Effects of extreme hot weather on the reproductive output of great tits (Parus major, L.) in urban and natural habitats

Abstract

Climate change and urbanization are among the most salient human-induced changes affecting Earths biota. Extreme weather events can have high biological impacts and are becoming more frequent recently. In cities, the urban heat island can amplify the intensity and frequency of hot weather events. However, the joint effects of heat events and urban microclimate on wildlife are unclear, as urban populations may either suffer more from increased heat stress or become adapted to warmer temperatures. Here we test whether the effects of hot weather on reproductive success of great tits (Parus major) are exacerbated or dampened in urban environments compared to forest habitats. By studying two urban and two forest populations over six years, we show that 14-16 days-old nestlings have smaller body mass and tarsus length, and suffer increased mortality when they experience a higher number of hot days during the nestling period. The negative effects of hot weather on body mass and survival are significantly stronger in forests than in urban areas, where these effects are dampened or even reversed. These results suggest that urban birds are less vulnerable to extreme hot weather conditions than their non-urban conspecifics, possibly by adaptively evolving or flexibly adjusting nestling physiology to tolerate heat stress, and/or by adjusting parental behavior in response to heat. This finding highlights that endothermic vertebrates may be able to adapt to heat stress, which may help their populations cope with the joint challenges of climate change and urbanization. Significance statementExtreme weather events are becoming more frequent due to climate change and can have substantial effects on reproduction and survival of wild animals. Urban heat island can amplify the frequency of extreme hot weather events, making it potentially more harmful for city-dwelling organisms. Alternatively, urban populations living in warmer environments may adapt to better tolerate heat. We investigated these alternatives by comparing nestling development and survival between urban and forest great tit (Parus major) populations. We found that urban populations are less vulnerable to heat: nestling body mass and survival decreased rapidly with the increasing number of hot days in forests, while these effects were dampened in urban broods. Thus, urban populations may become adapted to better tolerate heat events.