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Abstract and Summary

Previous studies that have looked at the aversive properties of simple, predator-related, eye-like patterns presented in an artificial context, have yielded conflicting results. The aim of the present series of experiments was to investigate whether such stimuli had potential for use as bird scarers. The starling (Sturnus vulgaris) was the subject species. The experimental procedure was designed to provide a relevant and objective, interval scale of aversiveness based upon the ability of eye patterns to deter hungry birds from a feeding area.

Each subject was deprived of food for 5 h and then tested in an apparatus which exposed an eye pattern over a food trough immediately after the bird alighted on that trough. 10 measures of the starlings' feeding behaviour were monitored over a 1-h trial. It was predicted that five of these variables would be positively and five negatively correlated with an increase in the fear evoking properties of the stimulus. The 10 measures were analysed using principal component analysis and the first component in every experiment had the signs of the latent vectors of the variables exactly in the predicted pattern and therefore appeared to be an ‘aversiveness index’. The scores from this first component were then used in a Latin square ANOVA to distinguish subject, test order and stimulus effects.

The main findings from the eight experiments are as follows. Simple eye-like patterns can indeed deter hungry starlings from feeding in their vicinity. The presence of a pair of eyes painted on a white card caused an 88% reduction in time spent on the nearby food trough and a 65% reduction in actual feeding time. The presence of ‘pupils’ in the patterns is essential for simple circles alone are not significantly more aversive than the control card. Once a pattern has the ‘pupil/iris’ distinction then a circular outline becomes important although the shape of the ‘pupil’ appears relatively unimportant. Eyes with coloured irises are more effective than black and white patterns but the degree of contrast between the ‘pupil’ and ‘iris’ appears irrelevant. Increasing the number of simultaneously presented eyes from one to three is correlated with an increasing trend in aversiveness which is destroyed if the same stimuli are presented within the context of a simple head outline. Changing the orientation of a pair of eyes from horizontal to vertical only slightly reduces their aversiveness. The size of the patterns appears to be unimportant within the range tested. The aversive properties of eyespots and of broadcast starling distress calls are positively additive. The effectiveness of a stimulus combining all the important features mentioned is as aversive as is a realistic model of the head of an owl.