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. 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. 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 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.