Experiments on Cuckoldry in the Mountain Bluebird

Abstract

Cuckoldry produces both genetic polygyny within otherwise socially monogamous species and cryptic altruism by cuckolds themselves, thus the genetic effects of cuckoldry are extremely severe, and should result in the evolution of both anticuckoldry behaviors and means of evading them in socially monogamous birds. Nonexperimental observations of mountain bluebirds revealed that (1) nesting males usually follow nest-building and foraging females, and escort them in and out of their nest cavities prior to hatching, thereby apprising themselves of their mates' whereabouts and companions at all times and preventing their own cuckoldry; (2) intruder males without adjacent territories proceed directly to nest sites when invading others' territories and sometimes display there before being evicted, thereby stationing themselves at the place where most copulations occur and giving themselves the opportunity to cuckold the males that occupy the nest sites; and (3) resident bluebirds defend their nest sites and terri...