Connecting the dots: Stopover strategies of an intercontinental migratory songbird in the context of the annual cycle

The phases of the annual cycle for migratory species are inextricably linked. Yet, less than five percent of ecological studies examine seasonal interactions. In this study, we utilized stable hydrogen isotopes to geographically link individual black‐and‐white warblers (Mniotilta varia) captured during spring migration with breeding destinations to understand a migrant's stopover strategy in the context of other phases of the annual cycle. We found that stopover strategy is not only a function of a bird's current energetic state, but also the distance remaining to breeding destination and a bird's time‐schedule, which has previously been linked to habitat conditions experienced in the preceding phase of the annual cycle. Birds in close proximity to their breeding destination accumulate additional energy reserves prior to arrival on the breeding grounds, as reflected by higher migratory condition upon arrival, higher refueling rates measured via blood plasma metabolites, and longer stopover durations compared to birds migrating to breeding destinations farther from the stopover site. However, late birds near their breeding destination were more likely to depart on the day of arrival (i.e., transients), and among birds that stopped over at the site, the average duration of stopover was almost half the time of early conspecifics, suggesting late birds are trying to catch‐up with the overall time‐schedule of migration for optimal arrival time on the breeding grounds. In contrast, birds with long distances remaining to breeding destinations were more likely to depart on the day of arrival and primarily used stopover to rest before quickly resuming migration, adopting similar strategies regardless of a bird's time‐schedule. Our study demonstrates that migrants adjust their en route strategies in relation to their time‐schedule and distance remaining to their breeding destination, highlighting that strategies of migration should be examined in the context of other phases of the annual cycle.     

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.     

Fall migration and breeding origins of Canada Warblers moving through northern Colombia

Canada Warblers (Cardellina canadensis) are long‐distance Neotropical migrants, but little is known about their migratory behavior and ecology. We examined the fall migration of Canada Warblers at two sites, Darién and the Sierra Nevada de Santa Marta, in northern Colombia from 2011 to 2015 using constant‐effort mist‐netting. Our objectives were to determine: 1) breeding origins and connectivity patterns, 2) migratory pathways, 3) the phenology of migration, 4) possible differences in movements between ages and sexes, 5) their body condition when arriving in Colombia, and 6) evidence of stopover and refueling. Stable hydrogen isotopes (δ²H_f) in flight feathers were analyzed to estimate breeding origins of captured Canada Warblers in North America. The δ²H_f values revealed that most Canada Warblers captured in the Darién likely originated from the central and northeastern regions of their breeding range. The capture of all but one of 162 Canada Warblers in the Darién also indicates a migration route through Central American rather than across the Caribbean Sea. Most captured birds were hatch‐year birds (91% vs. 9% after hatch‐year birds), and we captured more females (67%) than males (33%). Canada Warblers migrated through the Darién from 20 September to early November, with most arriving in mid‐October. Most (89%) individuals arrived with low fuel reserves. These results combined with estimated flight ranges revealed that 46% of the individuals captured in the Darién likely needed to refuel to continue migrating, whereas 31% could continue 50 to 200 km beyond our capture site. However, no individuals were recaptured so stopover duration could not be determined. Canada Warblers may adopt a strategy of 1‐d stopovers and short flights or, alternatively, the Darién may represent low‐quality habitat and birds quickly left our study site in search of suitable habitat. Further study is needed to determine the possible importance of other (montane) habitats for Canada Warblers in the Darién region to prioritize conservation actions.     

Fall migratory departure decisions and routes of blackpoll warblers Setophaga striata and red‐eyed vireos Vireo olivaceus at a coastal barrier in the Gulf of Maine

Each year, millions of songbirds concentrate in coastal areas during fall migration. The choices birds make at the coast about stopover habitat use and migratory route can influence both the success of their migratory journey and fitness in subsequent life stages. We made use of a regional‐scale automated radio telemetry array to study stopover and migratory flights and migratory routes of blackpoll warblers Setophaga striata and red‐eyed vireos Vireo olivaceus during fall migration in the Gulf of Maine, USA. We focused on differences between species, sexes, age groups, breeding origins, and time of year. Both species made within‐stopover relocations (i.e. ‘stopover flights’) from the coastal capture site. Stopover flights were primarily oriented inland, and were more frequent for blackpolls (87%) than vireos (44%). By studying migratory behavior at a broad spatial scale, we demonstrated that most blackpolls and vireos took coastal and offshore routes through the Gulf of Maine, despite initially relocating inland from the capture site. Though we captured blackpolls and vireos from a broad breeding range, more than 70% of migratory flights from the capture site were oriented for coastal or offshore travel for both species, suggesting that birds actively chose coastal and offshore routes, and were not simply displaced by wind drift. Later vireos oriented offshore more frequently during migratory flights from the coast, indicating that they may be more inclined towards time‐minimizing overwater flight routes and thus more exposed to coastal and offshore collision hazards than earlier conspecifics.     

