Why are the birds in the United States spending the winters farther north? Well, a new study by the Audubon Society has found the answer of the question. According to the study, it is "global warming" that is making the U. S. birds spend the winters farther north.

The Audubon Society study, released today, has found more that fifty percent of 305 birds species of North America, including robins, gulls, chickadees and owls, are now spending the winter about 35 miles farther north than they did 40 years ago.

The study analyzed the data collected over the past 40 years and found that 177 bird species in the U. S. are spending the winter farther north because of a warming world.

The purple finch, according to the study, is the main sufferer of the rising temperature. It is now spending its winters more than 400 miles farther north. In winter, the bird moves to the latitude of Milwaukee, Wisconsin, instead of Springfield, Missouri.

According to the study, the average January temperature in the United States rose about 5 degrees Fahrenheit over the 40 years. Terry Root, a biologist at Stanford University, said that the study shows a very, very large fraction of the wintering birds are shifting" northward.

Greg Butcher, the lead scientist on the study and the director of bird conservation at the Audubon Society, said, "It is not what each of these individual birds did. It is the wide diversity of birds that suggests it has something to do with temperature, rather than ecology."