Brood Parasitism Defense Behaviors Along an Altitudinal Gradient in the American Robin (Turdus Migratorius)

Abstract

Some host species accept eggs from brood parasites over parts of their range and reject them in other areas representing an “evolutionary lag” in the development of rejection behavior or the loss of an adapative behavior when the selection pressure of brood parasitism is removed. Hosts may deter brood parasitism through egg rejection and aggressive nest defense behavior specifically targetting female brood parasites during the egg incubation period. In areas where parasitism frequencies are spatially and temporally variable, anti-parasite behaviors may decline as costs outweigh the benefits. Along the Colorado Front Range, American robins (Turdus migratorius) breed from low elevations where the brood parasitic Brown-headed Cowbird (Molothrus ater) is abundant to near timberline (3700 m) where cowbirds are uncommon. We tested the hypothesis that egg rejection and nest defense behaviors decline with reduced probability of parasitism. We found that robins accepted 100% of immaculate (robin-like) experimental eggs at both low and high elevations, but were more likely to reject spotted (cowbird-like) experimental eggs at low elevations than high elevations. Response to egg size was more variable than to egg color. When presented with a mount of a cowbird and Song Sparrow (Melospiza melodia) near the nest, robins responded more aggressively to cowbird models than to sparrows (control), and nest defense behavior towards cowbirds was longer and more aggressive at the lower elevation sites where cowbirds are common. These results suggest that egg rejection and nest-site aggression are costly adaptations to cowbird parasitism, and these behaviors decline when the threat of parasitism is reduced.