Thus, one reason robins are common in the suburbs is that the suburbs, with their widely spaced trees, open lawn areas, and occasional weedy spots, from the robin's perspective, have a great deal in common with a forest edge, or a part of the forest that has been ravaged by fire or a storm.
Of course, it shouldn't be thought that man's destruction of nature has in general benefited wild plants and animals. Many other species whose niches occurred only in deep forest suffered severely from these same changes. The Veery is a bird nesting in moist forests, and its numbers have diminished drastically in step with the disappearance of moist forests.
Another reason American Robins are common in our suburban neighborhoods is that there's a certain flexibility in their behavior. The Kirtland's Warbler occurs only in large tracts of Jack-pines growing six to eighteen feet tall. Consequently, Kirtland's Warblers are among the rarest of birds, breeding only in a small area of Michigan. Contrast this persnicketiness with the robin's behavior.
One moment you might see a robin gulping Poison Ivy fruits, and then the next it may be on the ground tugging at an earthworm. A study of the American Robin's diet showed just how flexible -- how broad-niched -- it is. During the fall when many fruits and seeds are available, plant matter makes up about 80 percent of its diet. However, in the spring, when insect eggs hatch and earthworms work close to the soil's surface, plant matter drops to only about 20 percent!
A few other bird species are even more broad-niched than American Robins. If you've ever seen House Sparrows, European Starlings, or Domestic Pigeons fight over a piece of hamburger bun tossed onto the pavement outside a car at a fast-food place... you can guess what some of the species are.
Other wild animals surviving in human neighborhoods likewise show tremendous flexibility in what they eat. We think of squirrels as eating nuts, but Eastern Gray Squirrels, for instance, the common park and backyard squirrel of eastern North America, eats maple seeds, pine seeds, dogwood berries, mulberries and fungi, as well as caterpillars and insect cocoons, beetles, ants, and even birds' eggs and live nestlings! American Toads can turn up under any backdoor light, and you can bet that they gobble up practically anything moving and small enough to fit into their mouths.
Therefore, the two main reasons American Robins are so common in our backyards are: