Ecophysiological aspects of rapid population growth in a novel migratory blackcap (Sylvia atricapilla) population: an experimental approach

Summary Blackcaps (Sylvia atricapilla) that breed in central Europe have usually migrated to Mediterranean or African wintering grounds. In the past several decades, a portion of this breeding population has started migrating to the British Isles to overwinter and this population has increased dramatically. Several factors, including higher annual survivorship (due to supplemental feeding and reduced migratiry distance), assortative mating, and enhanced reproductive success may be involved in this rapid population growth. As part of an intensive, long-term study of this population, we tested the hypothesis that the differences in photoperiod experienced by British-wintering versus Mediterranean-wintering blackcaps might lead to relatively early vernal (i.e., migratory and/or reproductive) physiological condition in members of the former group. We found that birds exposed to photoperiodic conditions that simulated migration to Britain to overwinter generally initiated vernal migratory activity earlier than birds held under conditions simulating migration to traditional wintering areas in central Spain. This difference, coupled with the shorter migratory distance to the British Isles, leads to significantly earlier estimated arrival dates for blackcaps that winter in Britain compared to central Spain. Bimodality in arrival times suggests that assortative mating on central European breeding grounds might occur between members of the different wintering populations. Males exposed to British-winter photoperiods showed significantly earlier testicular development than males kept under Spanish-winter photoperiods. Early arrival on the breeding grounds, coupled with accelerated reproductive condition, should lead to a relatively early reproductive effort, perhaps increasing average reproductive success. In general, these results support the hypothesis that differences in photoperiod on the wintering grounds may play an important role in the dynamic state of this population.     

Spring arrival along a migratory divide of sympatric blackcaps (Sylvia atricapilla)

The recent formation of a migratory divide in the blackcap (Sylvia atricapilla) involves sympatrically breeding birds migrating to different overwintering quarters. Within the last 50 years, a novel migratory strategy has evolved resulting in an increasing proportion of birds now wintering in Britain instead of migrating to the traditional sites in the Mediterranean area. This rapid microevolution has been attributed to allochronic spring arrival of migrants from the respective wintering quarters leading to assortative mating. Moreover, blackcaps wintering in Britain may experience fitness advantages owing to improved local wintering conditions. We used stable hydrogen isotope signatures (δD) to scrutinize the degree of temporal segregation of blackcaps upon spring arrival and to test for carry-over effects in body condition associated with the disparate wintering environments. Although we found that migrants from Britain arrive significantly earlier on German breeding grounds than migrants from the Mediterranean region, we also found a considerable overlap in arrival times. In a resampling model, the mean probability of assortative mating of birds wintering in Britain is ≤28% in both years. These results suggest that allochrony alone is not a strong isolating barrier between the two subpopulations. Migrants from both wintering locations did not differ in terms of body mass, mass-tarsus residuals or mass-tarsus ratio and arrived in a similar reproductive disposition. Thus, blackcaps wintering in Britain do not gain an apparent fitness advantage on spring migration due to carry-over effects in body condition. Future studies should explore additional factors such as differences in song quality and habitat that might contribute to the rapid microevolution of the blackcap.     

Migration phenology and winter habitat quality are related to circulating androgen in a long‐distance migratory bird

In migratory birds, the timing of departure from wintering grounds is often dependant on the quality of habitat on an individual's territory and may influence individual fitness, resulting in an interaction of life history stages across large geographical distances. American redstart Setophaga ruticilla males who overwinter in high quality habitats arrive early to breed and subsequently produce more offspring than late arrivers. Since many migratory species overlap vernal migration with the physiological transition to breeding, we examined if breeding preparation plays a role in this seasonal interaction. We tested the hypothesis that early arriving male redstarts from high quality winter habitats are in superior breeding condition by simultaneously measuring winter habitat quality (stable‐carbon isotopes) and breeding preparation (circulating androgen, cloacal protuberance (CP) diameter) upon arrival at breeding grounds. Compared with late arrivers, early arriving males were from higher quality winter habitats and had higher androgen, but smaller CPs. Males arriving with higher androgen were in more advanced physiological migratory condition, as measured by haematocrit. Early arrivers were more likely to successfully breed, but there was no significant relationship between androgen upon arrival and breeding success. One possible explanation for these relationships is that androgen measured during arrival is most relevant in a migratory context, such that birds with high androgen may benefit from effects on migratory condition, positively influencing fitness through earlier arrival.     

