|Factors Influencing Where American Robins Overwinter|
During this past winter, many observers in the Northeast and Great Lakes regions reported American Robins in unusually high numbers. While it is not unusual for some American Robins to overwinter in these regions, the numbers in some areas did seem extraordinary.
Why were so many American Robins being reported? Food availability probably is the major factor influencing the number of American Robins that overwinter in an area. Snow cover may play an important role in that higher concentrations of snow and ice limit the availability of the fruits and berries that American Robins depend on during winter and early spring. During January and February, 1999, there was very little snow cover across most of the United States. The map below shows the snow cover across North America in late February. The map indicates that only the extreme northern portions of the United States and the higher elevations in the west had significant accumulations of snow.
BirdSource received over 10,000 reports of American Robin during the Great Backyard Bird Count held Februay 19-22, 1999. These reports provide an excellent snapshot of late winter American Robin distributions across North America.
The map above depicting American Robins during the 1999 Great Backyard Bird Count indicate that they were most common in the southeast through Texas, and in the North Pacific. By comparing the map of American Robin distributions with the snow cover map, it becomes apparent that in the northwest and in the western Great Lakes, American Robin observations decrease sharply where there is snow cover. Below is an enlarged map of the western Great Lakes. The dark line indicates the general demarcation between snow depths greater than 5 inches and areas with less than 5 inches. The red dots represent reports of American Robins, while the gray dots represent Great Backyard Bird Count reports that did not contain American Robins. There were only 3 reports of American Robin in Wisconsin, Minnesota, and Michigan where snow cover exceeded 5 inches, while in areas where snow cover was less than 5 inches there were many more American Robins-- including some areas with flocks of up to 1200 individuals. Note the influence of metropolitan areas such as Minneapolis, St. Paul, Madison and Detroit, on American Robin distributions. The ornamental fruit trees (e.g. hawthorns, mountain ash, etc) planted in those cities provide vast amounts of food for overwintering American Robins.
Is the occurrence of American Robins in such high numbers this year especially unusual? Yes and no. American Robins are opportunistic in where they overwinter. During years when fruits and berries are available and easy to acquire, many American Robins will remain in more northerly locations than in years when food is not as plentiful, or is more difficult to acquire. This behavior allows those birds that successfully overwintered in the north to return to their breeding grounds earlier than those that overwinter farther south. Those that arrive on the breeding ground earlier are able to select the most optimal breeding territories. The last time that large numbers of American Robins overwintered in the Northeast and Western Great Lakes was during the winter of 1994-1995 (Field Notes vol.49 no. 2). Even as recently as last winter, American Robins did overwinter in New England in large numbers, particularly in areas where there was little snow cover (Field Notes vol. 52 no. 2).
In conclusion, American Robins overwinter across North America in a patchy mosaic that primarily reflects their opportunity to forage on fruits and berries. When snow cover is high, making food 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.