Minor interspecies differences in breeding phenology and productivity between two co-occurring aerial insectivores

Aerial insectivore populations have declined significantly across Canada for the last fifty years. Although there are several suggested drivers of these population declines, including agricultural intensification, not all species show similar spatiotemporal population trends. Therefore, comparing interspecies differences in breeding productivity on the breeding grounds is vital to understand what is driving population variation among aerial insectivores. We examined breeding productivity and phenology in relation to habitat, weather, and insect availability for two co-occurring swallow species, Tree Swallow (Tachinyeta bicolor; â'2.79 regional annual trend index) and Barn Swallow (Hirundo rustica; â'3.33 regional annual trend index) over four years on the southern coast of British Columbia, Canada. We found only minor interspecies differences in breeding productivity (brood size, fledge success) comparing first broods, although the Barn Swallow is double-brooded, suggesting higher potential productivity than in the Tree Swallow. However, Tree Swallows had larger clutches, earlier lay dates, and more rapid cumulative laying than Barn Swallows. There was little effect of habitat on breeding productivity or phenology for either species, and we found no significant difference in total insect abundance between crop and pasture habitats. Overall, our study suggests there is little interspecies variation in breeding productivity for first broods between these co-occurring aerial insectivores in British Columbia. Given the higher propensity for double brooding and no differences in brood size or fledging success, Barn Swallows in this region may, in fact, have higher annual productivity than Tree Swallows, despite a more steeply declining regional population trend, suggesting that declines are caused by factors operating outside the breeding grounds or during post-breeding

Regional variability in trajectories of Barn Swallow populations across Canada are not predicted by breeding performance

Conservation of migratory species depends on an understanding of the drivers causing population declines across the annual cycle. Since it is difficult to track the entire annual cycle for long-distance migrants, measurements from part of the cycle may provide insights into overall drivers. Barn Swallows (Hirundo rustica) in Canada have undergone steep and long-term population declines. Using Breeding Bird Survey data and breeding performance data collected at field sites in three regions across Canada, we examined long- and short-term Barn Swallow population trends in Canada, differences in breeding performance across regions, if average breeding performance measures or colony size predicted regional population growth rates and if local breeding performance is driven by density dependence and/or recruitment. We found that long-term negative population trends were generally stronger in the east and west, and weaker in central Canada. Short-term population trends were generally weakly negative or neutral. Average breeding performance and colony size were poor predictors of regional growth rates observed in the subsequent year. We found little support for density dependence affecting local breeding performance; however, local breeding success, presumably through subsequent recruitment of young, was positively correlated to changes in local colony size in the following year. Since we found juveniles recruited into colonies located in the same region, but no link between average breeding performance and estimated regional population trajectories, our results suggest that extrapolating from local-scale studies to broader populations is challenging, and other demographic rates (i.e., survival) may need to be considered. Overall, our study suggests that contemporary Barn Swallow populations are below carrying capacity in Canada, further suggesting that food limitation is not currently regulating populations. Further study is needed to understand what factors are limiting and regulating Barn Swallow populations in Canada, across the annual cycle