by Mary Holland ·
Friday, January 16, 2026
A robin eats from a sumac tree. (Photo by Mary Holland)
Editor’s note: This column by longtime former Harvard Press contributor Mary Holland first ran in the Feb. 27, 2009, issue of the Press.
How exactly do they do it? American robins—earthworm connoisseurs—routinely overwinter in Massachusetts and in all of New England, for that matter, where they face at least three cold months, during which there is a stark absence of anything that resembles a worm. The secret to the robins’ survival? The same as white-tailed deer and snowshoe hares—they are opportunistic, making do with the food that is available. Their diet change, however, is much more drastic than that of herbivores going from eating herbaceous plants to woody plants. Robins switch from eating animal matter to plant material and back again every year.
For much of the spring, invertebrates such as insects, earthworms, spiders, and millipedes make up more than 90% of a robin’s diet (the rest consists of fruit). Summer is a mixture of both plant and animal matter, but by fall, sometimes even before insects disappear, robins shift over to a diet of more than 90% fruit through the winter. (Robins, after eating fermented fruit, are a particularly amusing sight, as they react in much the same way as humans do, becoming obviously intoxicated—flapping, fluttering, and staggering around, and occasionally even passing out temporarily.)
The actual shift in the kinds of food they eat takes anywhere from one to two months—a relatively brief amount of time, considering how drastic a change it is. Both seasonal diets include a wide range of food. N.T. Wheelwright, in an analysis of the U.S. Biological Surveys published in the journal The Auk in 1986, showed that robins eat fruit in more than 50 genera of plants and invertebrates from more than 100 families. A diet this broad enhances their chances for surviving a New England winter.
Not only does the spring/invertebrate, fall and winter/fruit diet make seasonal sense for a bird, but it also makes nutritional sense. Robins require significant amounts of protein when females are producing eggs in the spring and when both males and females are molting and growing new feathers prior to migrating. Thus, a protein-rich diet of earthworms, beetles, caterpillars, flies, sowbugs, spiders, snails, and millipedes suits the needs of a robin during the spring and summer, when you often find robins foraging on the ground for their meals. (They have been observed supplementing their traditional invertebrate diet with small fish, snakes, mice, shrews, and damselfly nymphs.) A carbohydrate-loaded diet of 90% fruit, including that of sumac, dogwood, grapes, elderberry, red cedar, blackberry, Virginia creeper, and poison ivy, usually provides adequate fuel for the colder months, when birds aren’t nesting or molting.
Foodwise, this time of year is the most stressful for robins, as well as other fruit-eating birds that occasionally overwinter here, such as eastern bluebirds. The branches of trees and shrubs bearing the most desirable fruits, such as that of red osier dogwood, were stripped clean long ago, leaving the fruits of last resort, such as acidic highbush cranberries and fuzzy staghorn sumac berries, for late-winter sustenance. Even though they aren’t the food of choice for many birds, these bitter fruits are the ticket to survival for many of the robins that have survived this far into winter and will tide them over until insects are once again available.
How can the relatively small amount of fruit that is available support the robin population? First of all, the winter population of robins in Harvard is much smaller than the summer population. Also, unlike during the spring and summer when they are nesting, robins are not territorial in winter. During these colder months they tend to travel widely in large flocks, tracking sources of food, and do not usually remain in one location for very long. It hasn’t been determined whether the robins we see on our hawthorns, crabapples, and sumacs during winter are members of our summer population that choose to stay here year-round, or whether these robins are from farther north and flew south to spend the winter in relatively balmy Massachusetts.
The next time you see a flock of robins, rest assured that they are not frantically looking for worms (scientists feel they find them by sight rather than sound) and shouldn’t be for a few more weeks. If you find a scattering of tiny, red, fuzzy fruits on the snow under sumac bushes, and it is early morning or late afternoon, look up. It’s highly likely that these shrubs are being visited by robins, which can make quite a mess on the ground as they pick and eat the individual seeds of staghorn sumac.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England,” “Milkweed Visitors,” and “Ferdinand Fox’s First Summer.” She has a natural history blog that can be found at www.naturallycuriouswithmaryholland.wordpress.com.