Flight responses by a migratory soaring raptor to changing meteorological conditions

Soaring birds that undertake long-distance migration should develop strategies to minimize the energetic costs of endurance flight. This is relevant because condition upon completion of migration has direct consequences for fecundity, fitness and thus, demography. Therefore, strong evolutionary pressures are expected for energy minimization tactics linked to weather and topography. Importantly, the minute-by-minute mechanisms birds use to subsidize migration in variable weather are largely unknown, in large part because of the technological limitations in studying detailed long-distance bird flight. Here, we show golden eagle (Aquila chrysaetos) migratory response to changing meteorological conditions as monitored by high-resolution telemetry. In contrast to expectations, responses to meteorological variability were stereotyped across the 10 individuals studied. Eagles reacted to increased wind speed by using more orographic lift and less thermal lift. Concomitantly, as use of thermals decreased, variation in flight speed and altitude also decreased. These results demonstrate how soaring migrant birds can minimize energetic expenditures, they show the context for avian decisions and choices of specific instantaneous flight mechanisms and they have important implications for design of bird-friendly wind energy.     

Sequential settlement and site dependence in a migratory raptor

Habitat selection models, such as the ideal free, ideal despotic,site-dependent, and conspecific cuing models, are of great importanceto behavioral ecologists given their capability to predict habitatdistributions and to link individual behavior to populationprocesses. However, there have been relatively few field testsof their predictions. We tested the 4 models by studying theprocess of sequential settlement on territory in 2 distant populationsof a migratory raptor, the black kite Milvus migrans. Resultswere mainly consistent with the site-dependent model: on arrival,kites settled on progressively lower quality territories, andearlier arriving individuals were older, larger, and in betterbody condition than later arriving ones, leading to a state-dependentarrival sequence also predicted by a previous theoretical modelof settlement pattern. Occupation of superior territories bysuperior phenotypes resulted in cascading advantages for earlierarriving individuals in terms of subsequent reproductive performance.At the population level, the populations expanded/retractedfrom lower quality sites during population increases/declines.The above scenario was consistent across the 2 populations,and a review of the literature uncovered a remarkably consistentpicture of state-dependent arrival, progressive monopolizationof best quality sites, and cascading effects on subsequent breedingperformance. We propose as a general paradigm of sequentialsettlement the following process: 1) arrival date is a reliablesurrogate of phenotype quality, 2) early-arriving individualshave preferential access to the best quality sites and partners,3) the above conditions cascade into a number of benefits ultimatelyrelated to higher fitness for earlier arriving individuals.     

Transition between moult and migration in a long-distance migratory passerine: organ flexibility in the African wintering area

Phenotypic flexibility of organs in migratory birds has been documented for a variety of species of different genera during the migratory period. However, very little is known about phenotypic mass changes of organs with respect to other events within the annual cycle. This seems particularly interesting when birds face different physiological challenges in quick succession. We investigated mass changes of 13 organs from garden warblers (Sylvia borin) during the transition from moult to migration. These long-distance migratory birds perform a complete moult within their wintering area just shortly before the onset of spring migration. Birds were sampled in three successive stages according to their moult status: group I consisted of birds with growing primary or secondary wing feathers, group II consisted of birds with completed wing moult but with still moulting body feathers, and group III consisted of birds that had completed wing moult and body moult. Size-corrected flight muscle, kidney mass, and pancreas mass differed significantly among the three groups. Flight muscle was heaviest in birds that were about to leave their wintering area (group III) compared with birds still in body moult (group II). Kidney and pancreas showed a pattern similar to each other, with the heaviest mass occurring in birds with moulting wing feathers (group I) and significantly reduced mass in birds that had completed wing moult (group II) or both wing and body moult (group III). Mass reductions of kidney and pancreas during the transition from moult to migration are considered to be related to the demands of moult, while increased flight muscle may be due to moult, migration, or both. Phenotypic mass changes of organs in birds occur during their migration, but they also occur during the transition between other phases of the annual cycle such as moult and migration and are not restricted to the flight muscle.     

