Wednesday, March 28, 2012

Warmer Winters Reflected in Bird Counts, Range Shifts

When bird watchers joined this year’s Great Backyard Bird Count (GBBC), they recorded the most unusual winter for birds in the count’s 15-year history. With 17.4 million bird observations on 104,000 checklists, this was the most detailed four-day snapshot ever recorded for birdlife in the U.S. and Canada. Participants reported 623 species, during February 17–20, including an influx of Snowy Owls from the arctic, early-migrating Sandhill Cranes, and Belted Kingfishers in northern areas that might normally be frozen over.

“The maps on the GBBC website this year are absolutely stunning,” said John Fitzpatrick, executive director of the Cornell Lab of Ornithology. “Every bird species has a captivating story to tell, and we’re certainly seeing many of them in larger numbers farther north than usual, no doubt because of this winter’s record-breaking mild conditions.”

Ironically, a few arctic species also moved farther south than usual as well. Participants recorded Snowy Owl sightings in record-breaking numbers throughout the Great Plains and Pacific Northwest of the United States. Canadian bird watchers saw four times the number of Snowy Owls they reported to the count last year. Experts believe that Snowy Owls move south from their usual arctic habitats in years when prey, such as lemmings, are scarce.

Some northern locations recorded high numbers of waterbirds such as Mallards, Ring-necked Ducks, Hooded Mergansers, and American Coots, that either never left or came back early to lakes, rivers, and ponds that remained unfrozen.

As warmer winter temperatures become more common, one way for some animals to adjust is to shift their ranges northward. But a new study of 59 North American bird species released this week indicates that doing so is not easy or quick — it took about 35 years for many birds to move far enough north for winter temperatures to match where they historically lived.

“This is a problem, because birds are among the most mobile of animals and yet they take decades to respond to warming,” said Frank La Sorte, a postdoctoral researcher at the Cornell Lab and lead author of the study, which was published online by the Journal of Animal Ecology this month. “Climatic conditions are steadily moving northward, whether particular animals come along or not. As conservation biologists we need to know how well animals are keeping up.”

Earlier studies of responses to climate change examined shifts in species’ geographic ranges. “Our work adds important realism and a temporal dimension to these models for a critical aspect of climate: minimum winter temperature,” said coauthor Walter Jetz of Yale University, where the research was conducted.

The researchers used 35 years of data from the North American Christmas Bird Count to match winter temperatures to where birds were seen. They tested 59 bird species individually and found that they responded differently to climate change. When summarized across bird species, there was evidence for a strong delay lasting about 35 years.

For example, Black Vultures have spread northward in the last 35 years and now winter as far north as Massachusetts, where minimum winter temperature is similar to what it was in Baltimore, Maryland, in 1975. On the other hand, the endangered Red-cockaded Woodpecker did not alter its range at all despite the warming trend, possibly because its very specific habitat requirements precluded a range shift.

Both of these scenarios could represent problems for birds, La Sorte said. Species that do not track changes in climate may wind up at the limits of their physiological tolerance, or they may lose important habitat qualities, such as favored food types, as those species pass them by. But they also can’t move their ranges too fast if the habitat conditions they depend on also tend to lag behind climate.

“When you think about it, it makes sense that species move slower than the rate at which climate is changing,” La Sorte said. “They’re not just tracking temperature—many of them need to follow a prey base, a type of vegetation, or they need certain kinds of habitat that will create corridors for movement.”

Variability in climate warming is likely to affect how species respond, too, La Sorte said. If warming trends weaken, as they did over the past few years, birds may be able to catch up. But accelerated warming, which is likely as global carbon emissions continue to increase, may put additional strain on birds. The study highlights these challenges and the high potential climate change has for disrupting natural systems. It also underscores the challenges ecologists face in predicting the long-term consequences of climate change for many species simultaneously.

Top 10 birds reported in New York in the 2012 Great Backyard Bird Count:

Canada Goose
Snow Goose
European Starling
Ring-billed Gull
American Crow
American Goldfinch
House Sparrow
Redhead
Mourning Dove
Brant

Photo: Snowy Owl by Ian Davies (used with permission).

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One Response

  1. Pete Klein says:

    Don’t expect a repeat next winter.

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