Experiments on Cuckoldry in the Mountain Bluebird

Abstract

Two experiments on wild bluebirds were conducted to provoke them into anticuckoldry behavior. The pilot experiment used live, caged male and female bluebirds as models. The main experiment used stuffed male and female bluebirds as models in addition to live bluebirds. Forty pairs and 2 lone males were tested in 87 trials, covering all nesting stages. Bluebirds responded much more strongly to live than to stuffed models. They did not show any responses to models that could be interpreted as having evolved solely to thwart cuckoldry. They did show a repertory of behaviors from defense of their nest sites to attacks on models that formed a gradient of least aggressive to most aggressive responses and would have driven models away from their nests, mates and territories (simultaneously ended several threats, including that of cuckoldry) had models been free to flee. That these responses were evolved at least partly to cope with cuckoldry threat was affirmed by their sex-biased occurrence. Probably because bluebird responses function to cope with several threats, there were no statistically significant changes in their responses in our experiments from nest stage to nest stage except for mounting attempts and singing which never occurred after hatching. The nonoccurrence of these behaviors after hatching parallels the time course of the much more common behaviors after hatching parallels the time course of the much more common behaviors of male following and escorting of females and all of these behaviors are related to courting and fertilization. Hatching may be the cue used by bluebirds to signal the end of the period of cuckoldry risk because females sometimes begin incubation prior to clutch completion, creating ambiguity as to when laying ends. Males attacked females during some trials but females attacked males as often and as intensely and mate attacks were fairly uncommon in the experiments and were unrelated to nesting stage. Mate attacks are only misplaced aggression. Because replacement females are not readily available, it would not be adaptive for a male to drive away his mate, as asserted by Barash (1976-1978), unless he had very little certainty of paternity. Barash''s (1977) claim that male aggression toward females is the species-typical response of mountain bluebirds to cuckoldry threat is rejected. Bluebirds respond to that threat by driving potential or actual cuckolders away in a manner that reduces or eliminates other threats and is variable in its intensity and form from pair to pair and encounter to encounter.