For the Birds: American Crows

Have you been seeing large numbers of American Crows converging from all directions shortly before dusk? Shoppers in Waterville and Augusta are often surprised as nocturnal roosts of crows start to build close to shopping malls. The roost in Waterville must number in the thousands of crows.

It often takes a spectacular aggregation of crows for us to even notice them. American Crows are common, seen every day by anyone with an eye for nature. We take crows for granted. Yet, these birds have some fascinating behaviors and traits that reward a careful look.

Let’s start with their roosting behavior. Night-time roosts may contain fewer than 100 birds to tens or hundreds of thousands. One well-known roost in Oklahoma contains around two million crows!

The reasons for roosting are unclear. A large flock of birds is certainly more vigilant against predators. A large bird of prey has little chance of sneaking up on a roost of crows.

A controversial explanation for roosting in birds is to provide a way to exchange information about food locations. However, no one has convincingly shown that crows exchange information at their roosts.

Roosts often occur, like the ones in Waterville and Augusta, in urban areas. We know that the minimum temperatures in the vicinity of human developments (houses, roads, parking lots) can be as much as ten degrees warmer than in undeveloped areas. On a cold Maine night, every degree of warmth can help birds make it through the night.

In the morning, crows leave their roosts but usually do not leave alone. Crows have a stable family structure. Groups of birds consisting of a mated pair (crows mate for life) as well as their offspring from several different years leave together. This group defends a territory against other flocks of crows.

American Crows are widely distributed in North America. They are found throughout the United States except for portions of southeastern California, western Arizona, and most of Utah and Nevada. American Crows are found broadly across the southern half of Canada.

American Crows occur in a variety of habitats but are most common in open woodland areas. With the clearing of eastern forests by European colonists, American Crows increased in abundance after European colonization. Regarded as agricultural pests, American Crows were persecuted for most of the 19th century and the early part of the 20th century. The crows found that towns and cities provided a refuge from hunters and began to use urban and suburban habitats. With their broad diet, American Crows have little trouble finding food to eat in urban and suburban environments.

Like most members of the family Corvidae (jays, crows and ravens), American Crows have a broad diet. One can aptly describe them as omnivorous because their diet includes terrestrial and marine invertebrates, amphibians, reptiles, small birds and mammals, the eggs, nestlings and fledglings of birds, seed crops of various types, fruit, carrion and the French fries and other food that humans discard.

Over 200 species of birds share the behavior of cooperative breeding. Offspring from an earlier brood or grandparents, aunts, uncles or cousins may assist a breeding pair in raising their young. Many of the best studied species that display cooperative breeding have exotic names to North American birders: Superb Starling, Superb Fairy-wren and Gray-crowned Babbler, to name a few. Yet, we have a cooperative breeder right in our midst. American Crows show this intriguing nesting behavior.

The nature of cooperative breeding varies across North America. In Massachusetts, 94% of American Crow pairs had helpers with an average of 4.2 birds helping raise a clutch of eggs. In Florida, the cooperating group was even larger with 7.2 birds on average helping to raise the young.

Young birds may help their parents for up to six years before they become parents for the first time. Helpers make themselves useful in a number of ways. Helpers may help with nest building, help keep the nest clean, feed the incubating female and the nestlings and guard the eggs and nestlings when the parents are away from the nest.

The vast majority of helpers are related to the young that they help raise to fledging. The helpers therefore share genes with the current brood. So by helping to raise brothers or sisters, they are perpetuating some of the own genes without reproducing themselves.

We are use to hearing the caw of American Crows but their vocal repertoire is actually quite large. Many variants of the caw exist and they can also produce other sounds including screeches, barks, rattles, grating sounds and clicks. Carefully study of the variety of crow vocalizations is amazing.

[Originally published on December 27. 2008]