Acoustic directionality of red-winged blackbird (Agelaius phoeniceus) song relates to amplitude and singing behaviours

Sound waves typically radiate from a vocalizing animal with unequal amplitude in different directions; this may profoundly affect communication, favouring adaptations that use or compensate for this directionality. Previous studies suggest that variation in directionality among vocalization types is related to function, with vocalizations such as alerts being relatively omnidirectional, thus maximizing the number of surrounding receivers, and vocalizations such as courtship calls being relatively directional, thus reducing eavesdropping by beaming sounds at the targeted receiver. Here we examine variation within a vocalization type, bird songs, to test hypotheses about how variation in song directionality relates to singing behaviour and the social context of singing. We measured the directionality and amplitude of red-winged blackbird songs in the field using calibrated microphones encircling the bird. We found variation in directionality among song types, possibly allowing males to select songs from their repertoires with directionality to suit the social context. We found support for the hypothesis that the amplitude and directionality of songs are related, with more-directional songs delivered at higher amplitudes. In addition, we found support for the hypothesis that directionality is negatively related to the degree of male rotation on the perch between vocalizations, suggesting that males may combine omnidirectional songs and rotation to broadcast their songs, and may face the receiver and produce directional songs during direct interactions. We also observed variation in directionality among note types within songs. These results suggest that directionality plays an important role in shaping song structure and singing behaviour in red-winged blackbirds.