Will it stay or will it go? The curious migration of robins
It seems I go through this mental exercise every spring, wondering where all the robins are going. Just outside the room where our home computer rests, stands a mature crabapple. Unlike some varieties, this tree is largely ignored by fruit-eating birds during the fall and winter months and usually comes into spring with a decent load of fruit. Whatever slow metabolic processes are taking place in crabapple fruit -- fermentation, sugar changing form, cellular breakdown, etc.-- it seems to take this tree exceptionally long to become palatable to birds. But now, suddenly, it's a smorgasbord open for business. It's feasting time for cedar waxwings and -- in particular-- American robins (Turdus migratorius).
The tree's proximity to our window allows for close and unhurried observations. After a time a person begins to notice and appreciate the subtle differences in individuals; like humans, no two are the same. Deep orange breasts to pale orange; all-yellow beak to somewhat dark; much white around the eye to little at all; some show scalloping in breast feathers, others none; considerable white at the tail corner vs. little to none. You get the idea. Still, the overarching question that bounces around my head every year is the same one, 'I wonder which birds are staying here, and which ones are still migrating?'
American robins enjoy a range in North America that can only be described as gigantic. It breeds from nearly the northernmost tip of Alaska down to Florida and everywhere in between. It mostly abandons Canada in winter, heading to nearly the entire lower 48 states. As one might imagine given the huge range, populations exhibit some variability. Seven subspecies are generally recognized by ornithologists measuring traits like bill length, leg length, overall size, and color, with slight differences among them. The robins of the high Rocky Mountains and the northern Great Plains, for instance, are the largest ones.
This thrush is at the top of the list of most recognized birds, it being so inured to humans. Rare is the yard or park in North America without a robin around either nesting or foraging or both. It is easily identified by its overall grayish back, its orange-red breast, its yellow beak, and its propensity to feed on lawns looking for earthworms in summer. Perhaps more known than its appearance is the bird's flute-like song sung loudly from prominent perches. Males begin singing very early in the morning, so early they'll beat the newspaper to your doorstep.
American robins are another one of those species which seems to have benefitted greatly from human presence. Apparently the bird did not occupy much of the Great Plains and other areas until after 1900 when the prairie was broken up, earthworms were introduced, and trees planted around homesteads and towns. The species did not occur in southern Arkansas, for instance, until 1955. Today it's safe to assume a person will encounter a robin during a summer day virtually anywhere in the country.
From Sallabanks and James' piece in Birds of North America comes this phrase, "The term 'routes' does not really apply to robin migrations." Interestingly, while individuals will return to nesting territories in ensuing years, winter ranges do not hold the same fidelity with birds scattering across the country willy-nilly. Some populations don't appear to migrate at all, preferring to spend the winter in place.
It's this highly variable migratory practice that brings us back to the ultimate destination question. Unless copulation or nest-building is witnessed, I'm not convinced we can assume the robins clustered around town right now are staying. It could very well be that at least some are bound for Hudson Bay or elsewhere. Until someone answers that question with GPS trackers attached to the birds or something like that I'm not confident we will ever know for sure.
Somewhere in that cluster of 25 robins feeding on my crabapple, I believe, are a couple of birds that will stay and build a nest on my patio. At least I think so. They will be the ones hopping around the lawn this summer with heads cocked, watching for the slightest movement before pouncing on that hapless worm and yanking it out of the ground. As for the rest of them, I'm just not sure.