Bird Ranges Move, but Is It Climate Change?
DALLAS -- Alice Hale remembers the day she first saw a northern cardinal, an unmistakable creature famed in bird watching circles for its brilliant scarlet plumage.
"I saw a northern cardinal in a tree at the end of my driveway about a dozen years ago and then they started coming to my bird feeders . They've been around the yard since then," the retired educator and long-time resident of Dartmouth, Nova Scotia said.
But according to Hale's 1980 edition of the "Peterson Field Guides: Eastern Birds," Nova Scotia should not have been a common part of the bird's range.
More recent field guides -- including the "Kaufman Field Guide to Birds of North America," (2000) and the "National Geographic Field Guide to the Birds of North America" (2006) -- have the cardinal firmly established in Nova Scotia.
Is this a tell-tale sign of climate change linked to greenhouse gas emissions, with the cardinal moving north in response to a warming planet?
Or are other factors at play, such as the proliferation of bird feeders in urban yards, which may be enticing species away from their historic ranges?
Such debates are bound to heat up with a draft U.N. report on climate change set for release in Paris on Friday.
The northern cardinal is one of several birds whose range has changed significantly enough in a short period of time to be caught in updated editions of field guides, which use maps to show the distribution of species.
Annual counts conducted by birding enthusiasts are also tracking species further from the equator.
The Audubon Christmas Bird Count, for example, has highlighted several species of birds which are now wintering further north than the 1980 Peterson guide shows.
These include the northern cardinal, American robin, Eastern bluebird, tufted titmouse and red-bellied woodpecker.
"These are all birds that hang out in suburban areas, so it may be that their ability to take advantage of what humans offer contributes as much to their ability to winter farther north as warmer temperatures do," said Greg Butcher, Director of Bird Conservation at the National Audubon Society.
HABITAT CHANGE OR CLIMATE CHANGE?
Similar debates are going on around the world.
Phil Hockey of the Cape Town based Percy FitzPatrick Institute of African Ornithology said several factors seemed to account for changing bird ranges.
He added that when it came to using field guides for comparisons it was best to use those from the 1960s or 70s when maps became more accurate with groups of people recording birds in specific grid squares.
"In the last 40 years in South Africa we have had 16 bird species invade the Cape peninsula - most are associated with wetlands or forest," he told Reuters by telephone.
"They have expanded their range westwards because of the invasion of alien trees, the planting of commercial forests or they have leapfrogged from one artificial water body to another. We can explain all these shifts without reference to climate change by looking at man-made habitats," he said.
Still, Hockey -- who has co-authored a number of bird field guides himself -- said climate change could not be ruled out in some cases.
He also said that climate-induced range changes would likely be more gradual than those sparked by habitat transformation or the availability of feed in urban areas.
A widely cited 2002 study in the journal Nature found that global warming was already affecting plant and animal life across the planet.
Among other shifts, it noted that the ranges of 12 bird species in Britain had moved northwards over a 20-year period.
Back in North America, it is instructive to look at National Geographic's 1983 field guide to birds of the region and its most recent edition, published late last year.
The northern cardinal's move into Nova Scotia and a northward thrust by the turkey vulture into southern Maine and Nova Scotia from Massachusetts are some of the shifts which can clearly be seen by looking at maps from the two editions.
The movements are there over short periods of time, suggesting that human induced changes in the environment are the cause. And they are not dramatic, which could point to gradual shifts from rising temperatures.
But the explanation, at least in the cardinal's case, may simply lie in the bird feeders found in Hale's garden and those of her neighbors.