After Dark

Blackbirds exposed to low levels of light at night undergo reproductive changes earlier than birds kept in the dark, researchers report in Proceedings of the Royal Society B. The results suggest that artificial light from cities may affect birds’ breeding cycles.

Previous studies have suggested that city-dwelling birds start breeding earlier, but researchers didn’t know whether this trend was due to differences in food supply, temperature, artificial light, or some other factor. Experiments have shown that night-time light levels affect reproductive patterns in some birds, but the light intensity used was “likely to be far above the intensity birds are exposed to in urban areas,” the team writes.

To get a more realistic idea of urban light exposure, the researchers attached light-detecting devices to 15 blackbirds from Munich or a nearby forest. The forest-dwelling birds’ average light exposure at night was only 0.00006 lux, while the urban birds’ was 0.2 lux. (The lux is a unit of illumination; a street lamp’s intensity is about 6 lux.)

Next, the team caught 40 male blackbirds from the same areas and monitored their responses to different light levels in the lab. Some birds were kept in near-darkness at night. Others were exposed to 0.3 lux, similar to the exposure seen in the previously-tracked city birds.

The birds exposed to more light at night became reproductively active 26 days earlier than the birds kept in the dark, the team reports. And city birds were even earlier than forest birds, beating them by about two weeks. The light-exposed birds also moulted 22 days earlier and sang earlier during part of their reproductive cycle.

It’s not clear whether earlier breeding will help the birds or not. The birds might have time to produce more chicks, but females might not be ready to mate so soon. Roberta Kwok | 13 February 2013

Source: Dominoni, D., M. Quetting, and J. Partecke. 2013. Artificial light at night advances avian reproductive physiology. Proceedings of the Royal Society B doi: 10.1098/rspb.2012.3017.

Image © Simon Bratt |