Communication of male quality in owl hoots.

The evolution of communication through intrasexual selection is expected to lead signalers to transmit honest information on their fighting ability. Here we studied the information encoded in the acoustic structure of the territorial calls of a nocturnal raptor. During territorial contests, male scops owls give hoots composed of a downward frequency shift followed by a stable plateau. We found that the frequency of the hoot was negatively correlated with the body weight of the vocalizer. We shifted the frequency contour of natural hoots in order to create re-synthesized calls corresponding to individuals of varying body weight, and used these stimuli in playback experiments simulating an intrusion into the territory of established breeders. Territory owners responded less intensely when played hoots simulating heavier intruders, and males with heavier apparent weight tended to give hoots with a lower plateau in response to playbacks simulating heavier intruders.