The northward and inland movement of North American birds, confirmed by thousands of citizen-observations, has provided new and powerful evidence that climate change is having a serious impact on natural systems, according to a new report by Audubon (BirdLife in the USA). The findings signal the need for dramatic policy changes to combat pervasive ecological disruption.
Analyses of citizen-gathered data from the past 40 years of Audubon’s Christmas Bird Count reveal that 58% of the 305 widespread species that winter on the continent have shifted significantly north since 1968, some by hundreds of kilometres. Movement was detected among species of every type, including more than 70% of highly adaptable forest and feeder birds. Only 38% of grassland species mirrored the trend, reflecting the constraints of their severely-depleted habitat and suggesting that they now face a double threat from the combined stresses of habitat loss and climate adaptation.
Population shifts among individual species are common and can have many causes. However, Audubon scientists say the ongoing trend of movement by some 177 species—closely correlated to long-term winter temperature increases—reveals an undeniable link to the changing climate.
"Birds are showing us how the heavy hand of humanity is tipping the balance of nature and causing ecological disruption in ways we are just beginning to predict and comprehend", said report co-author and Audubon Director of Bird Conservation, Dr Greg Butcher. "Common sense dictates that we act now to curb the causes and impacts of global warming to the extent we can, and shape our policies to better cope with the disruptions we cannot avoid."
Movements across all species averaged approximately 56 kilometres during the period. However, it is the complete picture of widespread movement and the failure of some species to move at all that illustrates the potential for problems.
- Purple Finch Carpodacus purpureus, Pine Siskin Carduelis pinus, and Boreal Chickadee Parus hudsonicus have retreated dramatically north into the Canadian Boreal, their ranges moving an estimated 500, 395, and 340 kilometres respectively over 40 years.
- Red-breasted Merganser Mergus serrator, Ring-necked Duck Aythya collaris, and American Black Duck Anas rubripes, normally found in southern-tier states, have all taken advantage of warmer winter waters and have shifted their ranges north by an estimated 393, 272, and 226 kilometres.
- Only 10 of 26 grassland species moved north significantly, while nine moved south. Species such as Eastern Meadowlark Sturnella magna, Vesper Sparrow Pooecetes gramineus, and Burrowing Owl Athene cunicularia were likely unable to move despite more moderate northern temperatures because essential grassland habitat areas have disappeared, having been converted to intensive human uses such as row crops, pastures, and hayfields. In combination, global warming and ongoing overuse of grasslands by humans will doom grassland birds to continued population declines.
"Birds provide some of the best evidence for species's responses to climate change, through their population size, distribution and timing of breeding and migration", said Dr Stuart Butchart, BirdLife's Global Research and Indicators Coordinator. "There is now plenty of evidence that bird species are shifting their ranges northwards and to higher altitudes, and that their timing of breeding and migration is shifting forwards in response to climate change."
Audubon anticipates that the new avian evidence will help attract attention and spark action among more than 40 million U.S. bird-watchers, including tens of thousands who contributed to the Christmas Bird Count data on which the studies are based. The 109-year-old census provides the world’s longest uninterrupted record of bird population trends. "Citizen Science is allowing us to better recognize the impacts that global warming is having here and now. Only citizen action can help us reduce them", said Butcher.
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