Migratory connectivity and differential patterns of decline in Barn Swallow (Hirundo rustica) in North America: Potential effects of factors on breeding and wintering grounds

Populations of several Neotropical migratory bird species have experienced declines in the recent decades but long-distance migrant aerial insectivores seem to be declining at greater rates than any other group. The Barn Swallow (Hirundo rustica) is a long-distance migrant and an aerial insectivore whose breeding populations have declined drastically in northern regions of North America but remain stable in certain areas of the southern United States. However, reasons for differential population trends and whether factors on breeding and/or wintering grounds could be causing these declines remain unclear. The main objective of this study was to investigate causes of differential population dynamics experienced by Barn Swallows in North America by 1) studying patterns of migratory connectivity of populations using a three stable isotope approach (δ2H, δ13C and δ15N) and 2) evaluating evidence for factors on breeding and/or wintering grounds causing regional differences in population trends. Tail feathers of Barn Swallow grown on the wintering grounds were collected during breeding season in 2009, 2010 and 2011 across the breeding range of the species. Feathers were examined isotopically (δ2H, δ13C and δ15N) and used to determine the relative strength of migratory connectivity of populations and to identify approximate wintering regions in South America. Long-term mark-recapture data from two breeding colonies in Washington State and Ontario were compared to the Southern Oscillation Index (SOI) for breeding and wintering months as a measure of El Niño Southern Oscillation (ENSO) to study the fundamental effect of large-scale climatic conditions on annual survival rates. Similarly, long-term reproductive parameters (i.e. fledging success, total number of eggs laid, chicks per nest) of breeding individuals in Washington State and Manitoba were used to determine the effects of local weather variation on reproductive performance. Winter-grown feathers grouped according to population sizes were additionally analyzed for costicosterone (CORT) concentrations to test whether declining northern populations have higher exposure to stressors on their wintering grounds in South America. Stable isotope (δ2H, δ13C and δ15N) analyses revealed evidence for a diverse pattern of migratory connectivity among breeding populations. Four isotopically distinct clusters corresponding to different wintering regions were identified and a progressive longitudinal shift in wintering grounds was detected. This study found evidence for a differential effect of ENSO-related weather conditions on inter-annual survivorship of two different breeding populations of Barn Swallows in North America. Annual survivorship of Barn Swallows breeding in Washington State was strongly correlated to ENSO during breeding and wintering months, while no correlation was found for birds breeding in Ontario, potentially due to the geographical variation of the effect of ENSO on weather conditions across North America and South America and the differential degree of migratory connectivity of populations. The length of cold periods (total number of consecutive days (≥ 2 d periods) with maximum daily temperatures lower than 11ºC) was found to have also a strong effect on annual survival rates of birds breeding in Washington State. Analyses of historical weather data showed that declining breeding populations have been exposed to more extensive cold periods than stable populations, suggesting a relationship between length of cold periods in spring and population trends. Annual reproductive performance was significantly affected by environmental conditions early in the breeding season related to ambient temperature, while precipitation variables had little effect. Wintering ground stressors related to CORT seem to have no significant effect on population size changes, since feather-CORT levels from declining populations were similar to those from stable populations. This study has provided a first attempt to identify factors occurring on breeding and/or wintering grounds that could have an effect on population sizes of Barn Swallows in North America. Weather conditions during breeding and wintering months and the degree of migratory connectivity of populations seem to be playing a significant role in the population trends of the species