Stable isotope ratios indicate that body condition in migrating passerines is influenced by winter habitat

Although predicted some time ago, there has been little success in demonstrating that the overall fitness of migratory birds depends on the combined influences of their experiences over all seasons. We used stable carbon isotope signatures (delta13C) in the claws of migrating black-throated blue warblers Dendroica caerulescens to infer their wintering habitats and investigated whether winter habitat selection can be linked to condition during migration. Resident bird species with low delta13C corresponded to selection of more mesic habitats, and migrating birds with low delta13C were in better condition than conspecifics with higher delta13C signatures. These findings concur with empirical observations on the wintering grounds, where dominants (mostly males) tend to exclude subordinates from mesic areas (considered to be high-quality habitats). We believe that variation in condition during migration may be one of the key factors determining differences in arrival times and condition at the breeding areas, which in turn have a major influence on reproductive success.     

Sex and age-specific differences in wing pointedness and wing length in blackcaps Sylvia atricapilla migrating through the southern Baltic coast

The blackcap Sylvia atricapilla shows a complex migratory pattern and is a suitable species for the studies of morphological migratory syndrome, including adaptations of wing shape to different migratory performance. Obligate migrants of this species that breed in northern, central, and Eastern Europe differ by migration distance and some cover shorter distance to the wintering grounds in the southern part of Europe/North Africa or the British Isles, although others migrate to sub-Saharan Africa. Based on ˃40 years of ringing data on blackcaps captured during autumn migration in the Southern Baltic region, we studied age- and sex-related correlations in wing pointedness and wing length of obligate blackcap migrants to understand the differences in migratory behavior of this species. Even though the recoveries of blackcaps were scarce, we reported some evidence that individuals which differ in migration distance differed also in wing length. We found that wing pointedness significantly increased with an increasing wing length of migrating birds, and adults had longer and more pointed wings than juvenile birds. This indicates stronger antipredator adaptation in juvenile blackcaps than selection on flight efficiency, which is particularly important during migration. Moreover, we documented more pronounced differences in wing length between adult and juvenile males and females. Such differences in wing length may enhance a faster speed of adult male blackcaps along the spring migration route and may be adaptive when taking into account climatic effects, which favor earlier arrival from migration to the breeding grounds.     

Individual variability and versatility in an eco-evolutionary model of avian migration

Seasonal migration is a complex and variable behaviour with the potential to promote reproductive isolation. In Eurasian blackcaps (Sylvia atricapilla), a migratory divide in central Europe separating populations with southwest (SW) and southeast (SE) autumn routes may facilitate isolation, and individuals using new wintering areas in Britain show divergence from Mediterranean winterers. We tracked 100 blackcaps in the wild to characterize these strategies. Blackcaps to the west and east of the divide used predominantly SW and SE directions, respectively, but close to the contact zone many individuals took intermediate (S) routes. At 14.0° E, we documented a sharp transition from SW to SE migratory directions across only 27 (10–86) km, implying a strong selection gradient across the divide. Blackcaps wintering in Britain took northwesterly migration routes from continental European breeding grounds. They originated from a surprisingly extensive area, spanning 2000 km of the breeding range. British winterers bred in sympatry with SW-bound migrants but arrived 9.8 days earlier on the breeding grounds, suggesting some potential for assortative mating by timing. Overall, our data reveal complex variation in songbird migration and suggest that selection can maintain variation in migration direction across short distances while enabling the spread of a novel strategy across a wide range.     

Carry‐over effects provide linkages across the annual cycle of a Neotropical migratory bird,the Louisiana Waterthrush Parkesia motacilla

Population limitation models of migratory birds have sought to include impacts from events across the full annual cycle. Previous work has shown that events occurring in winter result in some individuals transitioning to the breeding grounds earlier or in better physical condition than others, thereby affecting reproductive success (carry‐over effects). However, evidence for carry‐over effects from breeding to wintering grounds has been shown less often. We used feather corticosterone (CORT_f) levels of the migratory Louisiana Waterthrush Parkesia motacilla as a measure of the physiological state of birds at the time of moult on the breeding territory to investigate whether carry‐over effects provide linkages across the annual cycle of this stream‐obligate bird. We show that birds arriving on wintering grounds with lower CORT_f scores, indicating reduced energetic challenges or stressors at the time of moult, occupied higher quality territories, and that these birds then achieved a better body condition during the overwinter period. Body condition, in turn, was important in determining whether adult birds returned the following winter, with birds in better condition returning at higher rates. Together these data suggest a carry‐over effect from the breeding grounds to the wintering grounds that is further extended with respect to annual return rates. Very few other studies have linked conditions during the previous breeding season with latent effects during the subsequent overwintering period or with annual survival. This study shows that the effects of variation in energetic challenges or stressors can potentially carry over from the natal stream and accumulate over more than one life‐history period before being manifested in reduced survival. This is of particular relevance to models of population limitation in migratory birds.     

