Reproduction and survival in the city: which fitness components drive urban colonization in a reed-nesting waterbird?

Abstract

Processes of adaptation to urban environments are well described for relatively few avian taxa, mainly passerines, but selective forces responsible for urban colonization in ecologically different groups of birds remain mostly unrecognized. The aim of this article is to identify drivers of recent urban colonization (Łódź, central Poland) by a reed-nesting waterbird, the Eurasian coot Fulica atra. Urban colonizers were found to adopt a distinct reproductive strategy by maximizing the number of offspring (carryover effects of higher clutch size), whereas suburban individuals invested more in the quality of the progeny (higher egg volume), which could reflect differences in predatory pressure between 2 habitats. In fact, reduced predation rate was strongly suggested by elevated hatching success in highly urbanized areas, where probability of hatching at least 1 chick was higher by 30% than in suburban natural-like habitats. Coots nesting in highly urbanized landscape had considerably higher annual reproductive success in comparison to suburban pairs, and the difference was 4-fold between the most and least urbanized areas. There was also a constant increase in size-adjusted body mass and hemoglobin concentration of breeding coots from the suburbs to the city centre. Urban colonization yielded no survival benefits for adult birds and urban individuals showed higher site fidelity than suburban conspecifics. The results suggest that the recent urban colonization by Eurasian coots was primary driven by considerable reproductive benefits which may be primarily attributed to: (1) reduced predation resulting from an exclusion of most native predators from highly urbanized zones; (2) increased condition of urban-dwelling birds resulting from enhanced food availability.