Extreme spring conditions in the Arctic delay spring phenology of long-distance migratory songbirds

Arctic regions are warming rapidly, with extreme weather events increasing in frequency, duration, and intensity just as in other regions. Many studies have focused on how shifting seasonality in environmental conditions affects vegetation phenology, while far fewer have examined how the breeding phenology of arctic fauna responds. We studied two species of long-distance migratory songbirds, Lapland longspurs, Calcarius lapponicus, and white-crowned sparrows, Zonotrichia leucophrys gambelii, across five consecutive breeding seasons in northern Alaskan tundra. We aimed to understand how spring environmental conditions affected breeding cycle phenology, including the timing of arrival on breeding grounds, territory establishment, and clutch initiation. Spring temperatures, precipitation, and snow-free dates differed significantly among years, with 2013 characterized by unusually late snow cover. In response, we found a significant delay in breeding-cycle phenology for both study species in 2013 relative to other study years: the first bird observed was delayed by 6–10 days, with mean arrival by 3–6 days, territory establishment by 6–13 days, and clutch initiation by 4–10 days. Further, snow cover, temperature, and precipitation during the territory establishment period were important predictors of clutch initiation dates for both species. These findings suggest that Arctic-breeding passerine communities may have the flexibility required to adjust breeding phenology in response to the increasingly extreme and unpredictable environmental conditions—although future generations may encounter conditions that exceed their current range of phenological flexibility.     

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

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

Metabolic profile of long-distance migratory flight and stopover in a shorebird

Migrating birds often complete long non-stop flights during which body energy stores exclusively support energetic demands. The metabolic correlates of such long-distance travel in free-living migrants are as yet poorly studied. Bar-tailed godwits, Limosa lapponica taymyrensis, undertake a 4500 km flight to their single spring stopover site and thus provide an excellent model in which to determine the energy fuels associated with endurance travel. To this end, we evaluated plasma concentrations of six key metabolites in arriving godwits caught immediately upon landing near their stopover site. Initial metabolite levels were compared with levels after 5 h of inactive rest to determine how flight per se affects energy metabolism. Birds refuelling on the stopover site were also examined. Arriving godwits displayed elevated plasma free fatty acids, glycerol and butyrate, confirming the importance of lipid fuel in the support of extended migratory activity. Further-more, elevated plasma triglycerides in these birds suggest that fatty acid provisioning is facilitated through hepatic synthesis and release of neutral lipids, as previously hypothesized for small migrants with high mass-specific metabolic rates. Finally, elevations in plasma uric acid suggest that protein breakdown contributes to the support of long-distance movement, to possibly maintain citric acid cycle intermediates, gluconeogenesis and/or water balance.     

Energetics of fattening and starvation in the long-distance migratory garden warbler,Sylvia borin,during the migratory phase

Garden warblers (Sylvia borin) were subjected to starvation trials during their autumnal migratory phase in order to simulate a period of non-stop migration. Before, during and after this treatment the energy expenditure, activity, food intake and body mass of the subjects were monitored. Assimilation efficiency was constant throughout the experiments. The catabolized (during starvation) and deposited body tissue (during recovery) consisted of 73% fat. Basal metabolic rate was decreased during the starvation period and tended to a gradual increase during the recovery period. The reduced basal metabolic rate can possibly be attributed to a reduced size/function of the digestive system, which is consistent with the sub-maximal food intake immediately after resuming the supply of food to the experimental birds. The observed reductions in basal metabolic rate during starvation and activity during recovery can be viewed as adaptations contributing to a higher economization of energy supplies. The experimental birds were unable to eat large quantities of food directly after a period of starvation leading to a comparatively low, or no increase in body mass. Such a slow mass increase is in agreement with observations of migratory birds on arrival at stop-over sites.Abbreviations BM body mass - BMR basal metabolic rate - LBM lean body mass - RQ respiratory quotient     

Divergent patterns of impact of environmental conditions on life history traits in two populations of a long-distance migratory bird

Some areas have experienced recent dramatic warming due to climate change, while others have shown no change at all, or even recent cooling. We predicted that patterns of selection on life history would differ between southern and northern European populations of a long-distance migratory bird, the barn swallow Hirundo rustica, because global patterns of weather as reflected by large-scale weather phenomena such as the North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO) have different effects on environmental conditions in different parts of the world frequented during the annual cycle. We investigated relationships between mean arrival date, dispersal rate and yearling survival rate among years, using two long-term population studies in Spain and Denmark. We found evidence of a difference in the effects of normalized difference vegetation index in North and West Africa on mean arrival date of male barn swallows, with the effect differing significantly between populations. Second, there was a significant interaction between ENSO and population on dispersal rate, showing that conditions in Africa during winter differentially affected dispersal in the two populations. Finally, the NAO index in winter had an effect on yearling survival that differed between populations. These findings highlight the divergent patterns of response to climate change among populations, and they suggest that climate change can differentially affect important life history traits with potential implications for maintenance of viable populations and gene flow among populations.     