Haemosporidian parasitism in the blackcap Sylvia atricapilla in relation to spring arrival and body condition

Parasites can exert strong selection on hosts. Throughout the year migrants are exposed to different sets of parasites, which may affect life history traits such as migratory schedules. Here, we studied the relationship between parasite infection and arrival date of blackcaps Sylvia atricapilla to their breeding grounds in Germany throughout a period of six years (2007–2012). We used two data sets, one that included all blackcaps and one that included only recaptured birds. We assesed whether parasites influence spring arrival to breeding grounds, and for the recaptured data set, we analysed temporal variation in parasitism (i.e. infection status and parasitaemia) throughout the breeding season. We used both microscopy and PCR (a fragment of ? 479 bp of the mtDNA cyt b) to determine haemosporidian infection. Blackcaps were mostly infected with Haemoproteus parabelopolskyi (lineages SYAT01 and SYAT02). Infection status, but not parasitaemia, was constant through time for individual birds; meaning that once a bird is infected, it most likely will retain the infection for life. We found that infection by haemosporidian parasites has no relationship to arrival date in this blackcap population; however, infection by H. parabelopolskyi has a marginally significant effect on arrival date of recaptured blackcaps, somewhat delaying their arrival to breeding grounds. Birds captured later in the season were more likely to be infected than those from early spring, and parasitaemia was frequently lower in birds captured earlier in the season compared to those captured later (summer).     

Overtaking on migration: does longer distance migration always incur a penalty?

For many migratory bird species, the latitudinal range of the winter distribution spans thousands of kilometres, thus encompassing considerable variation in individual migration distances. Pressure to winter near breeding areas is thought to be a strong driver of the evolution of migration patterns, as individuals undertaking a shorter migration are generally considered to benefit from earlier arrival on the breeding grounds. However, the influence of migration distance on timing of arrival is difficult to quantify because of the large scales over which individuals must be tracked. Using a unique dataset of individually‐marked Icelandic black‐tailed godwits Limosa limosa islandica tracked throughout the migratory range by a network of hundreds of volunteer observers, we quantify the consequences of migrating different distances for the use of stop‐over sites and timing of arrival in Iceland. Modelling of potential flight distances and tracking of individuals from across the winter range shows that individuals wintering further from the breeding grounds must undertake a stop‐over during spring migration. However, despite travelling twice the distance and undertaking a stop‐over, individuals wintering furthest from the breeding grounds are able to overtake their conspecifics on spring migration and arrive earlier in Iceland. Wintering further from the breeding grounds can therefore be advantageous in migratory species, even when this requires the use of stop‐over sites which lengthen the migratory journey. As early arrival on breeding sites confers advantages for breeding success, the capacity of longer distance migrants to overtake conspecifics is likely to influence the fitness consequences of individual migration strategies. Variation in the quality of wintering and stopover sites throughout the range can therefore outweigh the benefits of wintering close to the breeding grounds, and may be a primary driver of the evolution of specific migration routes and patterns.     

Migration and winter distribution of Iberian and central European black storks Ciconia nigra moving to Africa across the Strait of Gibraltar: a comparative study

This paper compares the migratory movements of Iberian and central European satellite‐tagged black storks Ciconia nigra moving to Africa across the Strait of Gibraltar. Results show that the populations differ in departure dates from breeding areas (central European birds start to move 15 d before Iberian birds), cross the Strait of Gibraltar together and reach the Sahel on similar dates. This synchronic arrival to the Sahel may be related with the onset of suitable conditions for the species after summer rains, when many pools are available for fishing. In this area, Iberian birds occupied westernmost localities compared to central European birds crossing the Strait of Gibraltar, which were distributed closer to those storks arriving in Africa across the Bosporus. This suggests that the parallel distribution of breeding and wintering areas results from posterior rearrangements of the two populations crossing Gibraltar. These patterns appear to be linked to the increasing population of central European black storks located on the western side of the migratory divide that moves throughout the western flyway to sectors of the Sahel close to their ancestral wintering grounds.     

Temperature‐related increases in grass growth and greater competition for food drive earlier migrational departure of wintering Whooper Swans

Understanding the departure decisions of migratory birds is critical for determining how changing climatic conditions will influence subsequent arrival times on the breeding grounds. A long‐term dataset (1972–2008) of Whooper Swan Cygnus cygnus departure dates from a wintering site in Ireland was used to assess the factors determining the timing of migration. Early and late migrating swans showed different departure patterns. Earlier wintering ground departure was more pronounced for the first 50% of the population than the last 10% of departing individuals. Earlier departure was associated with an increase in February temperatures at the wintering site for all departure phases except the date when the last individual departed. The date by which the first 50% of Swans had departed was earlier with increasing numbers of wintering Swans, suggesting that competition on the wintering grounds may further influence the timing of departure. The results also suggested that departure is mediated by the influence of spring temperature on food resources, with increased February grass growth in warmer years enabling earlier departure of migrating Swans. To determine why arrival dates in the breeding ground have altered, environmental conditions in the wintering grounds must be taken into account.     

