It used to be relatively simple. In spring you waited for the first robin to arrive in early March. They'd sing, breed, nest, raise fledglings and gather up in flocks to fly south in fall.
Now the robin algorithm has changed. While there have always been small flocks of robins that stayed the winter most years, the numbers of birds that overwinter here in Illinois have grown. Winter bird counts document these increases, that can be credited to a number of factors affecting seasonal numbers.
Food matters most
The first is food supplies. Robins are obviously able to access increased levels of food supplies that enable them to stay alive during the months of November, December, January and February when most songbirds head south. Being omnivorous helps. Robins can subsist on berries and some forms of seeds. If those food supplies remain edible due to warmer weather and less snow cover, then robins can sustain themselves through the coldest months of the year.
Warmer overall winters
The trend toward warmer winter weather helps robins in several ways. Food supplies, availability of open water and less stress from cold temperatures make it possible for these already hardy birds to develop winter territories.
Where once you might find a robin and perhaps a hermit thrush on a winter bird count in Illinois, now bird census teams head out for Christmas counts in early December expecting to find flocks of 50 to 200 robins.
Robins not alone in staying north
Other species such as Carolina Wren and Eastern Bluebirds will overwinter during milder winters. These species also have shown a propensity to "stay north" in the past decade. Hearing their songs on a cold winter morning is an interesting addition to the Illinois winter.
As one of our most common and well-known songbird species, robins do create a bit more confusion among non-birders who find them during the winter months. Some will ask if a robin in December is a sign of spring. Well, not exactly. They are instead a signal of an altered climate. Whether that trend is part of an advancing dynamic remains to be seen. But the clear and regular presence of robins year-round is no longer considered an aberration, but the norm in Illinois and many other northern states.
Some of the overwintering robins are migrants from further north who set up territories here. But there are also likely many birds that nest in Illinois and stay the winter months. The normal range of the American robin extends all the way up North America to the taiga and Alaskan forests extending from one side of the continent to the other. Out west robins tend toward darker coloration and are rarely seen migrating through the Midwest or east.
A varied relative
Another species of thrush, the Varied thrush closely resembles the robin in coloration, shape and behavior. And in fact Varied thrushes are regularly found in Illinois even though their natural range is along the Pacific coast from California to Alaska. It can be presumed that some of these birds hook up with migrating robins and follow them east, where they might stay the winter with robin flocks or continue on south for the winter. Careful birders always check large robin flocks for the occasional Varied thrush that might be hanging about. But it can be tough to find when flocks of winter robins pile up in woodland hideouts. It takes patience.
Know your robins well
Robins are a "baseline" species when it comes to bird identification. In the field robins provide crucial "measuring points" for comparison to other species present. When someone says a bird they've seen was "robin-sized," almost every birder knows what they means.
Robins have a distinctive flight and perching pattern that also helps birders recognize them in the field. As a thrush, robins adopt a familiar horizontal posture with their bodies while keeping their heads tall and beaks slightly turned up. These diagnostic postures can help birders distinguish robins from other similar-sized birds such as catbird, towhee, thrasher, cardinal, grackle and blackbirds. In some respects, the highly common presence of robins in the woodlands and fields is a ready-made measuring stick for all other birds present.
There are three basic coloration patterns in American robins to identify. The adult male has a gray back and brick-red breast, with white under the tail and a black head with white markings around the eye. The beak is yellow while under the chin is a white patch with black streaks.
The female robin is a muted version of the male, with paler back and breast, almost orange at times. The female's head is brown, not black, but still has the striking eye pattern like the male. Both sexes have white tips on the outer feathers of the tail, a handy field mark when the bird is flying away into the woods.
Young birds more thrushlike
Young robins show their thrush roots with a heavily spotted plumage, especially when they first fledge. Then the birds lose their fluffy look and thin out, but first year robins retain an overall mottled look with deeply spotted breasts similar to other thrush species in their adult stages. This spotted coloration in young robins is a considerable advantage in terms of cryptic coloration or camouflage in the field. It affords the birds a better ability to go undetected by predators like Cooper's hawks that hunt them in the forests. Young robins retain this plumage through the winter months and into spring. With an already high mortality rate in young birds, this extra time in juvenile plumage increases their odds for survival.
There are millions of robins in America. They are a successful species because of their adaptability and to some degree, their native persistence. Anyone who has ever had a robin decide to build a nest on top of their light fixture near the front door of the house knows that robins can be maddeningly determined to set up shop where and when they want.
Robins build wonderfully constructed nests using thick mud and straw that is formed into a robin-sized bowl. Often these nests persist throughout the winter months, but they are never re-used.
Sometimes in winter a flock of robins will sing their familiar caroling notes, bringing spring a little closer with each refrain. But be careful not to get your hopes up too soon. A robin singing in November may now be a sign of winter, not spring.