Ecological conditions during winter predict arrival date at the breeding quarters in a trans-Saharan migratory bird

We studied variation in arrival date to the breeding colonies in Italy of a trans‐Saharan migratory bird, the barn swallow Hirundo rustica, in relation to variation in ecological conditions, as reflected by the normalized difference vegetation index (NDVI), in the winter quarters. Arrival date of old but not young individuals captured during consecutive breeding seasons was earlier after winters with favourable conditions. Change in arrival date in relation to change in NDVI was similar in the two sexes. Change in arrival date significantly and positively predicted change in breeding date. As a result of increased frequency of second broods determined by earlier arrival, the number of fledged offspring per season was larger after African winters with good in comparison to poor ecological conditions for barn swallows. This is the first study demonstrating phenotypic plasticity in migration phenology of a long‐distance migratory bird in relation to ecological conditions during wintering.     

Ecological conditions in wintering and passage areas as determinants of timing of spring migration in trans-Saharan migratory birds

1.?Climate change has been associated with shifts in the timing of biological events, including the spring arrival of migratory birds. Early arrival at breeding sites is an important life-history trait, usually associated with higher breeding success and therefore, susceptible to selection and evolution in response to changing climatic conditions. 2.?Here, we examine the effect of changes in the environmental conditions of wintering and passage areas on the mean passage time of 13 trans-Saharan passerines during their spring migration through the western Mediterranean over the 15 years from 1993 to 2007. 3.?We found that most of the species studied have been advancing the timing of their passage in recent years. However, annual variation in the mean date of passage was positively correlated with vegetation growth (measured as the normalized difference vegetation index [NDVI]) both in the Sahel (the region of departure) and in northern Africa (the passage area). Thus, migration dates were delayed in years with high primary productivity in passage and wintering zones. All species seem to respond similarly to NDVI in the Sahel; however, late migrants were less affected by ecological conditions in northern Africa than those migrating earlier, suggesting differences based on species ecology. 4.?Mean timing of passage was not related to the North Atlantic Oscillation (NAO), El Ni?o-Southern Oscillation (ENSO), temperature or NDVI in the species-specific wintering areas (the overwintering region) when analysed in combination with the other covariates. 5.?Our findings show that ecological conditions in the winter quarters (specifically the Sahel) and en route are relevant factors influencing trends in the passage dates of trans-Saharan migratory birds on the southern fringe of Europe. Possible long-term consequences for late arriving spring migrants are discussed.     

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.     

Lifetime individual plasticity in body condition of a migratory bird

Body condition is an important trait that reflects the capacity of individuals to acquire food or resist disease, eventually allowing successful reproduction and survival. We first quantified the effects of condition on life history and a secondary sexual trait using long‐term lifetime data on condition in a migratory bird species, the barn swallow, Hirundo rustica. Second, we quantified sources of individual variation in condition by separating the within‐individual from the between‐individual components of variation of environmental condition and age on body condition. Therefore, we were able to partition variation as a result of selection and phenotypic plasticity. Within‐individual variation in body condition increased in early life until middle age (i.e. 3–4 years of age) in the two sexes followed by only a slight decrease in body condition during senescence in males but not in females. After accounting for age‐dependent variation, condition could be partitioned into a within‐individual plastic response to environmental conditions during migration and a nonplastic response (i.e. a between‐individual difference) to environmental conditions experienced in the African winter quarters. Specifically, there was a within‐individual increase in body condition as environmental conditions during migration improved in both males and females, independent of age. There was a between‐individual effect of condition found in the winter quarter in body condition of females, but not in males, which was attributed to the disappearance of females in poor body condition from the study population as a result of the higher natal dispersal of low‐quality females compared to high‐quality ones during years with favourable environmental conditions in the African winter quarters. Males and females also tended to be in better body condition during the warmer springs upon arrival at the breeding grounds. There was a temporal decline in female body condition during 1991–2007, whereas no significant trend was detected in males. Therefore, both intrinsic (e.g. age and sex) and extrinsic factors (e.g. climate) affected body condition. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 420–434.     

