How were Audubons scientists able to determine that many bird species are showing significant northward and inland movement?
A. Researchers pooled thousands of Christmas Bird Count observations and used them to calculate the center of abundance for each bird species in the study. (In simplified terms, half the individuals of a species are found south of the center of abundance, and the other half are found to the north.) They then measured the direction and distance of any changes in these centers of abundance over the last 40 years. The resulting numbers are approximations, but taken together, they give striking evidence that our wintering birds are indeed shifting their ranges toward the north and farther inland.
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