Growing in the city: urban evolutionary ecology of avian growth rates.

Rapid environmental change driven by urbanisation offers a unique insight into the adaptive potential of wildlife as it can induce distinct selective pressures on urban dwelling organisms. Despite mounting evidence for urban-driven phenotypic differentiation across taxa, knowledge of the impact of urbanisation on vertebrate developmental rates and subsequent survival is very limited. Importantly, the role of selection on urban-driven body mass divergence in juvenile organisms remains poorly understood. We studied nestling development in a gradient of urbanisation set in Warsaw, Poland, in two nestbox breeding passerine species: great tits (Parus major) and blue tits (Cyanistes caeruleus). Each nestbox in the study system was assessed for surrounding percentage of Impervious Surface Area (ISA). Within these nestboxes, weight measurements of individual nestlings were collected during three breeding seasons and at regular intervals after hatching. Although asymptotic mass and growth rate were not directly affected by ISA in a subset of frequently measured nestlings, the age of fastest growth (inflection point) was delayed in blue tit nests surrounded by greater ISA. Across the entire dataset, nestling body mass was negatively affected by increasing ISA at days 5 and 10 after hatching for great tits, and at days 10 and 15 for blue tits, respectively. Concomitantly, offspring survival at days 5 and 10 decreased with increasing ISA for both species. An analysis of selection differentials performed for two contrasting levels of imperviousness (low and high ISA) revealed a significant positive association between mass at day 2 and survival at fledging. Importantly, the strength of selection for heavier nestlings at hatching was greater for great tits in the more transformed, high ISA environment. This study confirms the considerable impact of imperviousness -a proxy for urbanisation level- on offspring development, body mass and survival, and highlights increased selection on avian birth weight in a high ISA environment.