Artificial light at night confounds broad‐scale habitat use by migrating birds

With many of the world's migratory bird populations in alarming decline, broad‐scale assessments of responses to migratory hazards may prove crucial to successful conservation efforts. Most birds migrate at night through increasingly light‐polluted skies. Bright light sources can attract airborne migrants and lead to collisions with structures, but might also influence selection of migratory stopover habitat and thereby acquisition of food resources. We demonstrate, using multi‐year weather radar measurements of nocturnal migrants across the northeastern U.S., that autumnal migrant stopover density increased at regional scales with proximity to the brightest areas, but decreased within a few kilometers of brightly‐lit sources. This finding implies broad‐scale attraction to artificial light while airborne, impeding selection for extensive forest habitat. Given that high‐quality stopover habitat is critical to successful migration, and hindrances during migration can decrease fitness, artificial lights present a potentially heightened conservation concern for migratory bird populations.     

The strength of migratory connectivity for birds en route to breeding through the Gulf of Mexico

The strength of migratory connectivity is a measure of the cohesion of populations among phases of the annual cycle, including breeding, migration, and wintering. Many Nearctic‐Neotropical species have strong migratory connectivity between breeding and wintering phases of the annual cycle. It is less clear if this strength persists during migration when multiple endogenous and exogenous factors may decrease the cohesion of populations among routes or through time along the same routes. We sampled three bird species, American redstart Setophaga ruticilla, ovenbird Seiurus aurocapilla, and wood thrush Hylocichla mustelina, during spring migration through the Gulf of Mexico region to test if breeding populations differentiate spatially among migration routes or temporally along the same migration routes and the extent to which within‐population timing is a function of sex, age, and carry‐over from winter habitat, as measured by stable carbon isotope values in claws (δ¹³C). To make quantitative comparisons of migratory connectivity possible, we developed and used new methodology to estimate the strength of migratory connectivity (MC) from probabilistic origin assignments identified using stable hydrogen isotopes in feathers (δ²H). We found support for spatial differentiation among routes by American redstarts and ovenbirds and temporal differentiation along routes by American redstarts. After controlling for breeding origin, the timing of American redstart migration differed among ages and sexes and ovenbird migration timing was influenced by carry‐over from winter habitat. The strength of migratory connectivity did not differ among the three species, with each showing weak breeding‐to‐spring migration MC relative to prior assessments of breeding‐wintering connectivity. Our work begins to fill an essential gap in methodology and understanding of the extent to which populations remain together during migration, information critical for a full annual cycle perspective on the population dynamics and conservation of migratory animals.     

Migratory stopover conditions affect the developmental state of male gonads in garden warblers (Sylvia borin)

Long-distance migrants face the challenge of a short window for reproduction that requires optimal timing and full functional gonads. Male garden warblers (Sylvia borin) meet these demands by initiating testicular recrudescence during spring migration, enabling them to reproduce immediately after arrival at the breeding grounds. In a combined field and laboratory study, we investigated testicular size, plasma levels of luteinizing hormone (LH), androstenedione (AE), 5α-dihydrotestosterone (DHT), testosterone and nocturnal migratory restlessness (Zugunruhe) under different stopover conditions. We manipulated food availability, the duration of stopover and simulated migration by food deprivation. Garden warblers showed significantly retarded testicular development after nine days of stopover under limited food conditions compared to birds that had ad libitum access to food. However, there was no significant difference in Zugunruhe between the two groups. Thus, the degree of Zugunruhe was unaffected by the quality of the stopover site and migration continued independent of the developmental state of the testis. We suggest that male garden warblers face the necessity to either compensate for slowed testicular recrudescence during the subsequent leg of migration and delay arrival at the breeding grounds, or arrive with less developed testes. Either of these may reduce annual reproductive success.     