Ecological conditions during winter affect sexual selection and breeding in a migratory bird

Populations of migratory birds have undergone marked declines, although the causes and mechanisms remain unknown. Because environmental effects on population dynamics are mediated by the effects of ecological factors on individuals, understanding changes in individual phenotypes in response to ecological conditions is key to understanding population trends. We show that breeding individuals of a declining population of trans-Saharan migratory barn swallows, Hirundo rustica, were affected by environmental conditions, as estimated from the normalized difference vegetation index (NDVI), reflecting primary production, in their winter quarters. The breeding dates of the same individuals in consecutive breeding seasons were advanced and clutch sizes were larger after winters with high NDVI in the winter quarters. Feather moult was also affected by winter conditions, with consequences for male sexual attractiveness. Length of tail ornament was positively correlated with NDVI during the previous winter, and males with large tail ornaments reproduced earlier and had larger clutches. The mean annual breeding date of the population was earlier and breeding success was increased after favourable winters, but this result was mainly determined by a single winter with very low NDVI. Thus, ecological conditions in Africa influence individual performance and productivity in a barn swallow population.     

Territorial occupancy and breeding performance in a migratory raptor do not follow ideal despotic distribution patterns

It has commonly been argued that many territorial species select their breeding sites following an ideal despotic distribution model, in which the most productive, high-quality territories are more frequently occupied. Theoretical and empirical studies have shown that this occupancy pattern may have population regulatory consequences, leading to density dependence in heterogeneous habitats. During a 9-year research project in a forested area of south-eastern Spain, we tested some of the predictions of the ideal despotic distribution model and the site-dependent population regulation theory in a migratory raptor species, the booted eagle Hieraaetus pennatus . Contrary to the predictions of the despotic model, our results showed that the temporal pattern of territorial occupation did not differ from randomness, and that the territory occupancy rate was not significantly related to the reproductive parameters considered. At population level, the breeding variables were density independent, suggesting the absence of site-dependent regulation. In addition, we were unable to find significant differences in the habitat characteristics between high-quality and low-quality territories, classified according to the criteria of both occupancy frequency and average productivity. Overall, our results suggest that booted eagles select their territories at random, probably due to the lack of strong environmental heterogeneity, and that occupancy rate is not a good measure of territory quality for the population studied.     

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

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

Food availability during migratory stopover affects testis growth and reproductive behaviour in a migratory passerine

Long-distance migratory passerines initiate testicular recrudescence during spring migration to meet the demands of timely reproduction upon immediate arrival on the breeding grounds. The degree of testicular development is known to depend on environmental factors like stopover habitat quality; reproductive performance may be strongly impacted by testicular maturation upon arrival on the breeding grounds. We investigated the effect of stopover food availability on subsequent reproductive performance in garden warblers (Sylvia borin). Spring migration was simulated by repeated food deprivation and re-feeding to imitate the alternation of flight and stopover periods. During the two final stopover periods, males were either kept under ad libitum food (ad libitum males) or under limited food conditions (limited males). After simulated arrival in the breeding area, manipulation of previous stopover food availability resulted in significantly slower testicular recrudescence (p < 0.001) and decreased plasma testosterone (p < 0.01) in limited males compared to ad libitum males. Body mass change was not significantly different between the two groups (p = 0.38). Limited males also exhibited reduced performance in reproductive behaviours employed in territorial and sexual contexts. Limited males had a longer ‘freezing’ interval (p < 0.05) and decreased activity (p < 0.01) when challenged with a live male decoy. In direct confrontation between limited and ad libitum males in the presence of a female, limited males exhibited significantly fewer behavioural traits in sexual context, i.e. directed to the female (p < 0.001). Therefore, we suggest that conditions encountered during previous migratory stopover may affect subsequent annual reproductive success by influencing key reproductive behaviours.     

Plumage brightness predicts non-breeding season territory quality in a long-distance migratory songbird, the American redstart Setophaga ruticilla

Many species of birds exhibit brilliant ornamental plumage, yet most research on the function and evolution of plumage has been confined to the breeding season. In the American redstart Setophaga ruticilla , a long-distance Neotropical-Nearctic migratory bird, the acquisition of a winter territory in high-quality habitat advances spring departure and subsequent arrival on breeding areas, and increases reproductive success and annual survival. Here, we show that males holding winter territories in high-quality, black mangrove habitats in Jamaica have brighter yellow-orange tail feathers than males occupying territories in poor-quality second-growth scrub habitats. Moreover, males arriving on the breeding grounds from higher-quality winter habitats (inferred by stable-carbon isotopes) also had brighter tail feathers. Because behavioral dominance plays an important role in the acquisition of winter territories, plumage brightness may also be related to fighting ability and the acquisition and maintenance of territories in high-quality habitat. These results highlight the need for further research on the relationships between plumage coloration, behavior, and the ecology of over-wintering migratory birds.     

