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

Abstract

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; −2.79 regional annual trend index) and Barn Swallow (Hirundo rustica; −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. Différences interspécifiques mineures dans la phénologie de nidification et la productivité de deux insectivores aériens cooccurrentsRÉSUMÉ. Les populations d'insectivores aériens ont diminué de façon importante au Canada au cours des cinquante dernières années. Bien que plusieurs facteurs soient avancés pour expliquer ces baisses de population, notamment l'intensification de l'agriculture, les espèces ne présentent pas toutes des tendances spatio-temporelles similaires. Par conséquent, il est essentiel de comparer les différences entre les espèces en matière de productivité sur les lieux de nidification si on veut comprendre ce qui fait varier les populations d'insectivores aériens. Nous avons examiné la productivité et la phénologie en fonction de l'habitat, des conditions météorologiques et de la disponibilité d'insectes pour deux espèces d'hirondelles cooccurrentes, l'Hirondelle bicolore (Tachinyeta bicolor; indice de tendance annuelle régionale de -2,79) et l'Hirondelle rustique (Hirundo rustica; indice de tendance annuelle régionale de -3,33) pendant quatre ans sur la côte sud de la Colombie-Britannique, au Canada. Nous n'avons trouvé que des diff...