| Summary: | Nocturnal species may communicate by visual signals more frequently than previously thought. In fact, such species are habitually active around sunset and sunrise, when light conditions are still suitable for visual communication. We investigated the communication function of a visual cue in the eagle owl Bubo bubo, a nocturnal predator. In this species, territorial and courtship displays peak during the sunset and sunrise periods and involve the display of a white badge located on the throat whose reflectance properties are sex and period dependent. Experimental intrusions were conducted at 30 eagle owl territories in order to understand the function of the white badge during contests. We analyzed the reactions of both male and female owners toward a taxidermic mount with a normal brightness and a brightness-reduced white badge, with both male and female territorial calls. Our results indicate that the white badge of eagle owls plays an important role in visual communication during contests. Males displayed more frequently toward male low-brightness mounts, which were also approached more closely or attacked. Female behavior did not differ between experimental groups. Furthermore, a positive relationship between male badge brightness and breeding output suggested a potential role of the white badge as an honest signal of male quality. The need to convey information by visual communication in a nocturnal species may have promoted the evolution of visual signals employed at crepuscule. © The Author 2006. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved.
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