A test of the dear enemy hypothesis in female New Zealand bellbirds (Anthornis melanura): female neighbors as threats

The "dear enemy" hypothesis proposes that the level of territorial aggression toward conspecific neighbors is lower than that shown toward strangers primarily because of differences in "threat." Individual recognition is considered to be an important component of this hypothesis. Among songbirds, recognition is often based on vocalizations and male song playbacks have been used to test this dear enemy hypothesis. However, territorial song is not an exclusively male activity and female song is more widespread than previously thought, although the functions are not well understood. The vocalizations of the endemic New Zealand bellbird (Anthornis melanura), a species in which both sexes sing prolifically, have recently been quantified, and bellbirds provide an exciting model for examining the function of female song. We experimentally tested the dear enemy hypothesis for territorial females using female neighbor--stranger playback. We found clear evidence that individual females discriminate between conspecific female neighbor and stranger song. Aggressive responses were strongest during the courtship and chick-rearing stages and involved rapid countersinging responses and movement toward the speaker. Most importantly, females were more aggressive toward the songs of neighboring females. This result is opposite to the dear enemy phenomenon and suggests that neighboring females pose a greater threat than strangers. A higher female neighbor threat may be linked to either competition for food resources or paternal care. We predict that these higher levels of aggression may play a role in sexual selection and polygny prevention and that neighboring females are the greatest threat to the loss of a mate. Copyright 2008, Oxford University Press.