ALBANY, N.Y. — Nearly 2 million robins ruled the roost in the 2010 Great Backyard Bird Count, outnumbering all other species in the 13th annual tally of North American birds reported by 63,000 volunteer bird watchers.
The large number of American robins was mainly because of a massive roost in St. Petersburg, Fla., where birders reported 1.4 million during the four-day event in February.
“They sometimes gather in a large roost like that before they start migrating,” Pat Leonard of the Cornell Lab of Ornithology said Tuesday. “There’s a big mangrove swamp in St. Petersburg that they like.”
The count also showed a dramatic increase in tree swallows and a decrease in winter finches such as redpolls, pine siskins and evening grosbeaks, which were unusually abundant in northern states in the previous two winters.
The count, sponsored by the Cornell Lab of Ornithology, National Audubon Society and Bird Studies Canada, helps document changes in the number and distribution of birds over time. Changes may reflect variations in food supply, reproduction, habitat loss, and other factors.
“We don’t want to overstate what the data show,” Leonard said. “You can’t draw conclusions from one count. The value is in looking at such a large area, an entire continent, and collecting long-term data.”
The 13-year-old Backyard Bird Count is relatively new compared to Audubon’s 110-year-old Christmas Bird Count and is more loosely structured, with volunteers recording birds they see in their yards over a four-day span. The Christmas count, from Dec. 14-Jan. 5 each year, takes a census under strict methodology in designated areas.
Both can help scientists spot trends that may warrant a closer look.
The introduction of the Eurasian collared dove from the Caribbean to Florida and its subsequent expansion across North America is illustrated in bird count data. In 1999, about 1,000 doves were reported in nine states. This year, more than 14,000 doves were found in 39 states.
Winter finch numbers can vary dramatically from year to year because of changes in food supply or reproductive success in the far northern part of their range. Annual fluctuations in the number of northern gulls along the Pacific Coast are less understood. The numbers were down markedly this winter in California, Oregon and Washington state.
Rare bird sightings recorded in this year’s backyard count included a black-legged kittiwake in Lake Havasu City, Ariz.; a crimson-collared grosbeak in McAllen, Texas; and a red-billed tropicbird along the San Diego coast.
In addition to robins, the most numerous birds were Canada geese, snow geese, American crows, and European starlings.
“GBBC data become more and more valuable with each passing year,” said Dick Cannings of Bird Studies Canada. “Over time we’ll be better able to see significant changes that may occur in the numbers and distributions of birds which may be tied to climate change, habitat loss, disease, or other factors.”