Meeting Abstract
22.1 Friday, Jan. 4 When and how do Tree Swallow chicks die during cold weather? BOYLE, W.A.*; WINKLER, D.W.; GUGLIELMO, C.G.; Kansas State U.; Cornell; U. Western Ontario aboyle7@mail.ubc.ca
Temperate-breeding altricial birds face strong selection to breed early with the consequence that chicks experience periods of inclement weather that both increase thermogenic stressors and reduce their food supply. Tree Swallows (Tachycineta bicolor) are aerially-foraging cavity-nesting insectivores that frequently suffer complete or partial nest failure during cold weather. We studied individual-level chick mortality risk in a population of Tree Swallows near Ithaca, NY during 2010. Using weather and food data collected locally, we measured environmental conditions chicks experienced between hatching and fledging. We also characterized the development of chicks at days 3, 6, 9, and 12 by weighing, measuring feather cover, capacity for endothermy, and body composition using quantitative magnetic resonance (QMR). During periods of cold weather, we weighed and conducted additional QMR scans daily, and determined chick fate. Two cold snaps occurred during the study, and due to nesting asynchrony, cold weather affected chicks at all stages of development. Of the 140 chicks we followed (32 nests), 65 (from 19 nests) died during or immediately following periods of cold weather. Mortality risk was highest for 6-9 day-old-chicks. At this age chicks have a moderate capacity for endothermy but have incomplete insulation from growing feathers. Chicks died with on average 9.6% fat which is 3.6% more fat than lean females carry at the end of the breeding. After controlling for body composition change with chick age (linear decrease in proportion of mass accounted for by fat), the last measurements prior to death revealed slightly higher fat loads than on chicks that survived. Our data suggest that chicks are most vulnerable midway through development, and that they do not die because they have exhausted their energy stores.
