Beak colouration of starling (Sturnus unicolor) males depends on the length of their throat feathers

Abstract

Within the context of complex sexual signalling, most research has focussed on exploring the associations between several signals and/or their relationships with different proxies of individual quality. However, very few studies have focused on checking whether the expression of one signal is conditioned by the expression of the others. Here, by experimentally shortening the throat feathers of male spotless starlings (Sturnus unicolor), we evaluated the influence of this trait on the colour expression of the bea19 base. In addition, we tested the relationship between these two sexually dimorphic characters with traits indicating individual quality such as body condition and colour reflectance at the wavelength related to carotenes in the tip of the beak. Our results show that the colouration of the beak base in males, but not in females, is positively related to body condition and to the length of ornamental throat feathers. Moreover, the experimental shortening of throat feathers in males had a negative effect on the blue chroma intensity of their beak base one year after manipulation. These results support for the first time a causal link between the expression of two sexually dimorphic characters, which is essential to understand their functionality in a multiple signalling framework