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- Explain key elements about crow biology important for their control.
- Understand federal and state laws and regulations restricting crow management.
- Explain management options for crows.
American crows (Corvus brachyrhynchos) can form large roosts and disturb nearby residents with their cawing and excrement. The size and number of winter crow roosts in urban areas has increased dramatically in the last decade. Crows also can damage crops, garden plantings, and fruit trees. Crows may also feed on the eggs and young of other birds.
Crows are protected by the Migratory Bird Treaty Act. Under the Act, crows may be controlled without a federal permit when found “committing or about to commit depredations upon ornamental or shade trees, agricultural crops, livestock, or wildlife, or when concentrated in such numbers and manner to constitute a health hazard or other nuisance.”
State wildlife agencies may require permits for the crow control, and may regulate the method of take. Federal guidelines permit states to establish regulations and crow hunting seasons. Regulations vary among states, and state or local laws may prohibit certain activities for control, such as shooting or trapping. Check with state wildlife officials for specific rules and regulations before initiating control.
The American crow (Figure 1) is the most common species of crow in North America. In the northeast, they may be confused with fish crows (Corvus ossifragus). The fish crow has a more nasal call than American crows. Northern ravens (C. corax) also look similar to crows, but are much larger, and typically inhabit mature forests away from urban areas.
American crows are easy to identify with their black plumage and relatively large size. They are 17 to 21 inches long and weigh about 1 pound. They are common in areas near people. Males and females are similar in appearance. Fish crows are slightly smaller than American crows, but appear similar in the field.
Common ravens are distinguished from crows by their larger size, call, wedge-shaped tail, and flight pattern that commonly includes soaring or gliding. In contrast, crows have a frequent steady wing-beat with little or no gliding.
Health and Safety Concerns
Crows and their roosts may threaten aircraft safety when their flight paths are near airports.
In some situations, large flocks of crows may become a factor in disease spread. When feeding in and around buildings that house swine, crows have been implicated in the spread of transmissible gastroenteritis (TGE). The scavenging habits of crows, and the apparent longer incubation time for avian cholera in crows, are factors that increase the potential for crows to spread this disease.
Roosts of crows, blackbirds, and starlings that have been in place for several years may harbor the fungus Histoplasma capsulatum, which causes histoplasmosis. This disease can infect people who breathe the airborne spores when soils at a roost site are disturbed. American crows are very susceptible to West Nile Virus (WNV) and are thought partly responsible for its rapid spread across the US. Fish crows also can contract WNV, but have a higher rate of survival.
General Biology, Reproduction, and Behavior
American crows are capable of breeding after 2 years. Crows nest in February to May in the northeast. Mates appear to remain together throughout the year, at least in non-migratory populations. They maintain pair bonds, even in large winter migratory flocks. Both sexes build the nest and feed the young. Occasionally, juveniles (nest associates) help with nesting activities. The female incubates the eggs, and is fed during incubation by the male and nest associates. Usually one brood is produced per year. The average clutch size is 4 to 6 eggs, which hatch in about 18 days. The young fledge about 30 days after hatching, and forage with their parents throughout the summer.
Nest success is lowest in populations in urban areas, which averages only one fledging per brood. Rural crows have the highest nest success with an average of 1.6 young per brood. Loss of broods may result from a variety of factors including predation by raccoons (Procyon lotor) and great horned owls (Bubo virginianus), starvation, and adverse weather.
Nests consist of twigs, sticks, and coarse stems and are lined with shredded bark, feathers, grass, cloth, and string. Usually they are in trees 18 to 60 feet above ground. Nests rarely are located in deep forests. Where few trees exist, crows may nest on the ground, or on the crossbars of telephone poles.
Crows are considered commensal because they thrive in environments impacted by humans. Crows rarely breed more than 3 miles from areas habituated by people. Historically, crow populations have benefited from agriculture as a source of food, and from fire suppression, which allows the growth of trees for roosting.
Crows are among the most intelligent of birds. American crows can count to 3 or 4, are good problem solvers, have good memories, use a diverse and complex range of vocalizations, and quickly learn to associate specific noises and symbols with food. One report describes an American crow that dropped palm nuts onto a residential street and waited for passing automobiles to crack them.
Crows are wary birds, as may be evidenced by the number of crows that scavenge along highways, versus how few are hit by autos.
Crows often post a sentinel while feeding. Studies indicate that the sentinel may be part of a family group, although unrelated crows and other birds in the area likely benefit from the sentinel’s presence.
Research indicates that roosting crows may have 2 distinct, daily patterns of movement. Some fly each day to a diurnal activity center, maintained by 4 or 5 birds. 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. The unattached birds, possibly migrants, are not faithful to a specific territory and feed at sites such as landfills.
