Description
Artificial light at night (ALAN) widely affects wildlife by blurring light-dark differences, including transitions such as sunrise and sunset, affecting the regulation of diel rhythms. As a result, activity onsets in many wild diurnal songbirds advance under ALAN. From chronobiological studies, it is known that the direction and strength of the response to light depends on when during the night exposure takes place. However, these experiments are mostly done under constant darkness conditions, when animals have free running rhythms. It remains unclear whether phase-dependence also holds in entrained, wild songbirds; i.e., does the effect of ALAN on activity patterns differ between exposure in the morning compared to the evening? This information is essential to assess the effects of mitigation measures by limiting ALAN to selected times of the night. We exposed incubating great tits inside the nest-box to four hours of light, of which one hour overlapped with night-time either before sunrise or after sunset. We found a small advancing effect of morning-light on activity onset and of evening-light on offset but not vice versa. While shifts occurred within individuals, effect sizes were small compared to the overall variation. Breeding success and chick condition were unaffected by the light treatments. However, light-treated females had lower weights 9-18 days after the end of the treatment compared to the controls, independent of whether ALAN occurred in the morning or the evening, indicating possible costs of ALAN. Despite the weak behavioural response, ALAN might have affected female’s circadian clock or physiology resulting in lower body condition.| Period | 15 Aug 2023 |
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