknown.     

Continent-wide tracking to determine migratory connectivity and tropical habitat associations of a declining aerial insectivore

North American birds that feed on flying insects are experiencing steep population declines, particularly long-distance migratory populations in the northern breeding range. We determine, for the first time, the level of migratory connectivity across the range of a songbird using direct tracking of individuals, and test whether declining northern populations have higher exposure to agricultural landscapes at their non-breeding grounds in South America. We used light-level geolocators to track purple martins, Progne subis, originating from North American breeding populations, coast-to-coast (n = 95 individuals). We show that breeding populations of the eastern subspecies, P. s. subis, that are separated by ca. 2000 km, nevertheless have almost completely overlapping non-breeding ranges in Brazil. Most (76%) P. s. subis overwintered in northern Brazil near the Amazon River, not in the agricultural landscape of southern Brazil. Individual non-breeding sites had an average of 91 per cent forest and only 4 per cent agricultural ground cover within a 50 km radius, and birds originating from declining northern breeding populations were not more exposed to agricultural landscapes than stable southern breeding populations. Our results show that differences in wintering location and habitat do not explain recent trends in breeding population declines in this species, and instead northern populations may be constrained in their ability to respond to climate change.     

Migratory connectivity and population-specific migration routes in a long-distance migratory bird

Knowledge about migratory connectivity, the degree to which individuals from the same breeding site migrate to the same wintering site, is essential to understand processes affecting populations of migrants throughout the annual cycle. Here, we study the migration system of a long-distance migratory bird, the Montagu''s harrier Circus pygargus, by tracking individuals from different breeding populations throughout northern Europe. We identified three main migration routes towards wintering areas in sub-Saharan Africa. Wintering areas and migration routes of different breeding populations overlapped, a pattern best described by ‘weak (diffuse) connectivity’. Migratory performance, i.e. timing, duration, distance and speed of migration, was surprisingly similar for the three routes despite differences in habitat characteristics. This study provides, to our knowledge, a first comprehensive overview of the migration system of a Palaearctic-African long-distance migrant. We emphasize the importance of spatial scale (e.g. distances between breeding populations) in defining patterns of connectivity and suggest that knowledge about fundamental aspects determining distribution patterns, such as the among-individual variation in mean migration directions, is required to ultimately understand migratory connectivity. Furthermore, we stress that for conservation purposes it is pivotal to consider wintering areas as well as migration routes and in particular stopover sites.     

Experimental and observational studies of seasonal interactions between overlapping life history stages in a migratory bird

Prior to reproduction, migratory animals are at the juxtaposition of three life history stages in which they must finish the non-breeding stage, initiate and complete migration, and prepare for the onset of breeding. However, how these stages interact with one another is not fully understood. We provide evidence that, for migratory birds that begin breeding development prior to departure from non-breeding sites, the level of breeding preparation can drive migration phenology, a critical behavioral determinant of reproductive success. Specifically, male American redstart (Setophaga ruticilla) plasma androgen levels, which increase in males during the period leading into migration, were positively correlated with energetic condition. We empirically tested the hypothesis that elevated androgen simultaneously supports migratory and breeding preparation in a hormone manipulation field experiment. Males with testosterone implants showed advanced preparation for migration and breeding, and ultimately departed on migration earlier than controls. It is assumed that early departure leads to early arrival at breeding areas, which increases breeding success. Collectively, our observational and experimental results demonstrate how overlapping life history stages can interact to influence important components of an individual's fitness. This highlights the critical need for understanding population processes across the full life cycle of an organism to better understand the ecological and evolutionary origins of complex life history events.     

Population-specific adjustment of the annual cycle in a super-swift trans-Saharan migrant

For migratory birds optimal timing of the onset of reproduction is vital, especially when suitable conditions for reproduction occur only for a short while during the year. With increasing latitude the suitable period becomes shorter and we expect the organization of annual cycle to be more synchronized to the local conditions across individuals of same population. This should result in low variation of arrival and departure date in breeding sites at higher latitudes. We quantify the temporal and geographical variation in pre- and post-breeding migration between individuals from four different populations of alpine swifts Tachymarptis melba along a latitudinal gradient. We tracked 215 individuals in three years with geolocators. The two western and two eastern populations showed separate migratory flyways and places of residence in Africa. Length of stay at the breeding sites was negatively correlated with latitude and differed by more than a month between populations. Duration of migration was similarly short in all populations (median 6.2 days in autumn and 8.7 days in spring). However, variation in timing of migration was unrelated to latitude and individuals everywhere arrived in the same asynchrony at the breeding site.