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Climate change is likely to have both direct and indirect effects on birds. Higher temperatures can directly alter their life cycles. The loss of wetlands, beaches, and other habitat could have an equally important indirect effect, by making some regions less hospitable to birds than those regions are today.
As temperatures warm, birds will tend to inhabit more northerly areas (in the Northern Hemisphere). Data collected by the National Audubon Society’s Christmas Bird Count show that during years with warmer temperatures, the majority of bird species do not have to fly as far south for the winter. Warmer temperatures also allow birds to spend their summers farther north. A 1997 study examined the impact of warmer summers on the Bobolink (a North American songbird). During summer, this bird is currently found throughout New England, the states that border the Great Lakes, and north of a line stretching from Missouri to Idaho. With the projected
climate changes under a doubling of carbon dioxide, the Bobolink would not be found south of the Great Lakes.
Warmer temperatures can also affect how birds respond to the change in seasons. Several types of birds that fly north to Michigan during spring now arrive two or three weeks earlier than in 1960. Scientists at the British Trust for Ornithology have found that 20 of 65 species of birds are laying their eggs an average of 9 days earlier today than in 1971. The earlier nesting appears to result in part because plants are flowering and growing leaves sooner, which in turn causes earlier availability of the insects that these birds eat.
Scientists do not know whether birds will benefit from these changes. Earlier nesting means that birds will be a week or so older when the time come to migrate south, which may improve their odds of survive their first winters. The changing climate, however, may impair the extent to which a bird’s life cycle is synchronized with its food supply. While birds can adjust to warmer temperatures by flying to more northern areas in any given year, the vegetation upon which they (or the insects they eat) rely may take decades or longer to adjust (see forests).
In some cases, the habitat upon which birds rely may not only fail to migrate north, it may be threatened in its current location. The loss of estuarine beaches caused by rising sea level would decrease available habitat for the least tern,  an endangered species; the loss of these beaches also would decrease feeding areas for shore birds that rely on horseshoe crabs and other organisms found in inter-tidal areas By decreasing estuarine fish and shellfish
populations, the loss of coastal wetlands would decrease available food supplies; and the loss of wetlands would also decrease available habitat. The loss of Louisiana’s wetlands could have a particularly adverse impact on international migratory birds that travel along the Mississippi flyway.
Similarly, the decline in prairie potholes would decrease duck populations. The prairie potholes in the northern Great Plains are responsible for breeding 50-80% of the nation’s duck population. A drier climate would decrease the amount of open water ponds in this region, with a commensurate reduction in duck populations. Scientists at the University of Michigan and Ducks Unlimited have estimated that a 1°C (2°F) warming would decrease duck populations by about 25 percent if rainfall were to remain constant. An 8 percent increase in precipitation would be required to offset the adverse effect of a 1°C (2°F) warming. The study also showed that a 15 percent increase in precipitation along with a 1°C warming would increase duck populations by 25 percent. As with many impacts of regional climate change, uncertainty about whether climate will be wetter or drier prevents scientists from knowing whether duck populations will increase or decrease. Nevertheless, most climate models suggest that rainfall in the Great Plains region will increase by less -- possibly far less -- than 8 percent for every degree by which temperatures warm. As a result, duck populations in this region seem likely to decline eventually.
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