Robins sing in the glow of city lights

By Elizabeth Quill

American Robins’ robust songs welcome day across the country. So when biologist Mark Miller heard the familiar chorus at his home in Schuylkill Haven, Penn., one night, he was surprised.

“It took me a while to realize this was unusual,” Miller said. “Wait a minute ... It is around one o’clock in the morning. Why are the robins singing?”

After hearing the same sounds a few nights in a row, Miller decided to go for a walk. He often works late into the night, and this time it was 3 a.m. But he saw robins everywhere – active and singing as if morning had broken. What could cause this odd, but consistent, behavior?

Miller suspected light pollution and decided to conduct a study. Though astronomers have studied the effect of increased light levels on star visibility, few studies have analyzed the effect on wildlife ecology.

The results, published in The Condor (Feb. 2006) supported Miller’s hypothesis. He found that human-created light radiating from populous cities and spacious suburbs not only hides the stars, but also appears to initiate bird song. In fact, the robins he studied started singing up to three hours earlier than expected.

The robin made an easy and convenient study subject because it’s common in both urban and rural habitats. Miller collected data in Pennsylvania, Virginia, Maryland and the District of Columbia and compared them with data collected in the early- to mid-20th century, when artificial light levels would have been lower.

Of course, there is always some variation in light levels from one night to another, depending on cloud cover and the moon’s phase, even when no artificial light exists. But artificial light levels were more than double levels of the brightest naturally lit night, and 20 times brighter on some nights. In these bright areas, robin song began between 30 minutes and three hours earlier than it had in the 20th century. The brightest sample area was the White House, where robins sang before 2:20 a.m. on all 10 sampled days. And those robins that started singing early didn’t quit. They still sang through twilight.

Sunlight obviously initiates song, but now artificial light from billboards, street lamps and buildings could play a larger role. Astronomers consider light pollution a growing problem and during the 1990s societies developed across the country to “protect the night sky.”

However, though early song may wake humans from their slumber, the effect on robin ecology are unknown. The robins could be exposing themselves to increased predation risks.

“An owl would have to be deaf, dumb and blind not to realize there was a robin there,” Miller said.

On the other hand, increased activity and foraging time could increase robin productivity. With only a few exceptions, robin populations are increasing or stable nationwide. And the species thrives in suburban parks and gardens. Robins may actually benefit from urbanization and agricultural development - lights and all.