Matching geographical assignment by stable isotopes with African non-breeding sites of barn swallows Hirundo rustica tracked by geolocation.

Knowledge on whereabouts within the annual cycle of migratory species is prerequisite for many aspects in ecology and biological conservation. Spatial assignments of stable isotopes archived in tissues allows for later inference on sites where the specific tissue had been grown. It has been rarely tested whether spatial assignments match directly tracked non-breeding residences, especially for migratory songbirds. We here compare assignments of stable isotopes from feathers of Palaearctic Barn swallows Hirundo rustica with their African non-breeding residence sites tracked by geolocation.Assignments based on δ2H, δ13C and δ15N isotope compositions delineate three main non-breeding regions: a main cluster in central Africa, a second in West Africa, and the third cluster in Northern Africa. Using δ13C, δ15N only, non-breeding sites ranged from clusters in West/Southwest Africa to South East Africa with a centre in Central Africa. The non-breeding areas (50% and 75% Kernel density estimates, KDE) of the birds tracked by geolocation stretched from West Africa via central Africa to southern Africa. We found little overlap of 0.3% (assuming a 1:1 odds ratio) to 1.4% (3:1 odds ratio) in the three element assignments and KDEs for only 2 and 13 individuals out of 32 birds. Assignment maps for two elements (δ13C, δ15N) and KDEs showed higher consistencies with an overlap of 3.6 and 8.5% for 12 and 18 birds. We argue that the low matching between stable isotope assignments and non-breeding sites in our study arise from insufficient baseline data for Africa (concerning both isoscapes and specific discrimination functions). However, other factors like aerial foraging habit of the species, and a potential mismatch of non-breeding site location and the spatial origin of aerial plankton might further hamper accurate assignments. Finally we call for concerted analyses of tissues i.e. feathers and claws of birds which are grown at known sites across the continent and from species with various ecological requirements (diverse habitats, foraging behaviours, and diet compositions) to establish isoscapes for general applicability.

Wing morphology, winter ecology, and fecundity selection: evidence for sex-dependence in barn swallows (Hirundo rustica).

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

Migration phenology and breeding success are predicted by methylation of a photoperiodic gene in the barn swallow.

Individuals often considerably differ in the timing of their life-cycle events, with major consequences for individual fitness, and, ultimately, for population dynamics. Phenological variation can arise from genetic effects but also from epigenetic modifications in DNA expression and translation. Here, we tested if CpG methylation at the poly-Q and 5'-UTR loci of the photoperiodic Clock gene predicted migration and breeding phenology of long-distance migratory barn swallows (Hirundo rustica) that were tracked year-round using light-level geolocators. Increasing methylation at Clock poly-Q was associated with earlier spring departure from the African wintering area, arrival date at the European breeding site, and breeding date. Higher methylation levels also predicted increased breeding success. Thus, we showed for the first time in any species that CpG methylation at a candidate gene may affect phenology and breeding performance. Methylation at Clock may be a candidate mechanism mediating phenological responses of migratory birds to ongoing climate change.

Sex-dependent carry-over effects on timing of reproduction and fecundity of a migratory bird.

Life of many organisms flows as a sequence of annual cycles. Timing of cyclical events is shaped by natural selection also via the domino effects that any life history stage has on the stages that follow. Such 'carry-over effects' have major consequences for evolutionary, ecological and demographic processes, but the causes that generate their individual-level variation, including the effect of sex, are poorly understood. We used light-level geolocators to study carry-over effects on the year-round life cycle of the long-distance migratory barn swallow (Hirundo rustica) and sex-dependent variation in their strength. Correlation analyses showed that timing of breeding influenced departure time for autumn migration in females but not in males. In addition, strong, time-mediated carry-over effects of timing of departure from the wintering areas in sub-Saharan Africa for spring migration on timing of arrival to the breeding grounds in Italy and Switzerland operated in both sexes. However, carry-over effects of spring migration phenology on breeding date and seasonal fecundity were observed among females but not among males. We used partial least squares path modelling to unveil the complex carry-over effects of phenology during the non-breeding season in combination with the ecological conditions experienced by individual swallows in the wintering area, as gauged by Normalized Difference Vegetation Index values (NDVI), on breeding performance. Phenology during the non-breeding season combined with NDVI during wintering accounted for as much as 65-70% of variation in subsequent seasonal fecundity in females, while such carry-over effects on breeding success of males were weaker. Intense, sex-specific carry-over effects can have impacted on evolutionary processes, including sexual selection, and affected phenological response to climate change, causing the large population decline observed in this species.

Clock gene polymorphism and scheduling of migration: a geolocator study of the barn swallow Hirundo rustica.

