Does dim city-like light at night change birds' melatonin levels and make them active earlier?

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Artificial light at night is known to affect a broad range of behaviors and physiological processes. In urbanized bird species, it advances biological rhythms, including daily activity/rest cycles and the timing of reproduction, but the underlying physiological mechanisms remain poorly understood. Because melatonin acts as a biological timing signal, it is a strong candidate for mediating these effects.

In a controlled experiment, researchers exposed urban and rural European blackbirds (Turdus merula) to two light treatments with the same photoperiod but different nighttime light intensities. The control group experienced 0.0001 lux, almost darkness, while the experimental group experienced 0.3 lux at night, simulating conditions previously recorded for free-living urban blackbirds. For all birds, researchers measured plasma melatonin profiles across the daily cycle in summer (July) and winter (January), while simultaneously recording locomotor activity.

Nighttime light clearly affected daily melatonin concentration patterns in both seasons. In winter, birds exposed to nighttime light had lower melatonin concentrations in the early and late night than birds kept in darkness. In summer, their melatonin concentrations were lower throughout the night than those of birds kept in darkness.

Birds exposed to nighttime light had higher overall locomotor activity than controls during both the day and the night, with a sharp increase before dawn. In winter, pre-dawn activity in the nighttime-light group correlated with the change in melatonin from midnight to late night: the greater the decrease in melatonin, the greater the pre-dawn activity. Urban and rural birds showed similar melatonin responses to nighttime light, but their behavior differed: rural birds showed more locomotor activity than urban birds.

The study points to reduced nighttime melatonin release as a potential physiological mechanism underlying the earlier onset of morning activity in urbanized birds. Based on the melatonin secretion pattern, the researchers suggest that birds exposed to nighttime light responded as though they were experiencing a longer day than birds kept under dark nights.