The United States and Canada just basked in an unusually mild winter. Temperatures ranked fourth warmest on record, according to the National Oceanic and Atmospheric Administration, and many spring flowers are already blooming. But did the birds notice? Definitely, according to the Great Backyard Bird Count (GBBC), an annual tally of bird sightings collected by amateur birders across the United States and Canada. The numbers reveal that the snowy owl population in particular boomed and that many other birds showed up in more northerly latitudes than usual.
GBBC, now in its 15th year, is a joint effort by the Cornell Laboratory of Ornithology in Ithaca, New York, and the National Audubon Society, headquartered in New York City. This year, birders, who were instructed to identify and record whatever birds they happened to see in their yards and neighborhoods between 17 and 20 February, tallied 17.4 million individual sightings. Pat Leonard, GBBC's director of communications, says that it's unclear how many individuals took part because each observer can submit more than one sighting checklist, but he estimates that between 65,000 and 70,000 volunteers participated.
Ornithologists working with GBBC analyzed the data and found a number of unexpected trends. One of the biggest surprises, says Marshall Iliff, an ornithologist at the Cornell lab who co-authored the report and leads a smaller, year-long project similar to GBBC called eBird, was an explosion in sightings of the snowy owl (Bubo scandiacus). In November, reports began trickling in to eBird that the snowy owl, which primarily lives north of the Arctic Circle, was showing up in unexpectedly large numbers in the United States and southern Canada, and GBBC's tally backs that up. Observers reported 428 sightings of the owl, which is four times the number from the same time last year. "This snowy owl thing is pretty surprising," Iliff says.
So what's driving the bird's southern influx? One possibility, Iliff says, might be that the owl's primary prey source, lemmings, could be sparser than normal in Arctic latitudes, which would lead more owls to seek food farther south. But that's unlikely because lemming populations have actually being doing quite well this year, writes Robert Ostrowski in an e-mail. Ostrowski is an experienced birdwatcher, a member of the Maryland Ornithological Society's records committee, and isn't involved with GBBC. "One possible explanation," he says, "is that the snowy owls had such an ample amount of food available that their reproduction success was higher than normal, causing an over-population of birds that were eventually forced south by competing individuals."
Other birds, such as the American coot (Fulica Americana), the pied-billed grebe (Podilymbus podiceps), and the belted kingfishers (Megaceryle alcyon), showed up in more northerly locations than ornithologists expect in a normal year. The mild winter meant that many areas that usually freeze over had more hospitable climates for these birds, leading some to shorten their southern migrations, Iliff says. Some may never have left northerly climes this year. As Earth's climate warms, these balmy years could become the norm, he says, meaning last winter's bird patterns could be a preview of the new normal.
Because GBBC observers are amateurs, ornithologists do have to be careful about interpreting the data, Iliff says. But because so many people participate, they excel at pointing out spikes and dips in established patterns that professional ornithologists can then verify with their own observations. "It's a big, messy data set with lots of differing skills among observers, but it's good for identifying outliers," he says.
Ostrowski agrees. "The beauty of the GBBC is in the high levels of participation," he says. "The GBBC produces an amazing amount of data that professionals would be hard-pressed to match. ... The high number of data points is capable of absorbing the comparatively small number of misidentifications which inevitably make it into the final report."
Jennie MacFarland, a lifelong birder and biologist at the Phoenix-based Arizona Important Bird Areas Program who lives in Tucson, participated in GBBC for the first time this year. In her count, she was most surprised that one of her favorite birds, the cedar waxwing (Bombycilla cedrorum), ventured farther into urban areas than usual—it's typically found only outside city limits. "It was really neat," she says. "I hadn't seen them all winter, but then I saw nine or 10 waxwings in the middle of the city."
GBBC is great for getting nonexperts interested in birds and the role they play in local and global ecology, MacFarland says. "It does yield great data, but it also gets lots of people looking into and learning about the natural world."