A team of animal biologists from the University of Glasgow and the Max Planck Institute for Orntihology fitted small devices known as light loggers to the backs of 100 blackbirds in Munich, Germany and a nearby forest.
The light loggers tracked the amount of light birds in both environments were exposed to each day and transmitted the information to the researchers via attached radio transmitters.
In a new paper published in the journal Philosophical Transactions of the Royal Society B, the researchers report that the additional light from artificial sources in the cities caused the city birds to perceive the days to be longer by 49 minutes on average.
The perception of additional light had an effect on the city birds’ photoperiod, the light fraction of a 24-hour day which organisms use to time their daily and seasonal biology. As a result, the birds in early March experienced the day length that their country counterparts experienced in late March.
The city birds’ perception that they were living at a later point in the year caused them to reach sexual maturity in anticipation of breeding season 19 days earlier than the forest birds.
The research was led by Dr Davide Dominoni of the University’s Institute of Biodiversity, Animal Health and Comparative Medicine.
Dr Dominoni said: “Artificial light at night is one of the most obvious environmental changes affecting the habitat of wild animals. The global increase in light pollution poses new challenges to wild species, but we’re very much in the early stages of understanding the effects of exposure to light at night.
“Our research suggests that light at night is the most relevant change in ambient light affecting the biological rhythms of urban blackbirds, most likely by a modification of their perceived photoperiod.
“We’re keen to continue our research and learn more about the effect of human activity on animals’ biological clocks.”
University of Glasgow: ‘Light pollution causing blackbirds to breed earlier, scientists suggest‘
The paper, ‘Does light pollution alter daylength? A test using light loggers on free‐ranging European blackbirds (Turdus merula)’, is published in Philosophical Transactions of the Royal Society B and is available online.