Why are these insect-eating birds declining across North America?

Populations of aerial insectivores such as Tree Swallows are declining across North America, and two new studies are detailing the factors which may be driving their disappearance. The results have shown that, although the specifics may vary between destinations, action is needed to address environmental changes across the entire geographic range of these insect-eating birds.

Peter Dunn is a Tree Swallow expert from the University of Wisconsin-Milwaukee who was not involved in the research.

“These studies are part of a growing number of studies addressing the causes of population declines among aerial insectivores,” said Dunn. “In terms of understanding population declines in aerial insectivores, both studies point to adult survival during migration or on wintering areas as an important factor. Thus, these models identify areas where we can focus our future research efforts.”

“Recent studies of Tree Swallow migration using geolocators suggest that populations breeding in eastern Canada are migrating along the east coast to winter primarily in Florida and Cuba, so these areas should probably be the next focus of study.”

A Queen’s University team led by Amelia Cox focused its investigation on a swallow population in southeastern Ontario, for which data was available from 1975 to 2017, while a Bowdoin College team led by Liam Taylor examined data from an island population off the coast of New Brunswick from 1987 to 2010.

Using the long-term data, the research teams were able to determine which life stages were having the largest impact on local population declines.

In Ontario, overall declines were mainly driven by a reduction in overwinter survival and the rate at which swallow chicks successfully left the nest. On Kent Island, the population depended on immigration from the mainland. During the study, the Kent Island population drastically dropped from 202 adult birds to only 12.

Cox and her colleagues believe that worsening weather conditions and declines in insect availability may be behind the demographic shifts.

“I hope that our results will spur more research into the environmental causes of Tree Swallow declines and declines of other similar species. Our research points the finger at poor survival overwinter and poor fledging as the probable demographic causes of population declines,” said Cox.

“The next step is to figure out exactly what has changed in their environment and why these birds are dying during these critical life stages. We have additional work in progress that is aimed at answering these questions. When we know that, I hope we will be able to start making changes to improve survival and fledging.”

“It was an incredible opportunity to analyze a dataset that started before I was born,” added Taylor. “Working with these long-term data had an emotional connection for me as a researcher and birder. My own fieldwork on Kent Island took place during the summers of 2014 and 2015, by which time most of the swallow nest boxes on the island were empty. For me, it was sobering to look back through the data and envision those nest boxes full of activity and life.”

The study is published in The Condor: Ornithological Applications.

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By Chrissy Sexton, Earth.com Staff Writer