Predator avoidance as a function of flocking in the sexually dichromatic Hawaii akepa

Hypotheses for joining a mixed-species bird flock consider each species as a single unit. In sexually dichromatic birds, differential conspicuousness between the sexes may result in differences in vigilance for predators. Aspects of the predator avoidance and foraging enhancement hypotheses for the selective value of joining a mixed-species flock were assessed for the strongly sexually dichromatic Hawaii akepa (Loxops coccineus coccineus). There was support for the primary predictions of the predator avoidance hypothesis: vigilance levels decreased with increasing group size, and with membership in a flock, but only for brightly colored adult males. There was little support for the hypothesis that the primary benefit of joining a mixed-species flock is to enhance foraging efficiency through "local enhancement".