Few wild crows live longer than 6 years, but some have lived to age 14, and crows in captivity have lived for over 20 years. One researcher reported a wild crow that lived 29 years. Adult crows have few predators, with the occasional exceptions of large hawks and owls.
One important and spectacular aspect of crow behavior is the congregation into huge flocks in fall and winter (Figure 2). Large flocks are the result of many small flocks gradually assembling as the season progresses, with the largest concentrations occurring in late winter. The Auburn area in central New York has a communal roost that holds tens of thousands of crows each winter. In several other states, crows commonly roost in towns, resulting in mixed opinions on how to deal with them. Flocks roost together at night and may fly 6 to 12 miles outward from a roost each day to feed.
American crows prefer habitats that include open fields where food can be found, and woodlots with trees for nesting and roosting. They commonly use woodlots, wooded areas along streams and rivers, farmlands, orchards, parks, and suburban areas. American crows select trees for roosting that are larger and have more canopy cover than other trees, in areas with high light levels at night that are less than 2 miles from sources of food.
Crows are omnivorous. They will eat almost anything, and they readily adapt foraging habits to changing seasons and available supplies of food. They appear equally adept at hunting, pirating, and scavenging. Crows consume over 600 different food items.
About ⅓ of the annual diet of crows consists of animal matter, including grasshoppers, beetles, beetle larvae (white grubs, wireworms), caterpillars, spiders, millipedes, dead fish, frogs, salamanders, snakes, eggs, young of birds, and carrion. Garbage is a primary source of food, particularly in urban areas. The remainder of their diet consists of vegetable or plant matter, particularly corn, much of which is obtained from fields after harvest. Crows also consume acorns, wild and cultivated fruits, watermelon, wheat, sorghum, peanuts, and pecans.
Voice, Sounds, Tracks, and Signs
American crows vocalize with several calls, including a warning call, “caw, caw.” Crows can mimic sounds made by other birds and animals, and have been taught to mimic voices of humans. The call of fish crows is a short, nasal “ca,” “car,” or “ca-ha.”
The gregarious behavior of crows accompanied by frequent vocalizations allow for easy identification. Tracks may be found in soft soils (Figure 3).
Damage to Landscapes
The amount and degree of damage is highly variable among places and years. Crows may tear up turf searching for worms and other invertebrates in the grass. Large roosts can foul areas with their excrement.
Damage to Crops and Livestock
Crows may damage crops, although many problems are more commonly associated with other species of wildlife. Crows damage seedling corn plants by pulling the sprouts and eating the kernels. Similar damage also may be caused by other birds (pheasants, starlings, blackbirds) and rodents (mice, voles, squirrels). Crows consume ripening corn during the milk and dough stages, although such damage also is commonly caused by blackbirds. Crows also damage grain sorghum, commercial sunflowers, fruits, and watermelons.
In rare situations, crows attack very young calves, pigs, goats, and lambs, particularly during or shortly after birth. Depredation is more often associated with ravens, and is most likely to be done by crows where livestock births occur in unprotected, open fields near large crow flocks.
Crows sometimes consume the eggs and young of waterfowl, pheasants, and other birds during nesting season. Depredation may be locally severe where breeding waterfowl are concentrated, and where little habitat exists to conceal nests. For example, crows find nests more easily when they are located in narrow fence rows, or at the edge of potholes that have little surrounding cover. Crows typically consume large eggs (2.3 x 1.7 inches or greater) at the nest, and may take smaller eggs away from the site.
Damage to Structures
Large winter roosts in urban areas may be objectionable because of the odor of droppings, concerns for human health, noise, and damage to trees. It is not uncommon for winter crow roosts to contain 10,000 to 30,000 crows or more.
Damage Prevention and Control Methods
Remove or modify roost trees by removing up to ⅓ of the branches to open up the canopy (Figure 4) and reduce protection from cold winds. If possible, reduce night time lighting near winter roosts, as this may change crow behavior.
Secure trashcans and dumpsters, and remove carrion. Provide alternate foods, such as broadcasting cracked corn through fields to protect newly planted corn.
Exclusion, generally, is not practical for controlling crows, but may be useful in some situations. Nylon or plastic nets might be useful for excluding crows from high-value fruit or vegetable crops or small areas.
Stretch cord or fine wire across gardens at a height of 6 to 8 feet to protect crops from crows. Strips of aluminum or cloth, or aluminum pie pans may be tied to the wires, but have variable results. Lines appear to represent an obstacle that is difficult for a flying bird to see, especially when rapid escape may be necessary. Species of birds respond differently to lines, and adult birds generally are more repelled by lines than juveniles. Other factors, such as season, activity of the birds, type of lines or wires, spacing, and height need further research and development.