Carry‐over effects of conditions at the wintering grounds on breeding plumage signals in a migratory bird: roles of phenotypic plasticity and selection

To understand the consequences of ever‐changing environment on the dynamics of phenotypic traits, distinguishing between selection processes and individual plasticity is crucial. We examined individual consistency/plasticity in several male secondary sexual traits expressed during the breeding season (white wing and forehead patch size, UV reflectance of white wing patch and dorsal melanin coloration) in a migratory pied flycatcher (Ficedula hypoleuca) population over an 11‐year period. Furthermore, we studied carry‐over effects of three environmental variables (NAO, a climatic index; NDVI, a vegetation index; and rainfall) at the wintering grounds (during prebreeding moult) on the expression of these breeding plumage traits of pied flycatcher males at individual and population levels. Whereas NAO correlates negatively with moisture in West Africa, NDVI correlates positively with primary production. Forehead patch size and melanin coloration were highly consistent within individuals among years, whereas the consistency of the other two traits was moderate. Wing patch size decreased with higher NAO and increased with higher rainfall and NDVI at the individual level. Interestingly, small‐patched males suffered lower survival during high NAO winters than large‐patched males, and vice versa during low NAO winters. These counteracting processes meant that the individual‐level change was masked at the population level where no relationship was found. Our results provide a good example of how variation in the phenotypic composition of a natural population can be a result of both environment‐dependent individual plasticity and short‐term microevolution. Moreover, when plasticity and viability selection operate simultaneously, their impacts on population composition may not be evident.     

Wing morphology,winter ecology,and fecundity selection: evidence for sex-dependence in barn swallows (<Emphasis Type="Italic">Hirundo rustica</Emphasis>)

Variation in wing morphology results from the combination of diverse selection pressures. Wing feather morphology within species varies with sex and ontogenetic effects, and also with ecological factors. Yet, the direction of causation for the wing morphology–ecology association remains to be elucidated. Under the ‘ecology-dependence’ hypothesis, wing morphology covaries with ecological conditions, because the latter affect feather molt. Alternatively, the ‘habitat choice’ hypothesis posits that individuals with different wing morphology choose different habitats because of the habitat-dependent advantages of a specific wing morphology. We tested these competing hypotheses in the migratory, aerially insectivorous barn swallow (Hirundo rustica). We quantified wing morphology (isometric size, pointedness, and convexity) on the same individuals during consecutive breeding seasons (i.e., before and after molt in sub-Saharan wintering areas) and located wintering areas using light-level geolocators. Wing pointedness of females but not males during 1 year negatively correlated with vegetation vigor (gauged by the Normalized Difference Vegetation Index; NDVI) in the African area where individuals spent the next winter. Partial least-squares path modelling showed that the association between wing morphology and NDVI was sex-dependent. Conversely, NDVI during wintering did not predict wing morphology in the next breeding season. Because wing morphology can have carry-over effects on subsequent performance, we investigated selection on wing traits and found strong positive fecundity selection on wing size of females. Our results suggest that female barn swallows choose their wintering habitat depending on their wing morphology. In addition, directional fecundity selection operates on females, suggesting sex-dependence of current selection on the flight apparatus.     

Central European Greylag Geese Anser anser show a shortening of migration distance and earlier spring arrival over 60 years

Global climate change can cause pronounced changes in species? migratory behaviour. Numerous recent studies have demonstrated climate‐driven changes in migration distance and spring arrival date in waterbirds, but detailed studies based on long‐term records of individual recapture or re‐sighting events are scarce. Using re‐sighting data from 430 marked individuals spanning a 60‐year period (winters 1956/1957 to 2015/2016), we assessed patterns in migration distance and spring arrival date, wintering‐site fidelity and survival in the increasing central European breeding population of Greylag Geese Anser anser. We demonstrate a long‐term decrease in migration distance, changes in the wintering range caused by winter partial short‐stopping, and the earlier arrival of geese on their breeding grounds. Greylag Geese marked on central Europe moulting grounds have not been recorded wintering in Spain since 1986 or in Tunisia and Algeria since 2004. The migration distance and spring arrival of geese indicated an effect of temperature at the breeding site and values of the NAO index. Greylag Geese migrate shorter distances and arrive earlier in milder winters. We suggest that shifts in the migratory behaviour of Central European Greylag Geese are individual temperature‐dependent decisions to take advantage of wintering grounds becoming more favourable closer to their breeding grounds, allowing birds to acquire breeding territories earlier.     

