Great Backyard Bird Count participants document a massive winter roost

By Miyoko Chu, science editor, Cornell Lab of Ornithology 

During the 2007 Great Backyard Bird Count (GBBC), American Robins ranked as the most numerous species reported, for the first time ever. Participants counted more than two million robins—an astounding number, considering that the highest GBBC count for robins previously was 337,973.

Does this mean that robins suddenly multiplied and inundated North America this year? No! One city—Saint Petersburg, Florida—reported nearly 1.7 million robins on just 13 checklists. That’s about 82 percent of the robins tallied in all of the United States and Canada. Most of these robins belonged to a massive roost in a mangrove forest near Weedon Island Preserve in northeast Saint Petersburg.

During the GBBC weekend,  American Robins streamed across the sky like beautiful red pepper spots for as far as the eye could see, said Lorraine Margeson, who viewed the spectacle from her fourth-floor deck. Beginning in the late afternoon, the robins flew overhead steadily for more than two-and-a-half  hours, she said. “We could hear the robins’ laughing sounds all over the sky as they flew.”

The robins—along with hundreds of Cedar Waxwings and Brown-headed Cowbirds, and thousands of European Starlings, Red-winged Blackbirds, and Common Grackles—descended into a dense mangrove forest on a spit of land in the bay.

To count the birds, Margeson and her husband, Don, picked a transect in the sky and calculated the number of birds passing by per second. They extrapolated these numbers to cover the entire time when the robins were passing by. Their highest single-day total during the GBBC was 720,000 robins. Margeson said she believes these estimates were conservative.

The size of the roost varies each year,  according to Margeson. She estimated there were 42,000 robins at the peak last year. She speculated that snow and ice in the Northeast and an ice storm in Texas this year may have pushed robins into Florida, accounting for the huge numbers. Robins returned to the roost for a month or so before their numbers began dwindling as they moved on in search of food.

Margeson said she experienced the true magnitude of the roost when she went to the mangrove forest about 15 minutes before daybreak. “The noise is deafening, with as many as 10,000 Common Grackles calling at once. Then, just before the birds take off, they all go silent,” she said. Suddenly, with an incredible rush of wings, the grackles come out, she recalled. The other birds follow—starlings, Red-winged Blackbirds, cowbirds, waxwings.

“Then the robins just pour out of there,” Margeson said. “It takes at least 15 minutes to empty the mangroves of robins. It’s spectacular with the sunrise on their red bellies. When you see it, you think this is what makes life worth living. It’s amazing.”

American Robins are the most widespread and abundant thrush species in North America. In autumn, they gather into flocks and migrate to the southern states. They roam in search of berries during winter, their movements varying depending on the availability of food. GBBC data have shown that robins avoid areas with heavy snow cover. Weather and food supply can determine which areas the robins may inundate in any given year.

This year, participants in Washington state also reported an influx of robins, with flocks as large as 37,000 reported in Yakima.

GBBC participants this year tallied robins on 22,722 checklists in 60 states and provinces, creating a continentwide picture of where the robins were. As the Margeson’s efforts show,  every checklist helps record the variation in concentration of robins across the landscape. How unusual is the Saint

Petersburg roost? Where will next year’s phenomenal flocks show up? By reporting your sightings of robins to the Great Backyard Bird Count, and to eBird year-round, you can help reveal the dynamic patterns of movement and abundance of robins through time.

To report your bird sightings to eBird at any time, visit www.ebird.org.