Spring Stopover Ecology and Physiology of the White-Throated Sparrow (Zonotrichia albicollis) in Western New York

Abstract

Stopover sites are an essential part of a North American migratory songbird’s journey between wintering and breeding grounds, but annual variation in use and habitat conditions make it difficult to determine which sites are most critical for conservation. By learning which factors influence a bird’s behavior when choosing and using a stopover site, we can target certain species or locations and more efficiently invest conservation efforts. In April-May 2013 and 2014, I studied stopover refueling rate in the White-throated Sparrow (Zonotrichia albicollis), a common northeastern spring migrant, at two locations near the south shore of Lake Ontario, a natural migratory barrier. To do this, I used morphological measurements and physiological techniques that measured the concentration of two important blood metabolites, triglycerides and ß-hydroxybutyrate. Blood triglyceride concentration is a measure of fat deposition and feeding efficiency, and ß-hydroxybutyrate concentration is associated with fat catabolism and energy loss. I found that birds captured at a location ~15 km from the shore had significantly higher blood triglyceride concentrations, as well as significantly higher body condition score, than birds captured at a stopover location within 0.5 km of the shore (1.737 mM > 1.361 mM). However, after using ANCOVA to control for the effects of body condition and time after sunrise, blood triglyceride levels did not vary significantly with location. ß-hydroxybutyrate levels were not significantly higher in lakeshore-captured birds either before or after ANCOVA. Lack of statistical significance in both cases may be due to effects of small sample sizes. My results imply that birds obtain food more efficiently at the inland location, and that birds that arrive in the area in better condition may begin their cross-lake journey directly from the inland site. Birds in poorer condition may “pile up” at the lakeshore and then must compete heavily with other migrants for available resources, slowing their fat deposition rate. These results reinforce the importance of protecting high-quality stopover habitat where birds congregate near geographic barriers, but also suggest that inland habitat patches are important stopover sites that may allow some migrants to bypass nearshore areas of intense competition.