A test of receiver perceptual performance: European starlings' ability to detect asymmetry in a naturalistic trait

There has been substantial interest in whether birds use small degrees of asymmetry (fluctuating asymmetry, FA) in visual communication. However, there is a scarcity of experimental evidence for the visual role of FA. Hence, there is still much debate as to whether FA could be a visual cue. We address this issue by exploring whether European starlings, Sturnus vulgaris, can perceive small asymmetries in digital representations of a visual communication trait: white plumage spots on dark throat and chest feathers. Through a series of operant learning trials, we trained starlings to discriminate symmetry from an initially large asymmetry (50% relative asymmetry in the position and number of dots) and then reduced the asymmetry through subsequent learning and unreinforced test trials. Six of seven birds could reliably detect a 25% asymmetry and one bird could detect a 15% asymmetry. There was no evidence for discrimination of a 10% asymmetry. Therefore, we propose that starlings express a limit for detection of asymmetry in this complex structured trait between 10 and 15% relative asymmetry. We discuss this limit in light of natural plumage asymmetries and conclude that most individuals in a wild population would probably be perceived as equally ‘symmetric’, rendering FA in such a trait an unlikely cue in visual communication. We also discuss the commonalities between this apparent limit to asymmetry detection and other reports of perception in European starlings and pigeons, Columba livia, and suggest that our findings could be applied cautiously to other avian systems.