Nesting near a streetlight brightens the chances of philandering for young male birds.
Among birds called blue tits in a forest on the outskirts of Vienna, a yearling male often fathers at least one illicit chick if he nests within 50 meters of a lamppost, says Bart Kempenaers of the Max Planck Institute for Ornithology in Seewiesen, Germany.
In dark nests in the woods, though, roving females generally overlook youngsters in favor of older males, Kempenaers and his colleagues report in the Oct. 12 Current Biology.
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The young males’ strange success may come from a quirk linked to light pollution, the researchers suggest. Males near night lighting start singing on average about three minutes earlier during the dawn chorus of bird song than naturally lit birds do, Kempenaers and his colleagues found.
That’s not a long time, but a female taking a quick break from her usual mate often flits over to visit a neighbor for only a matter of minutes. Kempenaers’ previous work has shown that early serenading has extra appeal for wandering females.
“Birds are very sensitive to light cues,” says Michael Murphy of Portland State University in Oregon. His own studies of eastern kingbirds in North America found that the large, early-singing males were especially successful in fathering chicks with other males’ mates.
From an otherwise unattractive male’s point of view, streetlights must be great. But Kempenaers says he doesn’t have data on the consequences for the blue tit population as a whole if artificial light inspires many females to mate with males that they would normally shun.
Unlike noise or noxious chemicals, light gets ignored because it’s “a gentle form of pollution,” he says. But he predicts that scientists will find its effects on reproduction to be widespread among species.
Streetlights affect blue tit females too, researchers found. In nests near lights, females started to lay eggs on average 1.5 days earlier than females deeper in the woods.
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A growing body of research documents that artificial light is a disruptive pollutant for wildlife, says biogeographer Travis Longcore, science director of The Urban Wildlands Group in Los Angeles. The new paper “deals with things that aren’t as obvious as dead bodies on the ground but are no less profound,” he says.
Kempenaers noticed the light effects “a bit by accident,” he says, based on seven years of data involving a total of several hundred nesting pairs. Long interested in avian promiscuity, he mapped the territories of males who fathered chicks with other birds’ mates. Streetlight guys appeared to be disproportionately prolific.
Setting out automatic recorders at the study site, researchers found that blue tits — as well as three out of four other species — sang earlier in nests near artificial light. European robins started about an hour earlier.
Blue tits nesting on a forest edge without artificial lighting didn’t have unusual reproduction, so edges alone don’t explain the results, Kempenaers says. Neither does noise, he argues. The neighborhood is quiet, and the biggest advances in dawn singing came from species that start long before people stir.