Predictors of nestling survival during harsh weather events in an aerial insectivore, the Tree Swallow (Tachycineta bicolor)

Extreme weather events influence the population dynamics of wild animals. For organisms whose food source is affected by environmental conditions, such as aerial insectivorous birds, periods of inclement weather can have devastating effects. Here, we examine predictors of survival of individual nestlings and whole broods in Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) during an extreme, 2-day harsh weather event in central British Columbia, Canada, which co-occurred with experimental reduction of nest ectoparasite loads using an antiparasite drug (ivermectin) or heat-treating nests. A curvilinear relationship existed between survival and brood age such that middle-aged broods were least likely to survive. Survival of broods and nestlings was higher when raised by males with bluer plumage, whereas nestling survival was lower when female parents had brighter and more UV-reflective plumage. Within broods, smaller nestlings had a lower chance of surviving than their larger siblings. Nestlings in broods where half of the offspring received ivermectin injections had significantly higher chances of surviving than nestlings from nonexperimental broods, suggesting that parasite loads can influence survival during inclement weather. Our results identify several factors influencing resiliency of nestlings to harsh weather and are particularly relevant in the context of declining aerial insectivorous bird populations and climate change.