General Biology, Reproduction, and Behavior Go back to Crows
Crows are among the most intelligent of birds. Experiments indicate that American crows can count to three or four, are good at solving puzzles, have good memories, employ a diverse and behaviorally complex range of vocalizations, and quickly learn to associate various noises and symbols with food. One report describes an American crow that dropped palm nuts (Washingtonia sp.) onto a residential street, then waited for passing automobiles to crack them. Crows are keen and wary birds. Consider the number of crows that scavenge along highways; how many have you seen hit by autos? Crows can mimic sounds made by other birds and animals and have been taught to mimic the human voice. The myth that splitting the tongue allows a crow to talk better, however, is not true and is needlessly cruel.
Crows often post a sentinel while feeding. Although studies indicate that the sentinel may be part of a family group, unrelated crows and other birds in the area likely benefit from the sentinel’s presence.
Crows begin nesting in early spring (February to May, with southern nests starting earlier than northern ones) and build a nest of twigs, sticks, and coarse stems. Crow pairs appear to remain together throughout the year, at least in nonmigratory populations, and pairs or pair bonds are likely maintained even within large winter migratory flocks. The nest, which is lined with shredded bark, feathers, grass, cloth, and string, is usually built 18 to 60 feet (5 to 18 m) above ground in oaks, pines, cottonwoods, or other trees. Where there are few trees, crows may nest on the ground or on the crossbars of telephone poles. The average clutch is 4 to 6 eggs that hatch in about 18 days. Young fledge in about 30 days. Usually there is 1 brood per year, but in some southern areas there may be 2 broods. Both sexes help build the nest and feed the young, and occasionally offspring that are 1 or more years old (nest associates) help with nesting activities. The female incubates the eggs and is fed during incubation by the male and nest associates. The young leave the nest at about 5 weeks of age and forage with their parents throughout the summer. Later in the year, the family may join other groups that in turn may join still larger groups. The larger groups often migrate in late fall or winter.
Few crows in the wild live more than 4 to 6 years, but some have lived to 14 years in the wild and over 20 years in captivity. Recently, a bird bander reported a crow that had lived an incredible 29 years in the wild. Adult crows have few predators, although larger hawks and owls and occasionally canids take some. Brood losses result from a variety of factors including predation by raccoons (Procyon lotor), great-horned owls (Bubo virginianus), and other predators; starvation; and adverse weather.
One important and spectacular aspect of crow behavior is their congregation into huge flocks in fall and winter. Large flocks are the result of many small flocks gradually assembling as the season progresses, with the largest concentration occurring in late winter. The Fort Cobb area in Oklahoma, a communal roost site, holds several million crows each winter. In Nebraska, Wisconsin, and possibly other states, crows appear to be roosting more commonly in towns near people, resulting in mixed opinions on how to deal with them. These flocks roost together at night and disperse over large areas to feed during the day. Crows may commonly fly 6 to 12 miles (10 to 20 km) outward from a roost each day to feed.
Recent radio-telemetry studies indicate that roosting crows may have two distinct daily movement patterns. Some fly each day to a stable territory, called a diurnal activity center, which is maintained by four or five birds throughout the winter and apparently then used as a nesting site in spring. Although these stable groups of crows may stop at superabundant food sources such as landfills, individuals within the groups typically fly different routes and make different stops. Other crows appear to be unattached and without specific daily activity centers or stable groups. Although they use the same roosts as the activity-cen-ter crows, these unattached birds, possibly migrants, are not faithful to any specific location or territory and more regularly feed at sites such as landfills.
Ongoing changes in land-use patterns may result in associated impacts on crow populations and behavior. Historically, crow populations have benefited from agricultural development because of grains available as a food supply and because trees became established in prairie areas where agriculture and settlement suppressed natural fires. The combination of food and tree availability favored crows, and in some areas with abundant food and available roost sites, large winter roosting concentrations became established. As the current trend toward sustainable agricultural systems continues, which may include a variety of crops and rotations with nongrain crops, food availability and associated patterns of crow roosts may change.
The growing number of crows that nest and roost in urban areas also raises questions. Are urban habitats now selected because of adaptive changes in crow behavior, or are changes in rural settings making urban sites comparably more suitable? One study described two neighboring but distinct crow nesting populations — one that was urban and somewhat habituated to people and another that was rural and relatively wary of people. Will crows that are hatched in urban areas be habituated to people to such an extent that they will be more difficult than their rural counterparts to disperse from problem sites? Understanding such factors may lead to better options for managing crows in ways compatible with the needs of people.
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