The authors have analyzed several hundreds of hoots recorded from 17 territorial males and demonstrated that the pitch of the vocalization reflects the body weight of the male: the heavier the male, the lower the pitch of his hoots. In order to see whether this information is actually used by male owls during territorial interactions, the authors conducted a series of playback experiments (commonly used in studies of animal communication in order to assess the function of vocal signals), monitoring the reaction of subjects to the broadcast of vocalizations.
To do this they modified the pitch of several hoots, creating stimuli that mimicked the hoots given by males from a range of body weights. They then played back these recordings to males with established territories, and observed and quantified their response (a combination of approaches and vocal responses). The results show that male owls respond more strongly to the high-pitched calls that simulate lighter individuals, confirming that territorial males attend to pitch information advertising body weight in the calls of their competitors. The authors also found that the territory owners give slightly lower pitched hoots in response to calls mimicking heavier males, probably indicating that they attempted to sound heavier when challenged by more threatening individuals.
"The fact that owls are essentially active during the night puts a strong emphasis on acoustic communication as a means of assessment, both during male competition and during mate choice," says Loïc Hardouin who recently completed a PhD on acoustic communication and territoriality in owls. "The next step is to see whether females use these quality cues when they choose their mating partner."
You can find the abstract here.