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.