Plasma metabolites and migration physiology of semipalmated sandpipers: refueling performance at five latitudes

Long-distance bird migration is fueled by energy gathered at stopover sites along the migration route. The refueling rate at stopover sites is a determinant of time spent at stopovers and impacts the overall speed of migration. Refueling rate during spring migration may influence the fitness of individuals via changes in the probability of successful migration and reproduction during the subsequent breeding season. We evaluated four plasma lipid metabolites (triglycerides, phospholipids, β-OH-butyrate, and glycerol) as measures of refueling rate in free-living semipalmated sandpipers (Calidris pusilla) captured at non-breeding areas. We described the spatial and temporal variation in metabolite concentrations among one winter site in the Dominican Republic and four stopover sites in the South Atlantic and Mid-Atlantic Coastal Plain regions of North America. Triglycerides and β-OH-butyrate clearly identified spatial variation in refueling rate and stopover habitat quality. Metabolite profiles indicated that birds had higher refueling rates at one site in the Mid-Atlantic Coastal Plain than at three sites on the South Atlantic Coastal Plain and one site in the Dominican Republic. Temporal variation in lipid metabolites during the migration season suggested that male semipalmated sandpipers gained more weight at stopovers on the South Atlantic Coastal Plain than did females, evidence of differential migration strategies for the sexes. Plasma lipid metabolites provide information on migration physiology that may help determine stopover habitat quality and reveal how migratory populations use stopover sites to refuel and successfully complete long-distance migrations.     

Cold tolerance,and not earlier arrival on breeding grounds,explains why males winter further north in an Arctic‐breeding songbird

Sex biases in distributions of migratory birds during the non‐breeding season are widespread; however, the proximate mechanisms contributing to broad‐scale sex‐ratio variation are not well understood. We analyzed a long‐term winter‐banding dataset in combination with spring migration data from individuals tracked by using geolocators to test three hypotheses for observed variation in sex‐ratios in wintering flocks of snow buntings Plectrophenax nivalis. We quantified relevant weather conditions in winter (temperature, snowfall and snow depth) at each banding site each year and measured body size and condition (fat scores) of individual birds (n > 5500). We also directly measured spring migration distance for 17 individuals by using light‐level geolocators. If the distribution pattern of birds in winter is related to interactions between individual body size and thermoregulation, then larger bodied birds (males) should be found in colder sites (body size hypothesis). Males may also winter closer to breeding grounds to reduce migration distance for early arrival at breeding sites (arrival timing hypothesis). Finally, males may be socially dominant over females, and thus exclude females from high‐quality wintering sites (social dominance hypothesis). We found support for the body size hypothesis, in that colder and snowier weather predicted both larger body size and higher proportions of males banded. Direct tracking revealed that males did not winter significantly closer to their breeding site, despite being slightly further north on average than females from the same breeding population. We found some evidence for social dominance, in that females tended to carry more fat than males, potentially indicating lower habitat quality for females. Global climatic warming may reduce temperature constraints on females and smaller‐bodied males, resulting in broad‐scale changes in distributional patterns. Whether this has repercussions for individual fitness, and therefore population demography, is an important area of future research.     

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

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

Factors influencing plasticity in the arrival‐breeding interval in a migratory species reacting to climate change

Climate change is profoundly affecting the phenology of many species. In migratory birds, there is evidence for advances in their arrival time at the breeding ground and their timing of breeding, yet empirical studies examining the interdependence between arrival and breeding time are lacking. Hence, evidence is scarce regarding how breeding time may be adjusted via the arrival‐breeding interval to help local populations adapt to local conditions or climate change. We used long‐term data from an intensively monitored population of the northern wheatear (Oenanthe oenanthe) to examine the factors related to the length of 734 separate arrival‐to‐breeding events from 549 individual females. From 1993 to 2017, the mean arrival and egg‐laying dates advanced by approximately the same amount (~5–6 days), with considerable between‐individual variation in the arrival‐breeding interval. The arrival‐breeding interval was shorter for: (a) individuals that arrived later in the season compared to early‐arriving birds, (b) for experienced females compared to first‐year breeders, (c) as spring progressed, and (d) in later years compared to earlier ones. The influence of these factors was much larger for birds arriving earlier in the season compared to later arriving birds, with most effects on variation in the arrival‐breeding interval being absent in late‐arriving birds. Thus, in this population it appears that the timing of breeding is not constrained by arrival for early‐ to midarriving birds, but instead is dependent on local conditions after arrival. For late‐arriving birds, however, the timing of breeding appears to be influenced by arrival constraints. Hence, impacts of climate change on arrival dates and local conditions are expected to vary for different parts of the population, with potential negative impacts associated with these factors likely to differ for early‐ versus late‐arriving birds.     

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.     