Feather mass and winter moult extent are heritable but not associated with fitness-related traits in a long-distance migratory bird

In birds, the allocation of resources to plumage production may have important fitness consequences. However, we have only a limited understanding of how plumage traits respond to natural selection, making it difficult to predict how variation in plumage traits may contribute to the adaptation of birds to environmental change. In this study, we collected plumage-related data in a pedigreed population of a long-distance migratory bird (the Pied Flycatcher Ficedula hypoleuca) to estimate the heritability of two plumage traits: feather mass (as a proxy of feather quality) and the extent of winter moult. We further explored whether these plumage features were associated with some fitness-related traits. Variation in plumage characteristics could be explained by differences in sex, age and year, which indicates a high degree of plasticity in these traits. After controlling for these effects, however, feather mass and winter moult extent were highly repeatable (r = 0.58–0.82) and heritable (h² = 0.59–0.65), suggesting that additive genetic variation accounts for a significant proportion of the residual phenotypic variation of plumage traits in this population. Although the studied characteristics showed evolutionary potential, we did not find any relationship between plumage features and fitness-related traits like spring arrival date, egg-laying date, mating success or mating-time. We conclude that current selection on feather mass and moult extent, if existing, is weak, and that these moult-related traits are currently of minor importance for the adaptation of our study population to global warming.     

Highly pathogenic avian influenza virus H5N1 infection in a long-distance migrant shorebird under migratory and non-migratory states

Corticosterone regulates physiological changes preparing wild birds for migration. It also modulates the immune system and may lead to increased susceptibility to infection, with implications for the spread of pathogens, including highly pathogenic avian influenza virus (HPAIV) H5N1. The red knot (Calidris canutus islandica) displays migratory changes in captivity and was used as a model to assess the effect of high plasma concentration of corticosterone on HPAIV H5N1 infection. We inoculated knots during pre-migration (N = 6), fueling (N = 5), migration (N = 9) and post-migration periods (N = 6). Knots from all groups shed similar viral titers for up to 5 days post-inoculation (dpi), peaking at 1 to 3 dpi. Lesions of acute encephalitis, associated with virus replication in neurons, were seen in 1 to 2 knots per group, leading to neurological disease and death at 5 to 11 dpi. Therefore, the risk of HPAIV H5N1 infection in wild birds and of potential transmission between wild birds and poultry may be similar at different times of the year, irrespective of wild birds'' migratory status. However, in knots inoculated during the migration period, viral shedding levels positively correlated with pre-inoculation plasma concentration of corticosterone. Of these, knots that did not become productively infected had lower plasma concentration of corticosterone. Conversely, elevated plasma concentration of corticosterone did not result in an increased probability to develop clinical disease. These results suggest that birds with elevated plasma concentration of corticosterone at the time of migration (ready to migrate) may be more susceptible to acquisition of infection and shed higher viral titers—before the onset of clinical disease—than birds with low concentration of corticosterone (not ready for take-off). Yet, they may not be more prone to the development of clinical disease. Therefore, assuming no effect of sub-clinical infection on the likelihood of migratory take-off, this may favor the spread of HPAIV H5N1 by migratory birds over long distances.     

Continuous open flow-through system as a model for oil degradation in the arctic ocean

A continuous flow-through system incubated in situ was used to model oil biodegradation in Arctic coastal waters. High numbers of oil-degrading microorganisms were found in the Arctic coastal waters examined in this study. The microbial community underlying oil slicks increased and showed a population shift to a greater percentage of hydrocarbon-utilizing microorganisms. Microbial populations and oil biodegradation were increased by the addition of nitrogen and phosphorus. Both abiotic and biodegradative losses were lower than expected, perhaps due to the unusually harsh, ice-dominated Arctic summer, during which these tests were conducted. Chromatographic and spectrometric analyses showed that residual oils contained similar percentages of individual components and classes of hydrocarbons, regardless of the amount of degradation, indicating that most components of the oil were being degraded at similar rates.