Light at night reduces melatonin levels in urban birds

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 Urban birds wake earlier due to the suppression of melatonin production finds new research published in Frontiers in Zoology today. This suggests that birds living in urban environments could become desynchronized with the light-dark cycle which affects daily behaviour as well as annual breeding cycles.

It is known that urban birds are affected by artificial light at night, and have been observed to begin daily activity earlier and breed sooner in the year when living in urban environments. However, little has previously been known about the physiological mechanisms underlying this unusual behaviour.  

Working on a hunch that the hormone melatonin, which is often associated with daily biological rhythms, may be involved, researchers investigated the effects of light levels of 0.3 lux – which is similar to light levels experienced at night in urban environments. The team recorded profiles covering 24 hour periods of plasma melatonin of birds exposed to either almost darkness or urban-like light levels, as well as recording behavioural activity of the study birds. They found that melatonin levels are greatly reduced in birds exposed to light at night (melatonin production is suppressed by light), and this corresponds with an earlier start of daily activity.

The authors suggest that the reduced melatonin production has been interpreted by the birds as a longer day length, which explains their earlier morning activity. This can also have serious implications for the birds breeding season, as longer day length is a trigger to signal the start of the breeding season. “Our findings may have important implications for understanding the control of seasonal processes, such as reproduction, in urbanized birds,” said Davide Dominoni of Max Planck Institute for Ornithology and lead author of this paper.