Pervasive low-frequency vocal modulation during territorial contests in Eurasian Scops Owls (Otus scops)

Abstract

In several animal species, including birds, individuals are known to produce low-frequency vocalizations during aggressive interactions with conspecifics. In this study, I investigated territorial interactions between male Eurasian Scops Owls that occupy territories in a densely-packed area. The single-note hoot of the Scops Owl is generally assumed to be highly repeatable, however extensive recording of male-male interactions identified previously unrecognized variation in the hoots structure. Male Scops Owls gave hoots at a frequency lower than usual when engaging in short-distance contests with neighbouring males. Within-subject analysis revealed that the callers average hoot frequency was positively correlated with the distance from its rival. During contests, males gradually reduced their hoot frequency as they approached one another, perhaps reflecting changes in the degree of escalation. Male Scops Owls reduced their hoot frequency immediately after the rival initiated countersinging, and returned to their usual frequency range at the end of the contest. These findings demonstrate that vocal modulation is pervasive in social contexts in the Scops Owl, and suggests that individuals have full voluntary control of the structure of their vocalizations. This study confirms in part the findings of other authors experimental work, where male owls adjusted their vocal frequency when challenged by an opponent. However, that study suggested that vocal frequency would encode information about the callers body weight. In contrast, the present study revealed context-dependent fluctuations in hoot frequency, which may suggest that the hoot of the Scops Owl dynamically reflects the current motivational state of the caller during the contest.