Title: Limits to length asymmetry detection in starlings: implications for biological signalling
Abstract: Fluctuating asymmetry has received considerable recent attention in evolutionary biology as these small developmental asymmetries can be related to biological fitness and, hence, could be used as a visual cue (or signal) of quality among individuals. The ability of signal receivers to detect and respond to small asymmetries is a fundamental assumption of the symmetry–signalling hypothesis, but has not been experimentally investigated. In this study I have investigated the perceptual threshold to detect and respond to paired–bar length asymmetry in a common bird, the European starling Sturnus vulgaris, by means of operant–learning techniques. The threshold indicates how large the length asymmetry must be to be reliably discriminated from symmetry; birds could not detect an asymmetry of 1.25%. In nature, many asymmetries can be smaller than 1.25%, hence this initial study suggests that caution should be used when trying to invoke symmetry–signalling in natural populations.