Damage Prevention and Control Methods

Exclusion

American crow, Corvus brachyrhynchos (also called the common crow or eastern crow).

Exclusion generally is not practical for most crow problems, but might be useful in some situations. For example, nylon or plastic netting might be useful in excluding crows from high-value crops or small areas. Protect ripening corn in small gardens from crow or other bird damage 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 by the corn tassels, a necessary step in corn grain development.

Lines. Another excluding or repelling technique used historically to protect fields from crows is stretching cord or fine wire at intervals across the field at heights about 6 to 8 feet (1.8 to 2.4 m) above the ground. Sometimes aluminum or cloth strips or aluminum pie pans were tied to the wires. More recently, the concept of stretching widely spaced lines or wires over or around sites needing protection from certain birds has received increased attention. Crows were included in two studies at sanitary landfills, but results were somewhat conflicting. One report from South Carolina indicated that a 20 x 20-foot (6 x 6-m) wire grid repelled crows, but another from New York indicated that parallel wires stretched 10 x 10 feet (3 x 3 m) apart and 80 x 80 feet (24 x 24 m) above the ground did not repel them.

The reason this technique has worked for certain birds is not completely clear, but the wires appear to represent an obstacle that is difficult for a flying bird to see, especially when rapid escape may be necessary. Various species respond differently to lines, and generally adult birds are more repelled by lines than juveniles. Other factors such as season and/or biological activity of the birds, type of lines or wires, spacing, and height need further research and development to better understand the potential usefulness of lines in bird management.

Cultural Methods

Agricultural Crops. Some reports indicate that providing an alternate or decoy food source will reduce crop damage caused by crows. An example would be scattering a grain such as whole corn, preferably softened by water, through a field where crows are damaging newly planted corn seed-lings. Although this technique has been reported to be helpful in some situations, it has not been well tested.

Tree Roosts. Thinning branches from specific roost trees or thinning trees from dense groves reduces the avail-ability of perch sites and opens the trees to weather effects. Such vegetation management has effectively dispersed starling/blackbird roosts, and the same biological concepts indicate probable effectiveness in dispersing crow roosts. When roosts occur in a small number of landscape trees near homes or along streets, they usually are in fairly dense trees where thinning the branches will reduce the trees’ attractiveness as roosts. Roosts in tree groves or woodlots usually occur in dense stands of young trees. Thinning about one-third of the trees improves the tree stand, especially if marked by a professional forester. Such thinning successfully dispersed blackbird/star-ling roosts from research woodlots in Ohio and Kentucky, and from at least two problem roost sites in Nebraska. In dense cedar thickets, bulldozing strips through the roost site to remove one-third of the habitat has also been successful in dispersing birds, but soil disturbance with this method may be hazardous if soils harbor fungal spores of the human respiratory disease histoplasmosis. For further information on roost dispersal, see Bird Dispersal Techniques.

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Frightening

Frightening is effective in dispersing crows from roosts, some crops, and other troublesome sites. In a recent study in California, crows were successfully dispersed from urban crow roosts using tape-recorded “squalling” calls (given by a crow struggling to escape from a predator) and a portable tape player commonly used by hunters to attract animals. Such dispersal allows crows to be moved from problem sites to sites where they are less likely to interfere with people.

In addition to recorded distress or alarm calls, frightening devices include gas-operated exploders, battery-operated alarms, pyrotechnics, (shell-crackers, bird bombs), chemical frightening agents (see Avitrol^® below), lights (for roosting sites at night), bright objects, clapper devices, and various other noisemakers. Beating on tin sheets or barrels with clubs can help in scaring birds. Spraying birds as they land, with water from a hose or from sprinklers mounted in the roost trees, has helped in some situations. Hanging mylar tape in roost trees may be helpful in urban areas. A combination of several scare techniques used together works better than a single technique used alone. Vary the location, intensity, and types of scare devices to improve their effectiveness. Supplement frightening techniques with shotguns, where permitted, to improve their effectiveness in dispersing crows. Ultrasonic (high frequency, above 20 kHz) sounds are not effective in frightening crows and most other birds because, like humans, they do not hear these sounds. For a more detailed discussion of frightening techniques, see Bird Dispersal Techniques.

Animated “crow-killing” owl models can frighten crows from gardens and small fields. These are made from a plastic owl model with a crow model attached in such a way that the crow appears to be in the owl’s talons. Movement is supplied by mounting the model on a weather vane and by adding wind- or battery-powered wings to the crow.

Clapper devices (Tomko Timer-Clapper) have been reported by the Nebraska Game and Parks Commission as successful in dispersing crows from waterfowl concentration areas where crow roosting was destroying a multiple-row shelterbelt and where there was concern that crows were aggravating the spread of avian cholera. A clapper device intermittently “claps,” producing a sound much like a twig snapping or like two boards clapping together. The device can be placed up in trees or at other sites close to crow perches, making it perhaps more significant to crows as a frightening device. Clappers have also been used to frighten and disperse other birds (starlings, grackles, swallows) and to repel deer at night. Like many other frightening techniques, clappers appear to be most effective with wary populations. Populations that have habituated to people or disturbance to such an extent that they have lost their wariness, may not respond.