Advancement of spring arrival in a long-term study of a passerine bird: sex,age and environmental effects

In migratory birds, mistimed arrival might have negative consequences for individual fitness, causing population declines. This may happen if arrival time is not synchronized with breeding time, especially when earlier springs favour earlier reproduction. We studied spring arrival time to the breeding areas in a pied flycatcher Ficedula hypoleuca population in southern Norway during a 30-year period (1985–2014). We investigated trends in arrival both for the entire population and for different population fractions (e.g. early vs. late arrivals). We also studied sex and age class differences, along with repeatability of arrival. Finally, we explored how arrival is influenced by environmental conditions at the areas birds use throughout the year, using mixed-effects models and quantile regressions with individual-based data. Spring arrival advanced over five days, at a similar rate through the entire population. Males and adult birds arrived earlier than females and yearlings. Arrival was significantly repeatable for males and females. Birds arrived earlier in years with high temperature and rainfall at the breeding grounds, and low NDVI both on the Iberian Peninsula and in central Europe. Later fractions of the population showed a steeper response to these environmental variables. This intra-population heterogeneity in the responses to the environment probably stems from a combination between the different selection pressures individuals are subject to and their age-related experience. Our results highlight the importance of studying how migration phenology is affected by the environment not only on the breeding grounds but also on the other areas birds use throughout the year.     

Searching for potential wintering and migration areas of a Danish Barn Swallow population in South Africa by correlating NDVI with survival estimates

On the basis of correlation analyses between annual Normalised Difference Vegetation Index (NDVI) values in Africa and the annual survival rate estimated for a breeding population of barn swallows Hirundo rustica from Denmark, we identified potential wintering and migration areas in South Africa during December–February and March–May, when barn swallows commonly occur in South Africa. During December–February we identified potential wintering areas only in the western part of South Africa, in the Karoo. Potential areas in the central and eastern parts of the country were only identified during March–May. NDVI values in the Karoo during March–May explained most of the variance in annual adult survival rate of the population. The high ratio of European ringed barn swallows among controlled individuals in the Karoo was similar to the ratio that would be expected based on the number of ringed barn swallows and the population sizes of barn swallows in north-western European breeding populations. The level of this ratio in the Karoo was higher than in any other locality in the central and eastern parts of South Africa and Botswana, indicating that ringed birds from the eastern flyway are absent to a much smaller extent than ringed birds from the western flyway. This approach shows that the NDVI and survival method can focus ringing efforts to regions and areas that are likely to harbour specific breeding populations, thereby helping to identify potential wintering and migration areas for breeding populations of migratory birds.     

Rapid evolutionary change in a secondary sexual character linked to climatic change

The ability of organisms to respond evolutionarily to rapid climatic change is poorly known. Secondary sexual characters show the potential for rapid evolutionary change, as evidenced by strong divergence among species and high evolvability. Here we show that the length of the outermost tail feathers of males of the socially monogamous barn swallow Hirundo rustica, feathers that provide a mating advantage to males, has increased by more than 1 standard deviation during the period from 1984 to 2003. Barn swallows from the Danish population studied here migrate through the Iberian Peninsula to South Africa in fall, and return along the same route in spring. Environmental conditions on the spring staging grounds in Algeria, as indexed by the Normalized Difference Vegetation Index, predicted tail length and change in tail length across generations. However, conditions in the winter quarters and at the breeding grounds did not predict change in tail length. Environmental conditions in Algeria in spring showed a temporal deterioration during the study period, associated with a reduction in annual survival rate of male barn swallows. Phenotypic plasticity in tail length of males, estimated as the increase in tail length from the age of 1 to 2 years, decreased during the course of the study. Estimates of directional selection differentials for male tail length with respect to mating success, breeding date, fecundity, survival and total selection showed temporal variation, with the intensity of breeding date selection, survival selection and total selection declining during the study. Response to selection as estimated from the product of heritability and total selection was very similar to the observed temporal change in tail length. These findings provide evidence of rapid micro-evolutionary change in a secondary sexual character during a very short time period, which is associated with a rapid change in environmental conditions.     

