Climate change and urbanisation are among the most salient human-induced changes affecting
Earth's 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 may adapt to tolerate 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 760 broods from two urban and two forest populations over 6 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 nestlings are less vulnerable to extreme
hot weather conditions than their non-urban conspecifics. This difference might be
the result of adaptations that facilitate heat dissipation, including smaller body
size, altered plumage and reduced brood size. Alternatively or additionally, parental
provisioning and food availability may be less affected by heat in urban areas. Our
findings suggest that adaptation to heat stress may help birds cope with the joint
challenges of climate change and urbanisation.