Does winter region affect spring arrival time and body mass of king eiders in northern Alaska?

Events during the non-breeding season may affect the body condition of migratory birds and influence performance during the following breeding season. Migratory birds nesting in the Arctic often rely on endogenous nutrients for reproductive efforts, and are thus potentially subject to such carry-over effects. We tested whether king eider (Somateria spectabilis) arrival time and body mass upon arrival at breeding grounds in northern Alaska were affected by their choice of a winter region in the Bering Sea. We captured birds shortly after arrival on breeding grounds in early June 2002–2006 at two sites in northern Alaska and determined the region in which individuals wintered using satellite telemetry or stable isotope ratios of head feathers. We used generalized linear models to assess whether winter region explained variation in arrival body mass among individuals by accounting for sex, site, annual variation, and the date a bird was captured. We found no support for our hypothesis that either arrival time or arrival body mass of king eiders differed among winter regions. We conclude that wintering in different regions in the Bering Sea is unlikely to have reproductive consequences for king eiders in our study areas.     

Photoperiodic response may facilitate adaptation to climatic change in long-distance migratory birds

Recent climatic change is causing spring events in northern temperate regions to occur earlier in the year. As a result, migratory birds returning from tropical wintering sites may arrive too late to take full advantage of the food resources on their breeding grounds. Under these conditions, selection will favour earlier spring arrival that could be achieved by overwintering closer to the breeding grounds. However, it is unknown how daylength conditions at higher latitudes will affect the timing of life cycle stages. Here, we show in three species of Palaearctic-African migratory songbirds that a shortening of migration distance induces an advancement of springtime activities. Birds exposed to daylengths simulating migration to and wintering in southern Europe considerably advanced their spring migratory activity and testicular development. This response to the novel photoperiodic environment will enable birds wintering further north to advance spring arrival and to start breeding earlier. Thus, phenotypic flexibility in response to the photoperiod may reinforce selection for shorter migration distance if spring temperatures continue to rise.     

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.     

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.     

Age- and sex-dependent spring arrival dates of Eastern Kingbirds

ABSTRACT.   Among migratory passerines, the first birds to arrive on the breeding grounds are usually older males. Early arrival by older birds may be driven by experience, age-dependent changes in body condition, age-dependent access to resources during the nonbreeding period, or latitudinal segregation by age. Males may arrive earlier than females (protandry) because males maintain better condition due to greater access to resources during the winter, or because selection favors early arriving males that acquire the best territories or experience enhanced mating opportunities. During a 4-yr study (2004–2007) in Oregon, we found that older Eastern Kingbirds ( Tyrannus tyrannus ), regardless of sex, arrived nearly a week before younger birds and that males arrived about 5 d before females. Age- and sex-dependent arrival dates do not appear to be related to differences in body condition, social dominance in winter, or latitudinal segregation, and protandry is unrelated to the ability of early-arriving males to acquire high-quality territories. Instead, we propose that young birds have less to gain from early arrival because of their probable inability to displace experienced birds from prime territories and that protandry evolved due to enhanced mating opportunities for early arriving males that arise from the high rates of extra-pair matings in our population of Eastern Kingbirds.     

Carbon isotope turnover in blood as a measure of arrival time in migratory birds using isotopically distinct environments

Arrival time on breeding or non-breeding areas is of interest in many ecological studies exploring fitness consequences of migratory schedules. However, in most field studies, it is difficult to precisely assess arrival time of individuals. Here, we use carbon isotope turnover in avian blood as a technique to estimate arrival time for birds switching from one habitat or environment to another. Stable carbon isotope ratios (δ¹³C) in blood assimilate to a new equilibrium following a diet switch according to an exponential decay function. This relationship can be used to determine the time a diet switch occurred if δ¹³C of both the old and new diet are known. We used published data of captive birds to validate that this approach provides reliable estimates of the time since a diet switch within 1–3 weeks after the diet switch. We then explored the utility of this technique for King Eiders (Somateria spectabilis) arriving on terrestrial breeding grounds after wintering and migration at sea. We estimated arrival time on breeding grounds in northern Alaska (95% CI) from red blood cell δ¹³C turnover to be 4–9 June. This estimate overlapped with arrival time of birds from the same study site tracked with satellite transmitters (5–12 June). Therefore, we conclude that this method provides a simple yet reliable way to assess arrival time of birds moving between isotopically distinct environments.     