Earlier breeding,lower success: does the spatial scale of climatic conditions matter in a migratory passerine bird?

Following over 20 years of research on the climatic effects on biodiversity we now have strong evidence that climate change affects phenology, fitness, and distribution ranges of different taxa, including birds. Bird phenology likely responds to changes in local weather. It is also affected by climatic year‐to‐year variations on larger scales. Although such scale‐related effects are common in ecology, most studies analyzing the effects of climate change were accomplished using climatic information on a single spatial scale. In this study, we aimed at determining the scale‐dependent sensitivity of breeding phenology and success to climate change in a migratory passerine bird, the barn swallow (Hirundo rustica). For both annual broods, we investigated effects of local weather (local scale) and the North Atlantic Oscillation (NAO, large scale) on the timing of breeding and breeding success. Consistent with previous studies in migratory birds we found that barn swallows in Eastern Germany bred progressively earlier. At the same time, they showed reduced breeding success over time in response to recent climatic changes. Responses to climatic variation were observed on both local and large climatic scales, but they differed with respect to the ecological process considered. Specifically, we found that the timing of breeding was primarily influenced by large‐scale NAO variations and to a lesser extent by local weather on the breeding grounds. Conversely, climatic conditions on the local scale affected breeding success, exclusively. The observed decrease in breeding success over years is likely a consequence of scale‐related mismatches between climatic conditions during different breeding phases. This provides further evidence that a species' response of earlier breeding may not be enough to cope with climate change. Our results emphasize the importance of considering the response of ecological processes along different climatic scales in order to better understand the complexity of climate change effects on biodiversity.     

Age-related migration patterns in Larus fuscus spp.

Migration is a critical period in a bird’s life that can affect the fitness of individuals. Intra-population migratory patterns and the way different sex and/or age classes within a population differ in timing and/or distance of migration are not completely understood. The present study aims to observe inter- and intra-population migratory patterns in the western population of Lesser Black-backed Gulls (Larus fuscus spp.), shedding light on age-related differences of temporal patterns of occurrence in the Portuguese coastal areas during migration and winter. One thousand seven hundred and fifty-four colour ring records were analysed matching a 30-year period of observations on the Portuguese coast between 1975 and 2005. During migration, the graellsii population represents 90% of the migratory flow of L. fuscus through Portugal with the intermedius accounting for 9% and the fuscus population, being vestigial in this period, accounting for 1%. Nevertheless, interesting significant differences were observed between the age classes of the three populations during this period, the graellsii population having a large number of first winters (40% of the migratory contingent of this population) followed by immatures and adults whilst in the intermedius and fuscus populations, the largest age class is the adults. During winter, no inter-population differences were found. When comparing migration and winter periods, intra-population differences were found in the graellsii and fuscus populations regarding distribution and age classes. These results indicate different migratory routes amongst different populations suggesting a leapfrog migration in L. fuscus and also a differential age-related migration pattern that might result from first winters migrating further south in search of a wintering place since adults heavily occupy the closest wintering quarters in their attempt to arrive earlier at their breeding ground.     

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.     

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.     

