Spatial variation in White-throated Sparrow (Zonotrichia albicollis) refueling rate near a migratory barrier | La variation spatiale du taux de ravitaillement chez le Bruant à Gorge Blanche (Zonotrichia albicollis) près d'une barrière géographique

Stopover sites are essential to bird migration 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 studying physiological factors that may influence a bird's behavior when choosing and using a stopover site, researchers can target certain species or locations and more efficiently invest in conservation efforts. In spring 2013 and 2014, we studied stopover refueling performance in a common northeastern spring migrant, the White-throated Sparrow (Zonotrichia albicollis), at 2 locations near the south shore of Lake Ontario. We used morphological measurements and physiological techniques that assessed the concentration of the blood metabolite plasma triglyceride as a measure of fat deposition and feeding efficiency. We found that birds captured at an inland location had a significantly higher body condition score than birds captured at a coastal stopover site within 0.5 km of the shore; a trend toward higher triglyceride concentrations was also noted at the inland location. We found no significant differences in triglyceride concentrations between White-throated Sparrow color morphs, in contrast to previous studies performed during fall migration. Our results suggest that spring migrants arriving in the area in good energetic condition may begin their cross-lake journey directly from the inland site, while birds in poorer condition may “pile up” at the lakeshore and then compete with other migrants for available resources, slowing their fat deposition rate. Our results also reinforce the importance of protecting high-quality stopover habitat where birds congregate near geographic barriers but suggest that inland habitat patches are important stopover sites that may allow some migrants to bypass nearshore areas of intense competition.