Reproductive Differences Between Urban and Forest Birds Across the Years: Importance of Environmental and Weather Parameters

Urban environments pose many challenges to wildlife, not least for insectivorous passerines. Numerous studies have reported on the negative effects of urbanization on reproduction in these species. However, depending on the taxa and cities studied, no particular or positive effects have been reported. This may be related to the different levels of urban environmental stressors. As unfavorable weather can have deleterious effects on birds (e.g. lower prey availability and higher costs of pollutants), annual variations in the differences observed between sites could be related to synergetic effects between the urban environment and weather conditions. In this context, we studied the reproduction of great tit (Parus major) at two sites (urban and forest) over four years. First, we quantified pollution, prey availability, and vegetal cover at each site to characterize each environment. Second, we measured the effects of site and weather conditions on tit reproductive success to determine if the influence of weather is higher in the city. Except for the fledging rate, reproductive parameters were lower in the city than in the forest whatever the year probably because of poorer food availability and a predominance of non-vegetated areas in cities. The fledging rate and the nestling number in the urban environment were positively correlated to temperatures during rearing whereas there was no significant correlations in the forest. These results support the hypothesis of additive effects of urban constraints and weather that limit bird productivity in cities.