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City and Forest Great Tits Respond Differently to Artificial Light at Night

Wild Great Tit at Fairburn Ings nature reserve, July 2025
Research: Behaviour, Physiology

Artificial light at night can affect when animals are active. But do birds from cities and forests respond in the same way? Researchers compared great tits (Parus major) from the two habitats in a common-garden experiment, keeping the birds under the same controlled conditions and varying whether they experienced light at night.

During the breeding season, birds were kept in pairs for three months under either artificial light at night or dark nights. The following year, the same pairs took part again under the opposite treatment. Under artificial light, daily activity began earlier in both groups. Over time, this advance faded in city birds but persisted in forest birds.

The researchers also examined the birds’ circadian clocks—the internal timing systems that help organize daily activity. They measured when activity began under constant dim light and how light pulses shifted activity, using these responses to assess clock properties. The difference between city and forest birds in their behavioural response to night-time light was not reflected in the clock properties measured.

Because both groups were kept under the same conditions, the contrasting changes in activity suggest a difference associated with their city or forest origins. Yet the measured properties of the internal circadian clock did not differ in a way that explained their responses. The authors interpret this as a population difference in the behavioural response to night-time light, rather than in the clock properties measured. This may reflect inherited divergence, but the experiment does not identify its biological basis. The study is a bioRxiv preprint and has not yet been peer reviewed.

Sources and image credit

  1. City and forest great tits respond differently to artificial light at night: insights from a common garden studyPrimary source; bioRxiv preprint.

Image: Wild Great Tit in Fairburn Ings, England. Photo: Lloyd Tudor / Wikimedia Commons, CC BY-SA 4.0. Representative bird, not part of the light experiment. · CC BY-SA 4.0 · Source

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