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

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 examination. 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 individuals manipulated human food waste and gleaned from sealed surfaces more than exurban conspecifics (suggesting behavioural 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 exurban 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 appeared 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 eucalypts), size and isolation were similar in urban and exurban environments, but urban nests were significantly more concealed. We suggest that ‘preadaptation’, behavioural innovation and a relative lack of significant, interspecific food competition have contributed to urban colonization by Little ravens.

Keywords:

Foraging competition, group size, innovative feeding, recreational boating, nest site characteristics, ‘pre-adaptation’, urban colonization.