Male Response to Apparent Female Adultery in the Mountain Bluebird (Sialia currucoides): An Evolutionary Interpretation

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Abstract
The adaptive significance of the constellation of reproductive and parental behavior shown by each species is examined. Males, for example, should be strongly selected to avoid being cuckholded, ensuring that their females are not carrying strangers'' genes. Predictably, this characteristic would be especially well-developed among single-brooded, monogamous species and those in which males make a substantial investment in the success of their offspring. This report describes 1 aspect of such behavior in the mountain bluebird, S. currucoides. The male aggressive display is described. Aggression toward an intruding male would be especially advantageous early in the breeding season, when territories and nests are normally defended. The continuation of a relatively high level of aggression can be explained on a proximate basis by the operation of a releaser contained in the model and ultimately by the presumed advantage to reducing encroachment by strange males at any stage of the nesting cycle. The initial, aggressive response to the mated female is also adaptive in that, given a situation suggesting a high probability of adultery (i.e., the presence of the model near the female) and assuming that replacement females are available, obtaining a new mate would enhance the fitness of the male.