Morphological differences along a chronological gradient of urbanisation in an endemic insectivorous bird of New Zealand

Urbanisation constitutes one of the most rapid human-induced environmental changes, developing at the expense of natural and semi-natural habitats. It often implies alterations of many abiotic and biotic factors and contributes to create new environmental conditions, including temperature, food resources, competition and predation. Despite increasing empirical evidence of intra-specific divergence in phenotypic traits (e.g., physiological, behavioural or morphological) between urban and rural individuals, such patterns have often remained disconnected from the underlying mechanisms involved. In the current study, we tested for divergence in functional morphological traits that are related to feeding ecology (i.e., bill morphology, body mass and condition) and/or to the locomotory performance in escaping from predators (i.e., wing, tarsus and tail morphology, body mass and condition) along a chronological gradient of urbanisation (old urban, recent urban and rural areas), using the New Zealand fantail, an endemic insectivorous passerine species. We found divergences in phenotypic traits related to bill morphology along the urban–rural gradient: birds inhabiting the old urban area had stubbier bills (i.e., shorter, deeper and wider bills) than those inhabiting the recent urban and rural areas. We did not detect any difference in locomotion-related morphological traits. Our results suggest the urbanisation-induced alteration in food resources may drive morphological divergence in bird populations. We emphasized the need for mechanistic and experimental studies, with a particular focus on resource-based mechanisms, to identify more precisely the morphological responses of urban populations to changes in food composition, and the resulting implications for communities in urban ecosystems.