REDUCED PREDATION AT INTERIOR NESTS IN CLUSTERED ALL-PURPOSE TERRITORIES OF LEAST FLYCATCHERS (EMPIDONAX MINIMUS)

Abstract

Predator deterrence has been widely studied as a potential advantage of colonial breeding. We extend the predator-deterrence hypothesis to the Least Flycatcher (Empidonax minimus), a noncolonial species that clusters its all-purpose territories, leaving apparently suitable habitat unoccupied. Under the hypothesis that Least Flycatchers' alarm calls deter some predators from hunting inside their nesting clusters, we predicted that the rate of nest predation would be lower for interior nests than for those on the periphery. In 1995 and 1996 in north-central Minnesota, we monitored 157 Least Flycatcher nests from nine nesting clusters, with locations ranging from directly on the edge to 170 m in from the edge. Using proportional hazards regression, we assessed the effect of distance to cluster edge on nest success. The models best supported by the data all indicated higher success for interior nests, with 34–38% lower predation hazard. This result is the first evidence of reduced predation within nesting clusters of a species that defends all-purpose territories. Combined with earlier results demonstrating a dramatic response by Least Flycatchers to a live predator, our results lend substantial support to the predator-deterrence hypothesis.