Extreme hot weather has stronger impacts on avian reproduction in forests than in cities

Abstract

Extreme weather events are rare, but can have high impact on human societies and biological systems. As the frequency of extreme events are increasing with current climate change, it is important to understand its effects on fitness of individuals and on long-term viability of populations. In this study, we investigated the effects of extreme high ambient temperatures on breeding success of great tits in two urban and two natural forest populations during six years. We found that the number of hot days had habitat-specific effects on nestlings during their development period (from hatching to 15-days age). Average body mass of broods decreased with increasing number of hot days in both forest populations and one city population. However, the negative effect of hot days on chick mass was significantly stronger in forest compared to urban populations, suggesting that forest populations are more vulnerable to extreme hot weather conditions. Unlike other study populations, average chick mass increased with the number of hot days in the hottest urban study site, suggesting adaptation in heat tolerance in this population. Tarsus length of chicks and their survival until fledging was not influenced significantly by hot weather. Hot weather conditions may affect the reproductive output of birds at least two ways: indirectly through food availability and directly through nestling physiology. As the tarsus size and survival of chicks were not affected by hot days in our study, we suggest that the negative effect of hot days on body mass may emerge more likely from the direct physiological effects of heat. These results are one of the first demonstrations that the effects of extreme weather events can differ between wild populations living in different environment.

Urban nestlings have reduced number of feathers in Great Tits (Parus major)

“…This might especially hold for altricial bird species such as Great Tits, whose nestlings are exposed to the prevailing ambient temperatures in the nest and do not have established competent thermoregulation in their early life, so might be sensitive to the harmful effects of the UHI during summer heat days (Rodr ıguez & Barba 2016, Andreasson et al 2018). As Pipoly et al (2020) showed, heat days are indeed significantly more frequent at our urban than forest sites during the chick-rearing period: in 2013-2018 (6 years), 45.9% of the urban broods experienced at least one heat day (range: 1-13 heat days per brood), whereas this rate was only 20.8% in the forest sites (range: 1-5 heat days per brood). High temperatures can be especially problematic for hole-nesting species breeding in artificial nestboxes, as the temperature of the cavities and nestboxes can be up to several degrees higher than the ambient temperature (Maziarz et al 2017).…”

Anthropogenic food availability and body mass maintenance in urban Red-winged Starlings Onychognathus morio

“…Day status was marginally non-significant (p = 0.075), built-up index and sex were not significant et al 2019). However, other studies have shown the opposite effect during extreme weather events, with Great Tits Parus major in natural environments experiencing a decrease in nestling mass, because of heat stress (Pipoly et al 2020) and with urban Peregrine Falcons Falco peregrinus nesting in boxes having higher breeding success especially under cold and wet breeding conditions (Sumasgutner et al 2020). Finally, some anthropogenic structures might only exist temporarily and are removed before the end of the breeding cycle, which can again create an ecological trap situation for breeding birds (Reynolds et al 2019).…”

Differences in feather structure between urban and forest great tits: constraint or adaptation?

“…This food limitation is also reflected in nestlings' food composition (Sinkovics et al 2021) and has a strong negative impact on nestling development and survival (Seress et al 2020). In addition, the temperature and the number of heat days in our urban areas are also significantly higher than in the forest sites (Pipoly et al 2021). Thus, we expect that – similarly to many other traits – plumage characteristics should differ between urban and non‐urban populations, due to one or more of the reasons detailed above.…”

“…This results in a higher risk of heat stress (Ward et al 2016) that can affect nestling development (Pipoly et al 2013) and increase mortality (McKechnie and Wolf 2010). Although rapid adaptive changes in responses to the UHI have already been described in urban populations (in morphology: Kaiser et al 2016, Brans et al 2017, Merckx et al 2018; behaviour: Johnson et al 2020, Zhao et al 2021; or physiology: Diamond et al 2017, Campbell‐Staton et al 2020), such studies in birds are very rare (Pipoly et al 2021). Changes in environmental temperature may affect the structural properties of the plumage (e.g.…”