By David Spector Gazette Contributing Writer
In March many people notice American robins on lawns. As the ground thaws and earthworms become available, robins—both those returning from further south and those that spent the winter nearby feeding on fruit—reappear on lawns. The robin’s red breast is well-known; indeed it is the red breast shared with the European robin that inspired English immigrants to give our bird the same name. What about the rest of the bird? What does the bill look like? The face? The tail? How can we identify an American robin when we can’t see the breast, but can glimpse just a small part of the bird? Close examination of American robins can bring greater appreciation of the whole bird and open a window into aspects of the bird’s natural history.
The eye of most American robins is surrounded by a ring of white feathers. On most American robins this eye-ring is not continuous, but broken in the front and back with dark feathering, producing two eye-arcs. A small ˮhornˮ—a triangle of dark color—breaks the upper eye-arc on many individuals. The throat is usually white with some to many dark streaks, the pattern of streaking differing among individual birds. Close attention to the variable eye-rings and throat patterns of the birds that settle near your house might allow you to recognize individual robins.
At the other end of the bird, the tail (which is often the only part of a bird seen well as the bird flies away from an observer) is worth a close look. It is usually slightly darker than the back, and the tips of the outer tail feathers have small white spots that are adequate to identify this species, at least here in the East.
Many American robins that breed on the west coast of North America lack the white tail spots.
What about the famous red breast itself? The ˮredˮ varies considerably from a dark brick-red to a pale orange. In the later spring and summer, robins with little or no red in their breasts can be seen—young birds with spotted breasts. Some of the breast color variation is related to the sex of the bird. Females on average are paler than males, including in their breast color. During breeding season, a bird with a pale gray back, gray head and pale orange breast is likely to be a female, while an individual with a dark gray back, blackish head and dark red breast is probably a male. Males of this species are generally slightly larger than females, so size can also help to distinguish the sexes when members of a pair are seen next to each other.
In March and April, when robins that breed farther north pass through New England, sex determination at a glance is less reliable than later in the spring and summer. American robins from the humid forests of Canada’s Atlantic provinces are slightly larger and considerably darker than are our locally breeding birds. A female from Newfoundland can be as large and dark as a male from Massachusetts.
American robins replace their feathers once a year in the summer at the end of the breeding season. Appearance changes as feathers are bleached by the sun and abraded by contact with twigs and soil over the course of a year. A faded May bird can look different in many small ways than it had appeared in fresh September plumage. White parts of feathers are more fragile than are pigmented parts of the same feather, and the white tail spots can be damaged by a year of contact with soil and twigs. The robins themselves might pay attention to such details; a recent study of an Illinois population of robins suggests that robins tend to pair with mates that have a similar degree of abrasion to the tail spots.
The variation in robins’ appearance illustrates the types of biological variation. Differences due to age, sex, geographic population, individual genetic differences and interactions with the environment are common, not just in the American robin. Some of these differences are apparent on the outside of an organism, but there are many internal differences in biochemistry and details of internal anatomy.
These are just some of the variations that can easily be seen among American robins. Other details of appearance are equally interesting. Add to these the singing, feeding, nest-building, mating, flocking and roosting, and territorial behaviors and you can see how this common bird can provide uncommon fascination for the patient observer.
David Spector is a former board president of the Hitchcock Center for the Environment and teaches biology at Central Connecticut State University.
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