Breeding latitude and timing of spring migration in songbirds crossing the Gulf of Mexico

Each spring, millions of songbirds migrate across the Gulf of Mexico on their way to breeding sites in North America. Data from radar and migration monitoring stations have revealed broad patterns in the spatial and temporal course of trans-Gulf migration. Unfortunately, we have limited information on where these birds have previously spent the winter and where they are migrating to breed. Here we measure stable-hydrogen isotopes in feathers (δD_f) to infer the breeding latitude of five species of songbirds – hooded warblers Wilsonia citrina , American redstarts Setophaga ruticilla , black-and-white warblers Mniotilta varia , ovenbirds Seiurus aurocapilla , and northern waterthrushes S. noveboracensis – that were captured at a stopover site along the coast of southwestern Louisiana in spring 2004. Values of δD_f across all species ranged from −163 to −35‰ (n=212), and within most species the range was consistent with the latitudinal extent of known breeding sites in central and eastern North America. Individuals that arrived first along the northern Gulf coast had δD_f values indicative of southerly breeding sites in hooded warblers, American redstarts, black-and-white warblers, and ovenbirds, but no relationship was found between passage timing and δD_f for northern waterthrushes. Our findings suggest that spring passage is often timed to coincide with the emergence of suitable conditions on breeding areas, with southern breeding birds migrating first.     

Adcyap1 polymorphism covaries with breeding latitude in a Nearctic migratory songbird,the Wilson's warbler (Cardellina pusilla)

Understanding the genetic background of complex behavioral traits, showing multigenic control and extensive environmental effects, is a challenging task. Among such traits, migration is known to show a large additive genetic component. Yet, the identification of specific genes or gene regions explaining phenotypic variance in migratory behavior has received less attention. Migration ultimately depends on seasonal cycles, and polymorphism at phenological candidate genes may underlie variation in timing of migration or other aspects of migratory behavior. In this study of a Nearctic–Neotropical migratory songbird, the Wilson's warbler (Cardellina pusilla), we investigated the association between polymorphism at two phenological candidate genes, Clock and Adcyap1, and two aspects of the migratory phenotype, timing of spring migration through a stopover site and inferred latitude of the breeding destination. The breeding destination of migrating individuals was identified using feather deuterium ratio (δ²H), which reliably reflects breeding latitude throughout the species' western breeding range. Ninety‐eight percent of the individuals were homozygous at Clock, and the rare heterozygotes did not deviate from homozygous migration phenology. Adcyap1 was highly polymorphic, and allele size was not significantly associated with migration date. However, Adcyap1 allele size significantly positively predicted the inferred breeding latitude of males but not of females. Moreover, we found a strong positive association between inferred breeding latitude and Adcyap1 allele size in long‐distance migrating birds from the northern sector of the breeding range (western Canada), while this was not the case in short‐distance migrating birds from the southern sector of the breeding range (coastal California). Our findings support previous evidence for a role of Adcyap1 in shaping the avian migratory phenotype, while highlighting that patterns of phenological candidate gene–phenotype associations may be complex, significantly varying between geographically distinct populations and even between the sexes.     

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

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

Carry‐over effects of winter habitat vary with age and sex in yellow warblers Setophaga petechia

We use stable isotope data to investigate the role of winter habitat use in altering the breeding phenology of yellow warblers Setophaga petechia. We first confirm that δ¹³C and δ¹⁵N isotopic signatures vary with winter habitat use in this species. We then examine the relationship between winter habitat use, breeding phenology and productivity within four age‐sex‐classes, since life history theory would predict that carry‐over effects should vary with age and gender. The δ¹³C signatures of yellow warblers using riparian habitats over winter were more depleted than the signatures of those using agricultural or scrub habitat. Individuals on the Pacific coast of Mexico were also more δ¹⁵N enriched than those on the southern Gulf of Mexico. δ¹³C and δ¹⁵N signatures were only correlated with earlier clutch initiation and subsequent higher productivity in first‐breeding‐season females. We estimate that shifts in δ¹³C equivalent to a shift from scrub to riparian winter habitat would be associated with the production of 0.8 more fledglings by yearling females. Pre‐breeding events that influence the timing of breeding could also influence the reproductive performance of older males and females, but we found little evidence that winter habitat use influenced breeding season phenology in these birds.     

