Behavioural and Ecological Keys to Urban Colonization by Little Ravens (Corvus mellori)

Abstract

Avian urban colonization is thought to be facilitated by a capacity for innovative feeding, ecological generalism and social foraging. However, the relative importance in exploiting urban resources and avoiding urban predators of being inherently 'pre-adapted' to the urban environment or adjusting to it through phenotypic plasticity requires more examina- tion. These issues were explored in a native 'urban adapter', the Little raven Corvus mellori, by comparing its foraging ecology, group size and nest site use in Melbourne, Australia, and the surrounding exurban environment. Urban individu- als manipulated human food waste and gleaned from sealed surfaces more than exurban conspecifics (suggesting behav- ioural flexibility), but foraging behaviour and substrate use were broadly similar in both environments (suggesting 'preadaptation'). Little ravens foraged close to conspecifics and heterospecifics more frequently in the urban than the ex- urban environment, but some potential dietary competitors rarely foraged near urban Little ravens, possibly indicating some niche partitioning. Mean urban rate of agonistic interaction with other bird species was low (0.023 interactions per foraging raven observed). Although displacement of a raven >10 m occurred in 61-70% of such interactions, the displaced individual usually rapidly resumed foraging nearby. Thus aggressive, interspecific interference competition for food ap- peared limited. Large groups of Little ravens were twice as common in the exurban as the urban environment, which was inconsistent with the hypothesis that social foraging facilitated urban colonization. Nest tree type (predominantly euca- lypts), size and isolation were similar in urban and exurban environments, but urban nests were significantly more con- cealed. We suggest that 'preadaptation', behavioural innovation and a relative lack of significant, interspecific food com- petition have contributed to urban colonization by Little ravens.