Repurposing historical data to investigate aerial insectivore declines

Populations of aerial insectivores have decreased since the mid-1980s, possibly due to declines in their prey. However, long-term data on insect abundance in North America are lacking. I evaluated whether brood size manipulation experiments could be repurposed to assess changes in insect availability. A literature review found no evidence that parents’ ability to respond to a challenge has changed over time, but study methods varied widely. Therefore, I replicated a brood size manipulation experiment conducted on tree swallows in 1994/1995. Parents did not change how they responded to changes in brood size. However, delivery rates were consistently lower in 2017/2018 because parents delivered smaller boluses and tended to visit the nest less. These results are consistent with the hypothesis that aerial insectivores are declining due to reduced insect availability, but could also arise for other reasons. My thesis highlights the value of historical data for investigating aerial insectivore population declines.