The early bird supposedly catches the worm, but city-dwelling blackbirds (Turdus merula) may actually be rising too early before dawn. Night-time light pollution from artificial outdoor and indoor lighting has become so pervasive that it has actually upset their natural biological rhythms. Now their daily activity begins several hours earlier in the morning and the timing of their mating season has also advanced by up to a month. New research published in Frontiers in Zoology shows that the presence of light at night is actually triggering a physiological response in the birds by suppressing the amount of melatonin that they produce at night.
Melatonin is a hormone that naturally varies in a daily cycle and acts to regulate important functions such as sleep and body temperature. It is normally released at night and is suppressed by daylight, functioning as an internal biological clock for most animals. Because of this, light is a crucial cue for birds to control their daily and seasonal activities.
However, for blackbirds that live in cities, the bright nights are causing a reduction in their normal melatonin levels. This suppression appears to alter the birds’ perception of daylength, particularly in winter, so that they interpret the illuminated nights as being shorter than an actual dark night. As a result, blackbirds and other city birds are awakening earlier than their rural counterparts, while the amount of urban pre-dawn birdsong is also increasing.
But it’s not just songbirds that are fooled by artificial illumination. Light pollution is increasingly recognised as a serious problem that can lead to severe physiological and behavioural changes in everything from frogs and bats to moths and fish. Even humans are not immune to the detrimental effects of excess light on melatonin levels. Fortunately, light pollution is one problem that is easily solved. Simply switching off lights overnight, as is now required in France, is a huge step in the right direction, with the added benefit of massive savings on energy. So maybe we can choose to return to peaceful dark evenings once again.
Reference: Dominoni DM, Goymann W, Helm B, & Partecke J (2013). Urban-like night illumination reduces melatonin release in European blackbirds (Turdus merula): implications of city life for biological time-keeping of songbirds. Frontiers in Zoology, 10 (1) PMID: 24090446
Image: Jim Richardson