Starlings can categorize symmetry differences in dot displays

Fluctuating asymmetry is an estimate of developmental stability and, in som e cases, the asymmetry of morphological traits can reflect aspects of indiv idual fitness. As asymmetry can be a marker for fitness, it has been propos ed that organisms could use morphological asymmetry as a direct visual cue during inter- and intraspecific encounters. Despite some experimental evide nce to support this prediction, the perceptual abilities of animals to dete ct and respond to symmetry differences have been largely overlooked. Studyi ng the ability of animals to perceive symmetry and factors that affect this ability are crucial to assessing whether fluctuating asymmetry could be us ed as a visual cue in nature. In this study, we investigated the ability of wild-caught European starlings Sturnus vulgaris to learn to discriminate s ymmetry from asymmetry in random dot patterns through operant learning expe riments. The birds did not possess a spontaneous preference for either symm etry or asymmetry. The birds learned a symmetry preference, although the le arning process took longer than that previously reported for pigeons Columb a livia and was more error prone. After being trained to discriminate symme try differences in random dot patterns, birds successfully transferred thei r symmetry discrimination abilities to a set of novel stimuli that they had not previously seen. This indicates that starlings can form a mental categ orization of visual stimuli on the basis of a somewhat generalized symmetry phenomenon. We discuss these findings in relation to the probability that birds use fluctuating asymmetry as a visual cue.