Artificial light speeds sexual maturity in blackbirds

Blackbird singing with mouth open

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A study has found that urban nocturnal light makes European blackbirds develop reproductive systems a month before their woodland counterparts.

Birds exposed to higher light levels developed functional testes on average 26 days earlier than control birds.

Further results suggested that urbanisation may alter the physiological traits of songbirds.

Scientists say the findings highlight the consequences of artificial lighting on the ecology of urban animals.

The findings are published in the journal Proceedings of the Royal Society B.

Common blackbird

Davide Dominoni, from the Max Planck Institute for Ornithology in Germany and part of the research team told BBC Nature: "We knew that birds in urban areas sing earlier in the morning and breed earlier in season than conspecifics living in rural habitats."

"There was evidence that urban blackbirds showed not only advanced egg-laying date, but the whole reproductive physiology was turned on approximately three weeks before than in rural birds."

The scientists, from the Max Planck Institute and Konstanz University in Germany, measured the nocturnal light levels that eight "free-roaming" European blackbirds (Turdus merula) were exposed to in the field using miniature light loggers.

Light loggers are programmable, miniature devices, weighing 3g, which are able to record and store the light intensity over a long period of time.

Mr Dominoni explained: "We released the blackbirds equipped with light loggers and then we recaptured them after approximately 2 weeks.

"We used only the data recorded during the night, excluding the twilight values."

Into the lab

Researchers used data from the light loggers to simulate the nocturnal light environment in a controlled experiment in captivity.

Start Quote

Our data show that light at night can strongly modify one of the most crucial life-history traits, reproduction.”

End Quote Davide Dominoni Max Planck Institute for Ornithology, Germany

The scientists exposed birds captured from urban and woodland areas nearby to two different light treatments.

"Birds were exposed to either dark night conditions or to a night-time light level of 0.3 lux, a value representative of what we measured with the light loggers," said Mr Dominoni.

Birds were monitored for an entire breeding season.

Every month the scientists measured the size of the testes, the testosterone level in the blood and the singing behaviour in the early morning.

The study revealed that not only do birds exposed to light at night develop reproductive system up to one month earlier, but also that they moult earlier and have an earlier onset of the dawn song.

Mr Dominoni commented on the findings: "Although the effect we found was big, it is not fully clear what are the costs and benefits of being ready to breed during the early breeding season.

"Supposedly, birds which start to breed earlier could gain the best territories and eventually also increase the number of clutches per season, therefore increasing reproductive output, too."

Wider implications

"Our results hint at a very strong role played by artificial light in urban habitats to shape the physiological phenotypes of songbirds," said Mr Dominoni.

Baby blackbird in nest Nocturnal light can make blackbirds lay eggs earlier than rural birds

"[They are] much stronger than other cues like temperature and food availability, which are more likely to affect ultimate reproductive decisions such as egg-laying. On the contrary, the reproductive physiology seems to be under complete control of light."

The results have wider implications regarding the impact of human-induced lighting on the ecology for millions of urban animals and the scientists called for an understanding of the fitness consequences of light pollution.

According to Mr Dominoni: "Our data show that light at night can strongly modify one of the most crucial life-history traits, reproduction.

"However, this might come at a physiological cost because reproduction requires a great energetically investment which is traded-off with other physiological processes like immune-defence.

"Birds might particularly suffer from decrease immuno-competence in the winter when conditions are not optimal."

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