Q. I’ve noticed a lot more robins staying in my area during the winter in recent years. Why do some stay and some fly south?
A. Food availability is probably the major factor influencing the number of American robins that overwinter in an area, said Steve Kelling, a developer of BirdSource, a joint project of the Cornell Lab of Ornithology and the National Audubon Society. Heavy snow and ice may also play an important role, limiting the fruits and berries that the robins depend on during winter and early spring, said Mr. Kelling, now director of information science at the lab.
For example, a comparison of maps of snow-covered areas in January and February 1999 with maps showing the reports of robins during the Great Backyard Bird Count of Feb. 19-22, 1999, clearly showed a sharp decrease in reports in areas where there was snow cover.
A similar comparison for the western Great Lakes region found only three reports of robins in Wisconsin, Minnesota and Michigan where snow cover exceeded five inches, while some areas with less cover had many more robins, even flocks of up to 1,200.
Interestingly, in metropolitan areas like Minneapolis-St. Paul and Detroit, many robins did overwinter that year, because ornamental fruit trees like hawthorns and mountain ash provided a generous food supply.