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With assistance from streetlights, the early bird catches the female's favor

Male blue tits nesting near streetlights sing earlier than their peers.
Male blue tits nesting near streetlights sing earlier than their peers. (Madaboutgreys)

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Monday, December 13, 2010; 6:22 PM

With a little assistance from streetlights, the early bird catches the female's favor

Light pollution may give certain male songbirds a reproductive edge, according to German scientists who have been studying small birds called blue tits for the past decade.

The scientists found that males living near roads illuminated by streetlights along the edges of a forest were more successful at mating with more than one female than those living in the heart of the forest.

Because the natural light-dark cycle affects behavior, researchers hypothesized that the streetlights advanced the start of dawn singing in males living near them. Females nesting deep in the forest would hear the early-rising crooners and leave their social partners for dawn trysts.

"The blue tit is our long-term model species for our work on the evolution of promiscuity," said Bart Kempenaers, the lead author of the study, which was published in Current Biology. Although blue tits are socially monogamous, males often have offspring with other females, Kempenaers added.

Researchers from the Max Planck Institute for Ornithology in Seewiesen, Germany, had already collected data on blue tits' mating habits in the area of the streetlight study and had used molecular genetics to determine paternal lineages. For this study, they measured lamp position and light intensity. They also put recorders in the trees to listen to dawn songs.

Male blue tits residing within about 50 yards of a light began singing an average of three minutes earlier than their forest peers, and they were twice as likely to sire offspring with females that were not their social partners. Although artificial lighting affected all blue tits, adult males were the most promiscuous, fathering the most offspring outside their social pairings.

Yearlings with lighted nests were almost as promiscuous as adults lilving in the forest, while forest yearlings had the least success with females.

- Leslie Tamura


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