Personality is independent of morphological differences in common waxbills

Abstract

Personality differences may coexist because of their different benefits or, alternatively, because some personality types result from limitations of low individual quality while others require high physical or cognitive abilities. Recent work in a great tit, Parus major, population found that proactive personality is associated with larger head sizes, suggesting that it requires better cognitive abilities. This was never tested in other avian species, and other morphological correlates of personality have also seldom been studied. We used an open-field and a mirror test as proxies to assess personality differences in a large sample of wild common waxbills, Estrilda astrild, and tested whether behavioural differences are predicted by morphological traits that can relate to genetic quality or early life growth (body size), brain volume (head size), growth during moult (wing and tail length) or body condition (size-corrected mass). Exploration in open-field tests was related to size-corrected mass and to wing and tail size, perhaps because birds in better condition or with larger wings can move more or faster. These associations were very weak, with effects sizes close to zero, and, importantly, morphology did not predict behaviour in mirror tests. In waxbills, behaviour in mirror tests correlates with exploration in open-field tests, and provides a better assay of reactive-to-proactive personalities because it is repeatable in the long term. Our results thus indicate that avian personality can be independent of differences in quality between individuals.