The nest defense and nest-destroying behaviors of house wrens
Presentations of three potential nest intruders were used to investigate house wren nest defense behavior, while regular nest box checks provided information regarding house wren nest-destroying behavior. During nest defense trials, male wrens were typically more active in defense than females in the egg stage but no intersexual differences existed in the nestling stage. Males' greater defensiveness could be an extension of their territory defense role. Defense vocalizations were sex biased, with males giving primarily song calls and females giving chatter calls. Both sexes maintained or lowered their defense levels from egg to nestling stage, possibly due to decreased offspring vulnerability. House wrens altered their defense behaviors among intruder species, depending on the type of threat. Males exhibited greater defense levels against the house wren model compared to the bluebird or cowbird model in the egg stage but showed no differences among model species in the nestling stage. Females exhibited no defense differences among model species. Male defense levels were affected by their breeding strategy, as attentive males were more aggressive toward the house wren and cowbird models than non-attentive males. Attentive males discriminated among intruder species while non-attentive males did not. Lack of male attentiveness, due to polygamy or attempted polygamy, appeared to be potentially costly to females, regardless of their order of pairing with the male. House wren nest destroying behavior (NDB) varied with respect to the timing of house wren nest attacks, the abandonment and reacquisition of nests following a wren attack, and the status of neighboring house wren nests at the time of a nest attack. Some supportive evidence was found for the existing NDB hypotheses, but no single hypothesis was completely supported. House wren NDB may serve several functions but may ultimately depend on house wren population density.