Trying to estimate how many birds migrate through North America twice each year seems like an impossible task. However, Cornell University’s Laboratory of Ornithology has taken on this task. Using data from 143 weather stations scattered across the contiguous United States the lab has come up with the best estimates to date.
These data indicate that an average of 4 billion birds fly south out of Canada into the United States each fall. Some 4.7 billion leave the country heading on to their wintering grounds.
Far fewer birds return in the spring. According to Edward W. Rose, a postdoctoral fellow at the Laboratory of Ornithology, “in the spring, 3.5 billion birds cross back into the U.S. from points south, and 2.6 billion birds return to Canada across the northern U.S. border.”