Biology lettersSep 6, 2023

Light at night may explain why urban and forest great tits start their daily activity at different times

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 31, 2026

Abstract

Female great tits in urban areas exhibited an earlier onset of activity compared to forest females, with experimental artificial light at night (ALAN) further advancing this timing.

Urban female great tits started their activity earlier than their forest counterparts.
Forest females exposed to experimental ALAN showed similar early activity patterns as urban females.
ALAN exposure increased restlessness at night in forest females, unlike urban females.
The findings suggest a link between ALAN and altered biological rhythms in great tits.

Simplified

Artificial light at night (ALAN) is rapidly increasing and so is scientific interest in its ecological and evolutionary consequences. In wild species, ALAN can modify and disrupt biological rhythms. However, experimental proof of such effects of ALAN in the wild is still scarce. Here, we compared diel rhythms of incubation behaviour, inferred from temperature sensors, of female great tits () breeding in urban and forest sites. In parallel, we simulated ALAN by mounting LED lights (1.8 lx) inside forest nest-boxes, to determine the potentially causal role of ALAN affecting diel patterns of incubation. Urban females had an earlier onset of activity compared to forest females. Experimentally ALAN-exposed forest females were similar to urban females in their advanced onset of activity, compared to unexposed forest birds. However, forest females exposed to experimental ALAN, but not urban females, were more restless at night than forest control females. Our findings demonstrate that ALAN can explain the early activity timing in incubating urban great tits, but its effects on sleep disturbance in the forest are not reflected in urban females. Consequently, future research needs to address potential effects of ALAN-induced timing on individual health, fitness and population dynamics, in particular in populations that were not previously affected by light pollution. Parus major

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Light at night may explain why urban and forest great tits start their daily activity at different times

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