Effects of bill pigmentation and UV reflectance during territory establishment in blackbirds

During intrasexual interactions, the visual properties of conspicuous male sexually selected traits may be important in social signalling. We used intruder models to examine the effect of pigmentation and ultraviolet (UV) reflectance of a single male trait on the responses of male blackbirds, Turdus merula. Stuffed models of male blackbirds with brown, yellow or orange bills were presented to residents in their natural habitat; we altered UV reflectance from the bills by applying nail varnish. This UV reflectance did not appear to affect the response of resident males during simulated territorial intrusions under natural signal and viewing conditions. However, models with brown bills received less attention from resident males than models displaying yellow or orange pigmentation. Brown bills are typical of first-year males and may be an effective signal of subordinate status, reducing aggression from adult males. Furthermore, resident males came closer, and initiated aggressive displays more quickly, in the presence of orange- versus yellow-billed models, suggesting that orange-billed models may be perceived as more of a threat to territory ownership. In 44% of presentations, females also responded to models (a response rate half that of males). Females spent more time more than 10 m from models when they lacked UV, suggesting that UV may be more important during intersexual interactions. However, bill pigmentation did not affect female responses to the models. Males and females may be attentive to different properties of bill appearance when establishing the territory. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.