'Avitrol^®.' Avitrol^® (active ingredient: 4-aminopyridine) is a Restricted Use Pesticide and chemical frightening agent, available in a whole-corn bait formulation (Double Strength Whole Corn) for use in dispersing crows. It is only for sale to certified applicators or persons under their direct supervision and only for those uses covered by the applicator’s certification.

Avitrol^® baits contain a small number of treated grains mixed with many others that are untreated. Birds that eat the treated portion of the bait behave erratically and/or give warning cries that frighten other birds from the area. Generally, birds that eat the treated particles die. Overall, because of the type of damage problems associated with crows, Avitrol^® is unlikely to be used often. This product is included here, however, because situations may arise in which its use would be helpful. Before using this product for crow control, it is best to contact a qualified person trained in bird control work (someone from the Cooperative Extension or USDA-APHIS-Wildlife Services, for example) for technical assistance. For additional information on Avitrol^®, see Blackbirds and European Starlings.

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Repellents

Methyl anthranilate (Fruit Shield^®) is currently registered for ornamentals, corn, sunflower, a variety of fruit crops, and structures. In addition, polybutenes (tactile repellents) can be used to prevent crows from perching or roosting.

Recent studies show that conditioned aversion learning, a form of repellency, can reduce egg and possibly fruit and grain crop depredation by crows. Further work and registration of an appropriate agent for producing a conditioned aversion response are needed.

Toxicants

Some states have Special Local Needs registrations for DRC-1339 (3-chloro p-toluidine hydro-chloride) by USDA-APHIS-Wildlife Services for limited use in livestock facilities and staging areas.

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Trapping

Trapping is often less attractive than other techniques because of the wide-ranging movements of crows, the time necessary to maintain and manage traps, and the number of crows that can be captured compared to the total number in the area. Trapping and removing crows, however, can be a successful method of control at locations where a small resident population is causing damage or where other techniques cannot be used. Examples include trapping damage-causing crows near a high-value crop or in an area where nesting waterfowl are highly concentrated.

Two types of traps can be used to successfully capture crows. First, individual crows may be captured uninjured with No. 0 or No. 1 steel traps that have the jaws wrapped with cloth or rubber. These sets are most successful if placed at vantage points in areas habitually used by crows or if baited with a dummy nest containing a few eggs. Check such traps at least twice daily. Crows captured in this way might be used, if necessary, as initial decoys in the Australian crow trap described below, but the small number of captures is otherwise unlikely to affect a damage situation.

Figure 2. Australian crow trap: (a) completed trap, (b) end view, and (c) plan of “ladder” opening

A second and more commonly used trap for crows is the Australian Crow Trap (Fig. 2), a type of decoy trap. These traps are most successful if used during the winter when food is scarce. Australian crow traps should be at least 8 to 10 feet (2.4 to 3 m) square and 5 to 6 feet (1.5 to 1.8 m) high. If desired, construct the sides and top in panels to facilitate transportation and storage. Place the trap where crows are likely to congregate. The most attractive bait is meat (such as slaughterhouse offal, small animal carcasses) or eggs. Whole kernel corn, milo heads, watermelon, and poultry feed may also work and may be preferred where carnivores such as feral dogs might be attracted to the trap. Place the bait under the ladder portion of the trap. Also provide water. After the first baiting, the trap should not be visited for 24 hours. Once the birds begin to enter the trap, it should be cared for daily. Replace the bait as soon as it loses its fresh appearance. Remove all crows captured except for about five to be left in the trap as decoys. Remove captured crows after sunset when they are calm (to facilitate handling). Should any nontarget birds be captured, release them unharmed immediately. Euthanize captured crows humanely by carbon dioxide exposure or cervical dislocation. A well-maintained decoy trap can capture a number of crows each day, depending on its size and location, the time of year, and how well the trap is maintained.

A recent study in Israel of hooded crows (Corvus corone), which are about the same size as American crows, indicated that decoy crows were more important than bait to trap success. Using one hooded crow decoy bird, however, appeared to be as effective as using three to four, and fleshy baits did increase success in some cases. To prevent hooded crow escape, the ladder gap width of the American model was reduced from 18 to 12 inches (45 to 30 cm), and 1.5 x 0.8-inch (4 x 2-cm) square rungs were used instead of 3-inch (8-cm) diameter metal rods. The potential response of American crows to such trap modifications is unknown but merits study.

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Shooting and Hunting

Shooting is more effective as a dispersal technique than as a way to reduce crow numbers. Crows are wary and thus difficult to shoot during daylight hours. They may be attracted to a concealed shooter, however, by using crow decoys or calls, or by placing an owl effigy in a conspicuous location. Generally, the number of crows killed by shooting is very small in relation to the numbers involved in pest situations. However, shooting can be a helpful technique to supplement and reinforce other dispersal techniques when the goal is to frighten and disperse crows rather than specifically to reduce numbers. For more details on dispersal, see Bird Dispersal Techniques.

Crow hunting during open season can be encouraged in areas where crows cause problems. The helpfulness of hunting as a control technique varies depending on crow movements, the season in which the damage occurs, and other factors. Another consideration is that crows tend to be more wary of people when they are hunted and thus more easily dispersed from roosting or other areas where their presence is a problem. Further study is needed to better understand the relationships between hunting and wariness, and whether a pattern exists that might be used to improve crow management programs.

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