Ranging By Song in Carolina Wrens Thryothorus Ludovicianus: Effects of Environmental Acoustics and Strength of Song Degradation

Abstract

Territorial male song birds most frequently hear conspecific song that has been degraded (distorted) by transmission through the environment. Their ability to use this accumulated degradation in conspecific song to assess the distance of its singer requires a receiver to discriminate between different degrees of degradation by taking into account the acoustical properties of the habitat. Ranging accurately when acoustical properties change seasonally then requires a receiver to reassess previous associations of degradation with distance. Here I tested the possibility that Carolina wrens (Thryothorus ludovicianus) discriminate between different levels of song degradation and change their association of degradation with distance when the acoustical properties of their territories change. In response to playback of a single song, either undegraded or degraded (at two different levels), most subjects flew to the far side of the loudspeaker only in response to degraded songs. In addition, behavioral responses beyond the loudspeaker were consistently stronger to playback of degraded songs than to playback of undegraded songs. Responses indicate that wrens discriminated between different levels of degradation and suggest that they adjusted their association of degradation with distance as habitat conditions changed. Such adjustment of associating a given level of degradation with distance is an important requirement for accurate ranging, in particular under changing acoustical conditions of the environment. In addition, rapid ranging on the basis of only one song might facilitate processing of additional information such as a singers identity and motivation. Resulting selective attention to the closest rival might increase the reliability or speed of decoding such additional information.