Circannual rhythms often rely on endogenous seasonal photoperiodic timers involving 'clock' genes, and Clock gene polymorphism has been associated to variation in phenology in some bird species. In the long-distance migratory barn swallow Hirundo rustica, individuals bearing the rare Clock allele with the largest number of C-terminal polyglutamine repeats found in this species (Q8) show a delayed reproduction and moult later. We explored the association between Clock polymorphism and migration scheduling, as gauged by light-level geolocators, in two barn swallow populations (Switzerland; Po Plain, Italy). Genetic polymorphism was low: 91% of the 64 individuals tracked year-round were Q7/Q7 homozygotes. We compared the phenology of the rare genotypes with the phenotypic distribution of Q7/Q7 homozygotes within each population. In Switzerland, compared to Q7/Q7, two Q6/Q7 males departed earlier from the wintering grounds and arrived earlier to their colony in spring, while a single Q7/Q8 female was delayed for both phenophases. On the other hand, in the Po Plain, three Q6/Q7 individuals had a similar phenology compared to Q7/Q7. The Swiss data are suggestive for a role of genetic polymorphism at a candidate phenological gene in shaping migration traits, and support the idea that Clock polymorphism underlies phenological variation in birds.

Assessing costs of carrying geolocators using feather corticosterone in two species of aerial insectivore.

Despite benefits of using light-sensitive geolocators to track animal movements and describe patterns of migratory connectivity, concerns have been raised about negative effects of these devices, particularly in small species of aerial insectivore. Geolocators may act as handicaps that increase energetic expenditure, which could explain reported effects of geolocators on survival. We tested this 'Energetic Expenditure Hypothesis' in 12 populations of tree swallows (Tachycineta bicolor) and barn swallows (Hirundo rustica) from North America and Europe, using measurements of corticosterone from feathers (CORTf) grown after deployment of geolocators as a measure of physiology relevant to energetics. Contrary to predictions, neither among- (both species) nor within-individual (tree swallows only) levels of CORTf differed with respect to instrumentation. Thus, to the extent that CORTf reflects energetic expenditure, geolocators apparently were not a strong handicap for birds that returned post-deployment. While this physiological evidence suggests that information about migration obtained from returning geolocator-equipped swallows is unbiased with regard to levels of stress, we cannot discount the possibility that corticosterone played a role in reported effects of geolocators on survival in birds, and suggest that future studies relate corticosterone to antecedent factors, such as reproductive history, and to downstream fitness costs.

Light-level geolocators reveal covariation between winter plumage molt and phenology in a trans-Saharan migratory bird.

Contingent individual performance can depend on the environment experienced at previous life-stages. Migratory birds are especially susceptible to such carry-over effects as they periodically travel between breeding ranges and 'wintering' areas where they may experience broadly different ecological conditions. However, the study of carry-over effects is hampered by the difficulty of tracking vagile organisms throughout their annual life-cycle. Using information from light-level geolocators on the barn swallow (Hirundo rustica), we tested if feather growth bar width (GBW), a proxy of feather growth rate which depends on individual condition, and wing isometric size and shape predict the phenology of subsequent migration. GBW did not predict duration of wintering but negatively predicted the duration of spring migration and arrival date to the breeding sites, suggesting that migration phenology is not constrained by molt, and individuals in prime condition achieve both faster molt and earlier arrival. Wing morphology did not predict migration duration, as expected if wing shape were optimized for foraging, rather than migration performance, in this aerially foraging, insectivorous bird. Thus, we showed for the first time that migration phenology in a long-distance migratory bird covaries with body condition during wintering, as reflected by the growth rate of feathers.

Timing of molt of barn swallows is delayed in a rare Clock genotype.

Photoperiodic responses are major factors entraining circannual life-cycles, functioning to adaptively synchronize annual routines to seasonal fluctuations in ecological conditions. Photoperiodism in physiology and behaviour is enforced by genes, including the vertebrate Clock orthologues, which are associated, for example, with phenology of migration in fish and breeding in birds. However, the role of Clock in photoperiodic plumage molt processes is unknown. We analyzed variation in molt schedules in relation to Clock genotype, using the long-distance migratory barn swallow (Hirundo rustica) as a model and by identifying males and females using molecular sexing techniques. Consistently with previous studies, we found one very common (Q 7) and two rare (Q 6, Q 8) variants of a functionally significant Clock polyglutamine repeat. Molt schedules of primary wing feathers of swallows during their wintering period in Nigeria differed among Clock genotypes: rare (1.1%) Q 7/Q 8 heterozygotes had significantly delayed molt compared to the other genotypes. Molt schedules did not differ between males and females, and no differential association between molt and Clock in relation to sex emerged. The same rare Clock genotype that exhibited delayed breeding in Europe was here found to delay molt in Africa. Though based on a limited number of Q 7/Q 8 individuals from an otherwise very large sample, these novel results suggest that Clock is involved in the photoperiodic control of both molt and breeding, potentially also via reciprocal carry-over effects. If confirmed in species with higher Clock polymorphism, present results may have far-reaching consequences for the study of photoperiodic control of molt and expression of annual routines.