Weather and climate change drive annual variation of reproduction by an aerial insectivore

Abstract

For many bird species, but especially aerial insectivores, reproduction depends on weather. Climate change is likely to intensify effects, but with uncertain consequences. We report 22 years of data on Eastern Kingbird (Tyrannus tyrannus) reproduction for two populations located in different hygric environments undergoing climate change; mesic central New York, USA, (NY; 12 years) and xeric southeastern Oregon, USA, (OR: 10 years). Laying date became earlier with increasing temperature in the 30-day period preceding laying in identical fashion at both sites, and in years of early laying, clutch size was larger, length of laying season increased, and failed initial nesting attempts were more often replaced. High temperature in the 10-days preceding mean laying date was associated with shorter laying seasons, while a site by 10-day temperature interaction reflected an increase and decrease of clutch size with increasing 10-day temperature in NY and OR, respectively. Seasonal rate of clutch size decline was higher when the laying season was short but also slowed in xeric OR when rain was abundant in the 10-days prior to mean laying date. Nest predation drove annual variation in young fledged/nest, but the latter also increased and decreased with increasing maximum temperature during the nestling phase in mesic NY and xeric OR, respectively. Potential effects of climate change on kingbird populations are thus high given the dependence of reproduction on weather, and climate change likely contributed to declines of kingbirds in OR. Declines of kingbirds in NY appear unrelated to warming climates because higher temperatures advanced laying dates and yielded greater nest productivity. However, length of laying season declined across years at both sites, and thus early season gains may be negated by poor conditions late in the season that may be causing shorter laying seasons. Further work is needed to identify causes for the latter changes.