Aerial insectivores — birds that capture their prey while in flight themselves — are experiencing the steepest population decline in North America. One of the most widespread members of the group, Eastern Whip-poor-will, is no exception.
Population data from the Breeding Bird Survey indicate a range-wide decline of 2.76 percent per year in recent decades, and populations breeding in the northern portion of the range are falling even faster: about 3.5 percent per year.
Biologist Philina A. English of Simon Fraser University set out to determine what’s causing the ongoing reduction in numbers. In a paper published in February 2018 by Frontiers in Ecology and Evolution, she and two colleagues compared the chemical signatures in museum specimens with those of living whip-poor-wills to determine what the birds used to eat.
From 2011 to 2013, they took feather and tissue samples from birds at three Ontario breeding sites, and they obtained samples from 63 specimens that had been collected as far back as 1880, now held by museums in Toronto and Ottawa. Measuring the isotope ratios of carbon and nitrogen, they determined that “aerial insectivore populations are declining due to changes in abundance of higher trophic-level prey.”
In other words, there are fewer large bugs now than in the late 19th and early 20th centuries, leaving less prey for whip-poor-wills and other aerial insectivores.
English also tracked 22 whip-poor-wills with light-level geolocators. In her doctoral dissertation, published in 2017, she reports the birds wintered from the Gulf coast of Mexico to Costa Rica. “On southward migration, most individuals interrupted migration for periods of up to 15 days north of the Gulf, regardless of their subsequent route,” she writes. “Fewer individuals showed signs of a stopover in spring.”
The use of stopover sites in the southeastern U.S. and the determination of wintering sites farther south suggest the need to improve conservation efforts for the species in those regions, English says.
A version of this article appears in “Birding Briefs” in the May/June 2018 issue of BirdWatching magazine. Subscribe
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