Protect ripening corn in small gardens by placing a paper cup or sack over each ear after the silk has turned brown. The dried, brown silk indicates that the ear has been pollinated, a necessary step in the development of corn grain.
Frightening devices can be effective for dispersing crows from roosts, crops, and other sites. A combination of several frightening techniques used together may work better than a single technique. Vary the location, intensity, and types of frightening devices to improve their effectiveness. Supplement frightening techniques with lethal control, where permitted, to improve effectiveness. The addition of lethal control only has a short-term effect on the behavior of remaining birds.
Ultrasonic sounds (high frequency, above 20 kHz) are not effective in frightening crows and most other birds. Strips of Mylar® tape hung in roost trees may be helpful in urban areas.
Effigies may frighten crows from gardens and small fields. Effigies may be immobile (Figure 5) or animated, and animated models are often more effective. One of the animated effigies is a “crow-killing” model, made from a plastic owl with a crow model attached in the talons of the owl. Movement is generated by mounting the model on a weather vane, and by adding wind or battery-powered wings to the crow.
Lasers may cause crows to flee a roost, but their quick return, even after repeated treatments, suggests that lasers are not effective as a long-term technique. Crows can be dispersed from urban roosts using pre-recorded crow distress calls broadcast from a portable player. Play the sound before the flock lands at the roost.
Clapper devices that produce an intermittent “clap” can be placed in trees, or at other sites close to crow perches. As with other frightening techniques, clappers appear to be most effective with wary populations. Crows that have habituated to people and disturbance may not respond. Beat on tin sheets or barrels with clubs to scare birds. Spray crows with water from sprinklers mounted in the roost trees as they land to disperse roosts in some situations.
Long Range Acoustic Devices (LRAD) project a high decibel sound up to 153 dB at 200 to 300 yards. The LRAD may be useful for hazing birds out of trees or off surfaces. Check local noise ordinances in urban areas before using LRADs.
Methyl anthranilate is a grape-flavored food additive that also is a bird repellent in high concentrations. In aerosol form, it irritates the nasal passages of birds causing them to flee the treated area. Foggers and ultra-low volume devices are used to apply the product and disperse roosting birds. It is a general-use pesticide.
Tactile repellents made of polybutene are available to repel crows from roosts. Avoid applying the product directly to structural surfaces. Place tape or other removable material on the surface first. Polybutenes collect dust and lose effectiveness over time. They are most useful for indoor applications to repel birds. Polybutenes are general-use pesticides.
No methods are available for use on crows.
No toxicants are registered for controlling crows.
Crows are protected by the Migratory Bird Treaty Act. The USDI-Fish and Wildlife Service (USFWS) and state wildlife agencies may issue permits to shoot crows. Check your local and state regulations before shooting crows. Twelve-gauge shotguns set at full choke with No. 6 shot works well within 40 yards. Air rifles (.22 caliber or high-velocity .177 caliber) are effective on perching crows within 40 yards.
Shooting crows is more effective as a dispersal technique than as a way to reduce numbers, as crows are difficult to shoot during the day. They may be attracted to a concealed shooter by using decoys or crow calls. In general, the number of crows killed by shooting is small in relation to the numbers involved in damage or nuisance behavior. However, shooting can help to supplement and reinforce other techniques when the goal is to frighten and disperse crows rather than reduce numbers.
Crow hunting during open seasons may be effective in rural areas. The effectiveness varies depending on movements of crows, the season in which the damage occurs, and other factors. Crows tend to be more wary of people when they are hunted, and thus more easily dispersed from roosts or other areas. Further study is needed to better understand the relationships between hunting and wariness, and whether a pattern exists that might be used to improve management of crows.
Crows are protected by the Migratory Bird Treaty Act. The USFWS and state wildlife agencies may issue permits to trap crows. Check your local regulations before trapping crows. Trapping often is less useful than other techniques because of the wide-ranging movements of crows, the time necessary to manage traps, and the number of crows that can be captured. Crows are smart and wary, and quickly become trap-shy.
Crows may be removed with nets launched by rockets or compressed air. Crows must be prebaited to the site. It is critical for net operators to be hidden while firing the nets. Crows are wary and smart, and often difficult to trap with rocket or cannon nets.
Relocation of crows is not recommended except for rescues.
Translocation of crows is not recommended.
Euthanize crows with carbon dioxide or cervical dislocation.
Government or private agencies, universities, extension service.
Crows, NWCO, wildlife control, wildlife damage management