Clocks for the city: circadian differences between forest and city songbirds

Abstract

To keep pace with progressing urbanization organisms must cope with extensive habitat change. Anthropogenic light and noise have modified differences between day and night, and may thereby interfere with circadian clocks. Urbanized species, such as birds, are known to advance their activity to early morning and night hours. We hypothesized that such modified activity patterns are reflected by properties of the endogenous circadian clock. Using automatic radio-telemetry, we tested this idea by comparing activity patterns of free-living forest and city European blackbirds (Turdus merula). We then recaptured the same individuals and recorded their activity under constant conditions. City birds started their activity earlier and had faster but less robust circadian oscillation of locomotor activity than forest conspecifics. Circadian period length predicted start of activity in the field, and this relationship was mainly explained by fast-paced and early-rising city birds. Although based on only two populations, our findings point to links between city life, chronotype and circadian phenotype in songbirds, and potentially in other organisms that colonize urban habitats, and highlight that urban environments can significantly modify biologically important rhythms in wild organisms.

Early to rise, early to breed: a role for daily rhythms in seasonal reproduction

“…earlier chronotype) had earlier nest initiation dates than females that became active later in the morning. As previous findings have demonstrated that earlier chronotypes are manifestations of shorter circadian period lengths (Aschoff and Wever 1966;Duffy et al 2001;Dominoni et al 2013b), our findings of a relationship between early onset of activity and clutch initiation provide evidence that variation in endogenous circadian rhythms may act as a mechanism influencing individual variation in seasonal timing of breeding in wild populations. We observed consistent individual differences in daily onset of activity during the breeding season.…”

“…Captive great tits do show altered timing of sleep behaviors, including earlier awakening and nest departure times and shortened sleep duration, even when kept under naturalistic conditions compared to their free-living counterparts (Stuber et al 2015). A study in European blackbirds (Turdus merula) did find that chronotype measured during the breeding season under free-living conditions was related to the length of an individual's free-running rhythm measured in captivity later on in the same breeding season (Dominoni et al 2013b). While the study by Helm and Visser (2010) contained important findings, our study utilized measurements of rhythmic daily and seasonal behaviors in a natural environment.…”

“…Alternatively, we can measure the phenotypic output of circadian rhythms by measuring the chronotype, or the timing exhibited by an individual relative to a salient environmental cue (e.g. sunrise) and other population members (Lehmann et al 2012;Dominoni et al 2013b). Evidence in humans and birds has uncovered clear links between endogenous rhythms and chronotype; individuals with a shorter free-running circadian period exhibit an earlier chronotype and become active earlier in the day (Aschoff and Wever 1966;Duffy et al 2001;Dominoni et al 2013b;c.f.…”

Habitat urbanization and its effects on birds

“…A recent study on captive Eurasian Blackbird Turdus merula found that, when exposed to low light levels during nights, individuals started to moult and developed their reproductive system earlier compared to birds kept under dark night conditions, similar to forest nights (Dominoni et al 2013a). The underlying physiological mechanisms were investigated on urban-and city-dweller Blackbird kept under constant conditions (Dominoni et al 2013b). It turned out that forest and urban birds differed both in their chronotype and circadian clock, as the urban birds had longer daily activity (i.e.…”

Stressful colours: corticosterone concentrations in a free-living songbird vary with the spectral composition of experimental illumination

“…The repercussions of these behavioural and physiological changes in natural systems remain largely unknown and constitute a new and relevant focus for ecological research [6]. To illustrate, we are unaware of studies that look at the effects of experimental nocturnal light on stress physiology in free-living animals (see [7] for reproductive physiology), even though understanding these physiological effects is essential for developing measures to reduce potential impacts.…”