Nesting structures are important for successful reproduction in most birds, and, because of this, geographic variation in nest morphology and composition are usually interpreted as adaptations to breeding in different environments. We compared the structure of nests of Yellow Warblers (Dendroica petechia) breeding in Churchill, Manitoba, and Elgin, Ontario, Canada. Churchill is subarctic in habitat and typically much colder during the breeding season than Elgin. We compared temperature, rainfall, and wind speed at these two sites and then tested whether differences in nest structure corresponded to different environments. Yellow Warblers breeding in Churchill built larger, less porous nests that retained heat better but also absorbed more water and took longer to dry than Yellow Warbler nests from Elgin. We suggest that differences in the structure of Yellow Warbler nests represent adaptations to breeding in different environments because the differences in nest morphology and properties of heat retention and water loss correspond to differences between the sites in environmental challenges.
The Yellow Warbler is a small (∼10 g) migratory songbird that breeds throughout temperate North America and winters from Mexico to South America (Lowther et al. 1999). On the basis of plumage and morphology, Browning (1994) recognized 43 subspecies of the Yellow Warbler. He assigned those breeding at Churchill to D. p. parkesi and those breeding at Elgin to D. p. aestiva. Only females build nests, and away from the northern extremity of the breeding range, including Elgin, females readily renest if early nests are destroyed or depredated. Once a pair successfully fledges young, it typically does not attempt to raise additional broods within that season (Lowther et al. 1999).
During the breeding seasons of 2008 and 2009, we collected Yellow Warbler nests at the Churchill Northern Studies Centre (58° 40′ N, 94° 25′ W; elevation 20 m) about 20 km west of Churchill, Manitoba, and from the Queen's University Biological Station (44° 30′ N, 76° 19′ W; elevation 125 m) near Elgin, Ontario. We searched for Yellow Warbler nests by following females that were carrying nesting materials and by searching appropriate habitat. We monitored all nests found during building and laying so that we could be sure when nest construction and laying were completed. Nearly all active Yellow Warbler nests that we found were included in this study, reducing any potential bias in our selection of nests. Nests were collected immediately after being completed and prior to egg laying, in the early stage of laying, or during early incubation. We excluded nests that fledged nestlings from our analyses because nestlings change the shape of nests considerably (Holcomb and Twiest 1968, Calder 1973; V. G. Rohwer, pers. obs.).
We have observed no differences between Yellow Warbler nests built early in the breeding season and those built later, and female Yellow Warblers appear to build multiple nests with very consistent morphologies (Patrick 2009). Nonetheless, we collected all nests early in the breeding season to control for any unmeasured variation through the breeding season in nest composition or morphology.