Growing in cities: an urban penalty for wild birds? a study of phenotypic differences between urban and rural great tit chicks (Parus major).
Abstract
Urban sprawl is associated with deep and intense modifications of the natural habitats of
wild vertebrates. Although, many species are unable to cope with such an environment, a
few species can be found in cities and can help us assessing the impact of urbanization
on wildlife. Urban-related environmental modifications are multiple and some of them
seem beneficial while others seem rather detrimental to wild vertebrates. Moreover, the
impact of these modifications on wild vertebrates is likely to vary depending on the phase
of the annual life-cycle. Therefore, it is challenging to get a comprehensive picture of the
impact of urbanization on wild vertebrates. Overall, urbanization is usually associated
with reduced breeding performances in wild birds, but the impact of urbanization
on the phenotype and quality of developing offspring has been less studied. In this
study, we specifically investigated the impact of urbanization on several proxies of
individual quality in great tits (Parusmajor).We concomitantlymeasured body size (tarsus
length and body mass), plumage coloration, and telomere length in 14-days old chicks
issued from 4 populations (two pairs of urban/rural populations located in two different
geographical areas of France). First, rural chicks were significantly taller and heavier
than urban birds although this impact of urbanization on body size/body mass appears
only true for the most urbanized site. Interestingly, body size was also affected by the
geographical area of capture, suggesting that regional environmental conditions may
attenuate or exacerbate the influence of urbanization on nestling growth. Second, the
carotenoid-based yellow plumage of rural nestlings was more colorful than that of urban
birds, independently of the area of capture. This suggests that urban birds probably have
a low-carotenoid diet relative to rural birds. Finally, telomere length did not differ between
rural and urban chicks. These results suggest that urbanization probably imposes large
developmental constraints in wild vertebrates and that this impact may primarily be
related to constraining nutritional conditions.