Experimental Evidence for the Function of Mate Replacement and Infanticide by Males in a North-Temperate Population of House Wrens

Abstract

Male birds who replace other males on territories may kill, care for, or act indifferently towards offspring of the previous resident. In short-lived species, selection should favor adoption or indifference over infanticide only when the resident female is the only mate the replacement male is likely to obtain, and when this female will desert the male if her offspring are killed. We tested this hypothesis by removing 29 male House Wrens (Troglodytes aedon) from territories in a population in which unpaired females are readily available as mates. Ten of 16 males that were subsequently observed on experimental territories removed offspring from nests as predicted. At least four resident females bred with these males. All infanticidal males paired within 72 hr of replacement and most subsequently fledged young of their own. An eleventh replacement male fed young after eggs hatched but disappeared before nestlings fledged. The five remaining males appeared indifferent to resident offspring. Unlike infanticidal males, however, indifferent males did not remain on territories continuously, court resident females, or attempt to attract a mate to the territory. No indifferent male bred on the territory. Our observations suggest that selection should strongly favor a "replacement-then-infanticide" strategy among unsettled male House Wrens in this population. However, for reasons yet unknown, some males fail to complete this sequence of behaviors.