Many people consider the American Robin the first harbinger of spring. Data from RRBO’s annual Winter Bird Population Survey (WBPS) certainly prove that if one were to try to pinpoint the beginning of spring by the presence of robins, the date could occur any month of the year!
As the first graph below indicates, robins are present every winter. The bars represent the average number of robins counted per survey (visit) each year through winter 2011-2012. Overall, an average of 348 robins are counted each winter, or an average of 25 per visit.
The winter diet of robins is mainly fruit. If there are shrubs and trees available that have a good crop of fruit, robins will stick around until the food source is depleted. Temperature plays a more minor role on whether or not robins remain in northern areas. Prolonged cold requires more energy for the birds to stay warm. This could result in food crops being used up faster, in which case the robins might move on. Open water is probably attractive to them, even though birds will eat snow as a water source. If the ground is bare and temperatures are fairly warm, robins will forage for invertebrates much as they do in the summer, so they tend to move out of areas where there is a lot of snow cover.
Consequently, RRBO surveys show decreasing numbers of robins as the season progresses. The graph below depicts the average number of robins I’ve counted on our WBPS over each of the five 10-day periods spanning the survey period. By the time February rolls around, robin numbers have petered out.
Taking a broader view, we can look at the Great Backyard Bird Count, a citizen science project operated by the Cornell Lab of Ornithology. The GBBC takes place annually over four days in mid-February, providing a “snapshot” of the continent’s wintering birds. Maps reveal that even that late in the season, robins are still present and widespread in the Northeast and upper Midwest. In Michigan, robins are typically reported on 4 to 8% of the checklists submitted each year, with an average of 9 robins per checklist.
Christmas Bird Count and Project FeederWatch data also show the northerly distribution of wintering robins. In general, the wintering range of this species is considered to be most of the continent south of southern Canada.
Are there more wintering robins in the area in recent years, as my first graph seems to indicate? Much depends on weather conditions, as noted above. The proliferation of fruiting ornamental shrubs in developed areas may be contributing to what appear to be increased numbers of robins spending at least part of the winter in northern areas. If warmer winters are a result of global climate change, we could expect even more robins to find favorable winter conditions at increasingly higher latitudes.
If you are looking for a better signal that spring has arrived, it might be Common Grackles, which winter sparingly here (Red-winged Blackbirds, on the other hand, are more common). On the other hand, perhaps the remedy for the winter blues is to “think spring” each time you see a robin eating frosty berries on a gloomy winter day.
(A version of this appeared in the Washtenaw Audubon Society newsletter.)