The relative importance of conditions in wintering and passage areas on spring arrival dates: the case of long-distance Iberian migrants

Remote sensing data have been used in previous studies to assess the effects of winter ecological conditions in Africa on biological parameters recorded in bird populations during the following breeding season in Europe. Based on the results of these studies, we hypothesized that a high productivity of vegetation during the winter and, thus, high resource availability, should advance the arrival of long-distance migrants to the European breeding areas due to enhanced ecological conditions. To test this hypothesis, between 1982 and 2000 we examined the first arrival date to the Iberian Peninsula of five species (White Stork, Cuckoo, Common Swift, Barn Swallow and Nightingale) in relation to several explanatory variables: ecological conditions in their African wintering grounds and passage areas, as reflected by the normalized difference vegetation index (NDVI), temperature and precipitation in their passage areas and the winter North Atlantic Oscillation (NAO). Ecological conditions in the wintering areas were important for White Stork, Cuckoo and Barn Swallow phenology, while both NDVI in passage areas and NAO did not have an effect on any species. Migratory birds arrived earlier after winters with high vegetation productivity in Africa. Temperature in passage areas was important for the later species (i.e. Cuckoo, Common Swift and Nightingale), although in all cases the true relevance of this factor was scarce due to the poor explanatory capacity of the models. These species were recorded in the Iberian Peninsula earlier in the spring of those years with warmer temperatures in passage areas. The nexus between African NDVI and arrival phenology is hypothesized through increases in wintering survival rates and/or the faster acquisition of pre-migratory body condition and progression through sub-Saharan areas. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Climate change, migratory connctivity and changes in laying date and clutch size of the pied flycatcher

We examined long-term (1943–2003) variability in laying dates and clutch sizes in a Finnish population of the pied flycatcher Ficedula hypoleuca Pallas, and analysed whether potential changes were explained by changes in climatic factors at the wintering area in Africa, at migration route or at breeding grounds. Among-year variation in both mean and skewness of laying dates increased, which for mean laying date appeared to be explained by variability of temperatures at the breeding grounds and for skewness by variable temperature trends along the migration route. Pied flycatchers bred earlier in warm springs, but despite a warming trend in pre-laying temperatures, the laying dates tended to delay. Laying dates became continuously later in relation to the phenology of the environment. Mean clutch size decreased with time when mean laying date was controlled for, but the climatic factors did not appear to explain the decrease. The advancement of spring phenology may have shifted some food sources needed for egg-laying, thus leading to later laying and smaller clutches. Variation in clutch size increased when wintering conditions were favourable so that clutch size distribution was skewed with a tail of small clutches when there had been lot of rainfall (more vegetation and insects) in the wintering area. We suggest that when ecological conditions during winter were good, the tail of small clutches represented low-quality individuals that were not able to breed after bad winters. Our analyses demonstrate that measures of spread and symmetry give different information about population level changes than means, and thus complement the understanding of the potential influences of climate change on populations.     

Senescence in a short-lived migratory bird: age-dependent morphology, migration, reproduction and parasitism

1. Senescence reflects age-dependent changes in residual reproductive value. Annual survival rates of the barn swallow Hirundo rustica L. increased from 1- to 2-year-old individuals, but decreased among 5 years old or older individuals. Estimates of age-dependent reproductive value showed a similar pattern.
2. Longitudinal data from two long-term population studies were used to test whether a number of different measures of performance (condition-dependent morphological traits, migratory performance, reproductive success, intensity of parasitism) changed among individuals when reaching old age.
3. The length of the outermost tail feathers (a secondary sexual character) decreased among old individuals, while two measures of individual developmental instability increased with age. Migratory performance decreased in old barn swallows as reflected by a delay in spring arrival at the breeding grounds. Reproductive performance measured as seasonal reproductive success decreased with age. The intensity of infestations with an haematophagous mite and a mallophagous ectoparasite increased among old barn swallows.
4. These results suggest that the condition-dependent secondary sexual character, developmental stability, and measures of migratory and reproductive performance deteriorated, and the frequency of parasitism increased among old individuals. Ageing was thus associated with a general deterioration of performance.     

