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Birds and Snow
Does snow depth affect the distribution and abundance of birds? The primary factor influencing the winter distribution of many bird species is food availability. For example, the periodic and sometimes massive irruptions of winter finches, such as the Common Redpoll, into southern Canada and the United States are often attributed to a lack of food in their typical wintering grounds. But factors such as snow cover may also influence bird distributions in winter. We looked more closely into the relationship
between bird distribution and snow cover using as an example one of
the most common and easy-to-identify birds of North America, the American
Robin. Although most robins migrate south from northern latitudes in
the fall, some will overwinter on or near their breeding grounds. During
the winter of 1999, there were many reports of large numbers of American
Robins throughout the Great Lakes region and the Northeast. The abundance
of robins was so unusual that television news shows and newspapers ran
numerous stories about their commonness. To attempt to determine why
so many robins were being reported so far north, we used data collected
during the Great Backyard Bird Count (GBBC), a North American bird census
that occurs in late February and generates a snapshot of late-winter
bird distributions.
This map depicts the distribution of American Robins across North America, based on more than 10,000 individual observations of robins collected during the great Backyard Bird Count (GBBC) 1999 (February 19–22). Note the high number of robins reported (dark areas on the map) in the Great Lakes east through New England. FIGURE 2
This map depicts the distribution of American Robins across North America, based on more than 12,000 individual observations of robins collected during the great Backyard Bird Count (GBBC) 2000 (February 18–21). Note the low number of robins reported in the Great Lakes east through New England. Based on these maps, we wondered if weather conditions, such as snow cover, influenced the occurrence of robins. Snow depths greater than six inches were found in the northern Great Lakes and extended east near the Canadian border, which is at or above a 45-degree latitude. During GBBC 2000, we asked participants to measure snow depth, and we received more than 54,000 reports of snow depth across North America. Snow depth was greater than or equal to six inches across Minnesota, Wisconsin, Michigan, New York, and southern New England, or at latitudes between 40 and 45 degrees—the same regions where robin numbers were higher during GBBC 1999. To compare data from the 1999 and 2000 GBBC, we divided the eastern half of North America into five-degree-latitude by five-degree- longitude blocks and calculated the median number of American Robins for each block. We calculated the difference between years in the medians for each block; the difference in snow depth was calculated in the same manner. We found far fewer American Robins and much higher snow depths during GBBC 2000 between 40 and 50 degrees of latitude compared to GBBC 1999. We conducted the same analysis for Downy Woodpecker, a species that is primarily nonmigratory, and we found no difference in its distribution between the two years. Thus, food availability may be the primary factor influencing some species’ winter ranges, but snow cover may also play a role. Results from the GBBC and other continent-wide monitoring projects show that American Robins overwinter across North America in a patchy mosaic, primarily reflecting their opportunity to forage on fruits and berries. When snow cover is high and food is difficult to find, American Robins move farther south. When snow cover is low and food is more readily available, they seem to overwinter in northern locales in higher numbers. |