The migration of the great snipe Gallinago media: intriguing variations on a grand theme

The migration of the great snipe Gallinago media was previously poorly known. Three tracks in 2010 suggested a remarkable migratory behaviour including long and fast overland non‐stop flights. Here we present the migration pattern of Swedish male great snipes, based on 19 individuals tracked by light‐level geolocators in four different years. About half of the birds made stopover(s) in northern Europe in early autumn. They left the breeding area 15 d earlier than those which flew directly to sub‐Sahara, suggesting two distinct autumn migration strategies. The autumn trans‐Sahara flights were on average 5500 km long, lasted 64 h, and were flown at ground speeds of 25 m s^?1 (90 km h^?1). The arrival in the Sahel zone of west Africa coincided with the wet season there, and the birds stayed for on average three weeks. The birds arrived at their wintering grounds around the lower stretches of the Congo River in late September and stayed for seven months. In spring the great snipes made trans‐Sahara flights of similar length and speed as in autumn, but the remaining migration through eastern Europe was notably slow. All birds returned to the breeding grounds within one week around mid‐May. The annual cycle was characterized by relaxed temporal synchronization between individuals during the autumn–winter period, with maximum variation at the arrival in the wintering area. Synchronization increased in spring, with minimum time variation at arrival in the breeding area. This suggests that arrival date in the breeding area is under strong stabilizing selection, while there is room for more flexibility in autumn and arrival to the wintering area. The details of the fast non‐stop flights remain to be elucidated, but the identification of the main stopover and wintering areas is important for future conservation work on this red‐listed bird species.     

Fat stores of American redstarts Setophaga ruticilla arriving at northerly breeding grounds

Many long-distance passerine migrants arrive with more fat stores than necessary to have reached their northerly breeding grounds. Researchers have argued for adaptive advantages associated with arriving with 'surplus' migratory fat, including increased breeding performance and insurance against adverse weather, and reduced food resources during the days following arrival. The objectives of this work were to document fat stores in American redstarts Setophaga ruticilla arriving to breed in northern Michigan and to test predictions associated with the hypothesis that arrival fat serves an insurance role. Results suggest that redstarts arrived in northern Michigan with fat stores sufficient to have continued migrating an additional distance greater than 1000 km. Significant yearly variation in arrival fat corresponded in part with environmental conditions measured at the breeding grounds. Birds arrived with the most fat in the year with the coolest temperatures and the lowest food abundance at the time of arrival to the site. Further, an inverse relationship between arrival fat and arrival day in males indicated that early arrivals carried more fat than later arrivals. Birds that arrived early faced cooler temperatures and lower densities of terrestrial invertebrates, and arrival fat may have provided a mechanism to overcome poor early season foraging conditions. However, our results are not entirely consistent with the hypothesis that arrival fat serves only as insurance. Arrival fat appeared important even during the most benign year of this study. Further, evidence suggests that females may have derived more benefit from arrival fat than males. These results highlight the connection between phases of a bird's annual cycle. Migrants that do well en route , arrive at breeding grounds in better condition, which may contribute to survival and reproductive success.     

Climatic effects on timing of spring migration and breeding in a long-distance migrant, the pied flycatcher Ficedula hypoleuca

Climate change has advanced the breeding dates of many bird species, but for few species we know whether this advancement is sufficient to track the advancement of the underlying levels of the food chain. For the long-distance migratory pied flycatcher Ficedula hypoleuca the advancement in breeding time has been insufficient to maintain the synchrony with their main food sources. The timing of arrival in the breeding areas from their African wintering grounds is likely to constrain the advancement of breeding date. We hypothesise that this is because in Africa they cannot predict the advancement of spring in their breeding habitat. However, long-distance migrants may advance their arrival time by migrating faster when circumstances en route are favourable. In this study we show that both arrival and breeding date depend on temperatures at their main North African staging grounds, as well as on temperature at the breeding grounds. Male arrival and average laying date were not correlated, but the positive effect of temperature in North Africa on breeding dates suggests that breeding date is indeed constrained by arrival of females. Long-distance migrants thus are able to adjust arrival and hence breeding by faster spring migration, but the degree of adjustment is probably limited as timing schedules in spring are tight. Furthermore, as climate change is affecting temperatures differently along the migratory flyway and the breeding areas, it is unlikely that arrival dates are advanced at the same rate as the timing of breeding should advance, given the advancement of the underlying levels of the food chain.