Nest site selection by common eiders : relationships with habitat features, microclimate and incubation success

Abstract

Habitat selection theory presumes that organisms are not distributed randomly in their environments because of habitat-specific differences in reproductive success and survival; unfortunately, many previous studies were either unable or failed to look for evidence of processes shaping nest site selection patterns. Furthermore, little is known about adaptive nest site selection in northern environments where habitats often have little vegetation and time and climatic constraints may be pronounced. Therefore, I investigated patterns of nest site selection by common eider ducks (Somateria mollissima) at an island colony in Canada’s Eastern Arctic, and looked for evidence of selective processes underlying these patterns by employing experimental and observational techniques.I characterized physical features of (a) non-nest sites (b) active nest sites and (c) unoccupied nest sites that had been used in previous years. Habitat features that distinguished non-nest sites from unoccupied nest sites were also important in distinguishing between active and unoccupied nest sites during the breeding season. Active nest sites were closer to herring gull (Larus argentatus) nests, farther from the ocean and had organic substrates. In general, habitat features associated with nest use were not strongly associated with success after the onset of incubation. Nests near fresh water ponds were more successful in one study year, but in the other two study years successful nests were initiated earlier and more synchronously than were unsuccessful nests. Common eiders settled to nest first near the geographic centre of the colony, whereas sites near the largest fresh water pond were occupied later; distance to ocean had no observable effect on timing of nesting. Nest density was greater farther from the ocean, but timing of nest establishment did not differ between high and low density plots. I tested whether moss or duck down placed in nest bowls could increase nest establishment, or advance laying date. I placed this extraneous material in bowls before nesting and found no difference in likelihood of nest establishment; however, bowls containing duck down were initiated earlier (or had higher survival) than those containing no nesting material. To investigate the role of nest shelter and microclimate in nest site choices and female body condition, I placed plywood nest shelters over established nests. Temperature probes indicated that artificially-sheltered females experienced more moderate thermal environments and maintained higher body weight during late incubation than did unsheltered females. However, few eiders nested at naturally-sheltered sites, possibly because nest concealment increases susceptibility to mammalian predators. My results suggest that eider nest choices likely reflect trade-offs among selective pressures that involve the local predator community, egg concealment, nest microclimate and energy use.