American Robins rank third in numbers behind Red-winged Blackbirds and introduced Starlings as the most common bird in North America. To what do they owe their great success, compared to Cardinal and Bluejays, for example?
Note added after posting: Valerie Cunningham who writes a bird column for the Minneapolis Star Tribune uses a more reliable source for estimates of bird populations in the U.S. and Canada. According to the Partners in Flight Database, the American Robin is THE most numerous bird in North America, at an estimated 370,000,000 birds, far outpacing the Red-winged Blackbirds (160,000,000) and Starlings (86,000,000). So that makes what I have said below even more impressive!
One strategy for being prolific: breeding early and often — producing as many as three clutches of chicks during a breeding season lasting from April to July. Robins are one of the earliest to nest, and continue to raise broods until the flush of insects has diminished in late summer.
Another strategy for being versatile is their flexibility in changing diets as the seasons progress. We think of Robins as being primarily fruit eaters, and they do consume a lot of fruit in fall and winter — indeed, as much as 60% of their diet over the course of a year may be fruit.
This dietary switch from eating mostly animal prey to consuming mostly fruit is not trivial. There are major changes in gut anatomy, changes in types of enzymes synthesized for digestion, and amount of food to be consumed and the rate it is moved along the digestive tract daily that must take place during a short transition time of about two weeks. Ask any vegetarian what happens when they try to eat meat and you get a sense of what Robins must deal with twice a year as they switch food sources. So, we must credit their dietary versatility for their ability to survive and become one of the most common birds in North America.
But here’s a new wrinkle in the Robin’s key to success in surviving the food desert of the late winter landscape in northern latitudes — fishing! Recently someone posted photos (on Facebook’s Minnesota Birding site) of Robins fishing for minnows near the edge of a pond free of snow. One or more Robins poked a hole in thin ice, big enough for minnows to find as a place to gulp some oxygen in their severely anoxic swampy pond, and the patient Robin simply pulled them up for a meal.
This is not one isolated instance of American Robins eating fish. There are reports in the scientific literature as early as 1954 of Robins feeding on dead shiners, as well as newspaper articles documenting Robins hanging around bait shops for the dead minnows being thrown out.
American Robins are truly versatile and adaptable — and as a result are very successful in populating North America.