Directionality of the drumming display of the Ruffed grouse

Abstract

Directionality is a key feature of vocalizations in birds and can provide insight into vocalizations' function and propagation. Despite recent interest in birds' production of nonvocal acoustic signals, little is known about the directionality of these sounds. here, we examine the directionality of the drumming display of male Ruffed grouse (Bonasa umbellus), a nonvocal acoustic signal used in mate attraction and territorial defense. We recorded drumming males with a low-frequency microphone and measured sound-pressure levels with a soundlevel meter from three orientations: front (0°), behind (180°), and side (90°). The sound-pressure level was significantly higher in front of and behind a drumming male than on the sides, indicating that the drumming sound radiates primarily along the displaying bird's longitudinal axis. Spectral analyses of the recordings did not, however, yield consistent differences in frequency among the three orientations. The acoustic radiation pattern of the drumming display differs from that of vocalizations in that the sound-pressure levels are just as high behind the bird as they are in front of it. This directionality likely affects estimation of distance by predators and intended receivers as well as potentially limiting the number of receivers able to perceive the signal. © The Cooper Ornithological Society 2012.