Population-scale drivers of individual arrival times in migratory birds

1. In migratory species, early arrival on the breeding grounds can often enhance breeding success. Timing of spring migration is therefore a key process that is likely to be influenced both by factors specific to individuals, such as the quality of winter and breeding locations and the distance between them, and by annual variation in weather conditions before and during migration. 2. The Icelandic black-tailed godwit Limosa limosa islandica population is currently increasing and, throughout Iceland, is expanding into poorer quality breeding areas. Using a unique data set of arrival times in Iceland in different years for individuals of known breeding and wintering locations, we show that individuals breeding in lower quality, recently occupied and colder areas arrive later than those from traditionally occupied areas. The population is also expanding into new wintering areas, and males from traditionally occupied winter sites also arrive earlier than those occupying novel sites. 3. Annual variation in timing of migration of individuals is influenced by large-scale weather systems (the North Atlantic Oscillation), but between-individual variation is a stronger predictor of arrival time than the NAO. Distance between winter and breeding sites does not influence arrival times. 4. Annual variation in timing of migration is therefore influenced by climatic factors, but the pattern of individual arrival is primarily related to breeding and winter habitat quality. These habitat effects on arrival patterns are likely to operate through variation in individual condition and local-scale density-dependent processes. Timing of migration thus appears to be a key component of the intricate relationship between wintering and breeding grounds in this migratory system.     

Antioxidants and condition-dependence of arrival date in a migratory passerine

Early arrival at the breeding area after spring migration determines several fitness benefits to migratory birds, including an early start of reproduction and a greater seasonal reproductive success. Environmental conditions determine the onset of migration at the population level, while physiological costs associated with migration and imposed on individuals determine the level of asynchrony in the pattern of arrival. These costs include several physiological changes, suppression of immune response, and an increased risk of oxidation by production of free radicals due to energetically expensive activities. According to the condition-dependent hypothesis of arrival date, only healthy individuals in prime condition can afford to arrive and thus reproduce early in the season.
We investigated condition-dependence of arrival date in male barn swallow ( Hirundo rustica L.), a migratory passerine, in two years that differed in environmental condition, and the role of carotenoids in mediating arrival date. We show that male tail length, the main secondary sexual character, body condition, and hematocrit were negatively correlated with arrival date in the first year, indicating better quality of early arriving males. In the second year, better environmental conditions advanced arrival of the entire population by weeks. In this season we could show no relationship between arrival and tail length or body condition. Moreover, a lower value of sedimentation rate and a brighter colour of the red throat feathers indicated better health status of the population. Arrival date is a condition-dependent character in barn swallows, since we found high repeatability of arrival date, lower values of carotenoids in blood and a lower depletion of carotenoids in blood shortly after arrival in early arriving males. This suggests that early arriving males either use fewer antioxidants for free radical scavenging, or they have differential access to antioxidants.     

Barriers and distances as determinants for the evolution of bird migration links: the arctic shorebird system

We analysed migratory connectivity between different winter quarters and breeding sectors in the circumpolar tundra region for arctic shorebirds, in relation to migratory distances and ecological barriers. Total distances and barriers were calculated and measured for all potential migratory orthodrome links between 10 selected winter regions and 12 breeding sectors. The migratory segment between the northernmost stopover site and the breeding area, associated with the entry to and exit from the tundra during spring and autumn migration, respectively, was also identified and measured for each potential link.The analysis indicated that the evolution of migratory links among arctic shorebirds is constrained not by distance as such but by distance across ecological barriers, possibly because of the complex adaptations required for barrier crossing and extensive detour migration (and in a few cases because barrier distances exceed the birds' theoretical flight range capacity). A particularly pronounced barrier effect of the Arctic Ocean, as apparent from a sharp decline in migratory connectivity between the opposite sides of the Arctic Ocean, may reflect a crucial importance of favourable entry and exit conditions for successfully occupying different sectors of the tundra breeding area by shorebirds from winter regions situated at widely different total distances in both the southern and northern hemispheres.