Does avian nest site selection occupy an adaptive peak

Background/Question/Methods

Habitat selection is an ecological trait shaped by natural selection. Animals should select habitats that optimize individual fitness, in which case habitat selection is “adaptive.” However, the adaptive significance of habitat selection may be difficult to document in nature, particularly for populations within which all members occupy an “adaptive peak,” i.e., a range of habitats among which expressed relative fitness is uniform because the phenotypic variation in habitat choice necessary to detect habitat-fitness relationships no longer exists naturally. With this problem in mind, we investigated the adaptive significance of nest site selection in a population of Yellow Warblers. Given the general importance of nest predation to avian fitness, we specifically investigated nest site selection in the context of predation risk. We focused on site selection with respect to nest concealment, because despite being mechanistically relevant to nest predation, this nest site feature has not been consistently linked with predation risk in empirical studies. We measured concealment-predation relationships for both natural Yellow Warbler nests and experimental nests. Natural nest data were collected in 2000-2008, and experimental data were collected in 2006-2007. We measured concealment using ocular estimation, and we distributed experimental nests among sites both within and outside the concealment range (30-80%) identified for natural nests found prior to the experimental period (2000-2005).

Results/Conclusions

We found two lines of evidence indicating that Yellow Warbler nest concealment behavior is adapted for avoiding predation risk. During the experimental period, concealment-predation relationships for both natural and experimental nests were relatively weak within the natural concealment range, but experimental predation rates were substantially elevated in sites < 30% concealed. Since experimental and natural relationships were congruent among habitats in which both nest types occurred, we were satisfied that experimental relationships reflected real predation pressures relevant to natural nests. As such, experimental data provided evidence that Yellow Warblers occupy an adaptive peak along a nest concealment gradient, specifically avoiding elevated predation risk in highly exposed sites. In addition, natural nest data from a non-experimental year provide supplementary evidence. In contrast to other years, we found a strong positive natural concealment-predation relationship in 2001 coupled with increased concealment of natural nests. These data document an apparent adaptive response by Yellow Warblers to a shift in the form of natural selection, suggesting that Yellow Warbler nest site selection is not only shaped by predation risk, but also capable of dynamic responses to changes in predation pressure.