Too scared to settle: Predation risk shapes breeding bird community in ways predictable from functional traits

Background/Question/Methods

Predation risk is widely recognized to play a critical role in shaping ecological patterns and processes. Its role as an important force in structuring community assembly has long been proposed, but has only rarely been tested in terrestrial vertebrate systems. How animals respond to predation risk is thought to be generally predictable from species’ life-history and natural-history traits, but rigorous tests of such predictive relevance have remained scarce. We report a large-scale playback experiment with a forest bird system that asks two questions: 1) can predation risk shape the structure of a breeding bird community, and 2) can life-history and natural-history traits predict prey’s response to different predation risks? Over the breeding season of 2010, we manipulated cues of three avian predators that preferentially prey on either nests or adult forest birds in 4-ha plots (Blue Jay Cyanocitta cristata, BLJA, mainly depredates nests; Eastern Screech-Owl Otus asio, ESSO, mainly depredates adults; Cooper’s Hawk Accipiter cooperii, COHK, depredates both nests and adults; N=6/treatment), and conducted repeated community surveys to characterize community- and species-level responses.

Results/Conclusions

We found that increased cues of all three predators reduced community species richness by around 8.2% (ESSO) ~ 14.1% (COHK), caused distinct shifts in community composition, and led to negative responses in the abundance, occurrence and/or detection probability for most prey species (15 out of 18 focal species). In accordance with life-history theory predictions, we found that faster (i.e. shorter-lived and more fecund) species avoided settling in habitats with high nest predation risk more than they would avoid habitats with high adult predation risk, with the reverse for slower (i.e. longer-lived and less fecund) species. Natural-history traits including nest type and body size also predicted species’ risk responses: open-cup-nesting species showed stronger negative responses to BLJA and ESSO treatments but not to COHK, compared to cavity-nesting species; larger-sized species showed stronger negative responses to COHK than smaller species; smaller species showed stronger negative response to BLJA and ESSO than larger species. Our results provide strong evidence that predation risk can shape the structure of breeding bird communities, and species’ risk responses at the scale of habitat selection can be predicted from species’ key functional traits.