Rainfall and habitat interact to affect the condition of a wintering migratory songbird in The Bahamas

On the subtropical and tropical wintering grounds of migratory birds, variation in moisture levels and habitat can influence the availability of food resources and subsequently impact overwintering birds. Using stable carbon isotopes in blood samples as a measure of moisture, we assessed the interactive effects of rainfall, vegetation, and moisture on the demographics and condition of Prairie Warblers (Setophaga discolor) wintering in The Bahamas. Carbon isotopes in Prairie Warbler blood were more depleted in taller, wetter habitats; we additionally detected novel temporal effects of rainfall on isotope values. During a winter with more rainfall, most birds maintained mass and pectoral muscle regardless of the habitat type occupied. In a winter with less rainfall, birds lost mass and pectoral muscle, and this effect was more pronounced in birds with enriched isotope values and birds that occupied drier, shorter habitat. Prairie Warblers exhibited strong patterns of sexual habitat segregation with males disproportionately observed in areas with taller vegetation and females in shorter vegetation. During the drier winter, older males had better maintenance of pectoral muscle compared to females and younger individuals. Also in the drier winter, daily rainfall patterns explained more of the variation in body condition compared to the date of capture; pectoral muscle was best explained by recent precipitation (during the previous 30 days), while size‐corrected mass was more a function of longer‐term (90‐day) rainfall and habitat moisture. Our findings along with other studies suggest that Prairie Warblers and other migratory birds are sensitive to interactions between annual variation in winter rainfall, within‐season daily rainfall patterns, and habitat quality. Increasing drought and habitat loss in the Caribbean may be having a negative impact on wintering bird populations. To best conserve Nearctic–Neotropical migratory passerines in the region, we recommend prioritizing the protection of the least drought‐prone wintering areas.     

Migratory routes of red‐necked phalaropes Phalaropus lobatus breeding in southern Chukotka revealed by geolocators

The migration routes of red‐necked phalaropes breeding around the Bering Sea are poorly known, despite the fact that the Bering Sea could mark the boundary between the East Palearctic populations that winter in the Pacific Ocean around the East Indies and the West Nearctic populations that winter in the Pacific Ocean off the coast of South America. Geolocator data retrieved from two male phalaropes tagged in southern Chukotka, Far Eastern Russia, confirm that birds breeding in this region belong to the East Palearctic population and winter in the East Indies, suggesting that the division line with the West Nearctic population is farther to the east. The routes taken by the two phalaropes were almost entirely pelagic, totaling around 18 000–20 000 km round‐trip, with the birds continuously on the move during migration, rather than resident in any particular stopover site, contrary to most other migratory shorebirds.     

Effect of fuel deposition rate on departure fuel load of migratory songbirds during spring stopover along the northern coast of the Gulf of Mexico

Migrants are generally assumed to minimize their overall migration time by adjusting their departure fuel loads (DFL) in relation to anticipated and experienced fuel deposition rates (FDRs). We utilized a 21‐yr long migration banding station dataset to examine the relationship between FDR and DFL during spring migration in six Nearctic‐Neotropical migratory songbird species during stopover along the northern coast of the Gulf of Mexico (GOM) following trans‐gulf flight. Estimates of fuel stores, stopover durations, and FDRs from our long term migration data set were combined to determine DFL. We expected and found that migrants across all six species adjust their DFL to the rate at which they deposit fuel reserves. This robust finding suggests that songbird migrants are sensitive to time constraints during spring passage presumably to fine‐tune their stopover schedule in relation to experienced and anticipated habitat quality. Two of the species studied showed an effect of age on the FDR–DFL relationship: one was consistent with the expectation that older birds would be less sensitive to changes in FDR, while the second was contrary to our expectations and likely suggesting an age‐dependent response to habitat quality. We found sex‐dependent differences consistent with male DFL being more sensitive to FDR in only two of six species studied, and argue that both males and females are time constrained during spring passage in relation to arrival at breeding destinations. The positive relationship between FDR and DFL among all species and for age and sex groups in some species reflects a migration strategy sensitive to time.     

Implications of climate change for North American wood warblers (Parulidae)

Since 1912, Neotropical–Nearctic migrant birds may now have up to 20 fewer days to travel between Southern Illinois, where spring is arriving later, and Northern Minnesota, where spring is arriving earlier, to exploit optimal habitat conditions (expanding leaves and caterpillar activity) for refueling and breeding. As case studies of the effect of climate change on bird migration, I analyzed two long‐term data sets of arrival times for eight species of northern breeding migratory wood warblers (Parulidae) gathered over a 100 year period in east‐central Illinois (IL, USA) and a 40 year period in western Minnesota (MN, USA). Six (IL) and seven (MN) of the wood warbler species showed no significant tendency to migrate earlier in response to earlier springs in their breeding range. These results suggest that climate change may force many species of long‐distance migratory songbirds to become uncoupled in the spring from their food resources that are driven by temperature.