Northern Prairie Wildlife Research Center
Most economically severe blackbird damage to agricultural crops occurs in fields within 5 miles of roosts. Thus, one strategy is to plant crops that do not attract blackbirds such as soybeans, potatoes, or hay in fields that are within a few miles of a roost. If crops vulnerable to damage, such as corn or sunflower, are planted near a roost, it is important that sources of alternative food be available to reduce the feeding pressure on these cash crops. When crops are ripening, one strategy for providing alternative feeding areas is to delay plowing or tilling previously harvested cropland near the roost. Also, farmers should synchronize planting of fields of vulnerable crops near roosts so that all fields mature at about the same time. In general, as the number of suitable alternative feeding areas declines, maturing grain or sunflower fields become more attractive to blackbirds, and protecting the crop becomes more difficult.
The timing of harvest can be very important for reducing damage to crops from flocks of blackbirds. Although field corn generally becomes unattractive to birds when the kernels mature and harden, sunflowers continue to be attractive after they mature and thus should be harvested as soon as possible. Sweet corn should be harvested as early as possible in areas with extensive damage. Delaying harvest of sweet corn by only 1 or 2 days can result in substantially greater damage if flocks of blackbirds are in the area.
Planting crop varieties known to be resistant to blackbird feeding should be part of a grower's bird-damage management plan. Bird-resistant sunflower hybrids may soon be available from commercial seed companies for use in areas subject to a high risk of depredation. The seed of these hybrids is protected by morphological traits, such as concave-shaped heads, horizontally oriented heads, and long head-to-stem distance. These traits make it difficult for birds to position themselves on the plant so they can extract seeds from the head. These hybrids must be planted in north-south rows to maximize their birdresistant characteristics.
Corn hybrids vary greatly in their susceptibility to bird feeding. Hybrids of corn with long husk extension and thick husks have been shown to be more resistant to damage than other hybrids. Sweet corn is vulnerable to blackbirds for only a few days before harvest and, thus, may be easier to protect than field corn and sunflower.
Sunflowers with concave heads, horizontally oriented heads, and long head-to-stem distance appear to suffer less damage from birds than other varieties. (Photo courtesy of North Dakota State University; used by permission.)
Cattail Marsh Management
Cattail marshes used as roost sites for blackbirds can be sprayed with glyphosate (@Rodeo Aquatic Plant Herbicide) to reduce the density of the cattails, which reduction in turn disperses the birds. Rodeo is the only herbicide registered for controlling cattails growing in standing water. (Other herbicides are available for use on cattails growing in marshes without surface water.) Generally, cattails must be treated 1 year before vulnerable crops are planted in the vicinity of the marsh to allow time for the cattails to decompose, making the marsh unattractive to blackbirds for roosting for several years. For maximum effectiveness, the herbicide should be aerially applied at a rate of 4.5 pints/acre in August up to first frost. Sufficient cattails should be removed to create at least 70 percent open water in the marsh. Thinning out cattails in these marshes reduces blackbird use and improves the habitat for other, more desirable wildlife such as waterfowl.
Cattail marshes can be managed using a registered aquatic herbicide to disperse roosting blackbirds. (APHIS photo by David Bergman.)
For more detailed information about managing cattail marshes in the Dakotas, you may contact:
USDA, APHIS Animal Damage Control (ADC) Bismarck, ND (701 250-4405) Pierre, SD (605 224-8692)
or U.S. Department of the Interior (USDI) Fish and Wildlife Service Wetland Habitat Office Bismarck, ND (701 250-4403)
Frightening devices can be effective in protecting crops from flocks of blackbirds but require considerable work and long hours. The efficient use of these devices mandates that farmers persistently scare the birds before their feeding patterns become well established. Devices need to be employed especially in the early morning and in late afternoon, when the birds are most actively feeding.
Propane exploders are a popular frightening device. In general, there should be at least 1 exploder for every 10 acres of crop to be protected. Exploders should be elevated on a barrel stand to "shoot" over the crop, and they should be moved around the field every few days. Exploders should be turned off (manually or with automatic timers) each night to save propane and reduce objectionable noise levels. In addition, exploders should be reinforced occasionally with other scare devices because birds lose their fear of frightening devices over time.
A propane exploder and a popup scarecrow powered by carbon dioxide. (APHIS photo by John Cummings.)
By shooting a .22-caliber rifle just over the top of the crop, a person on a stand or truck bed can frighten birds from large fields (40 acres or more). Obviously, care must be taken when shooting in this manner, and the use of limited-range cartridges is recommended. Also effective are pyrotechnic devices such as cracker shells, bird bangers, and screamers. These pyrotechnic devices, fired from shotguns or specially designed pistols, explode after traveling up to 150 yards. Shooting birds with a shotgun, using standard bird shot, often can kill a few birds and reinforce other scare devices. However, this technique usually is not as effective in moving birds as the other devices that have greater range. Thus, a shotgun patrol should not be used as the sole means of frightening birds.
A variety of other bird-frightening devices, including electronic noise systems, helium-filled balloons tethered in fields, radio-controlled model planes, tape-recorded distress calls for birds, and various types of scarecrows, are also occasionally used to rid fields of blackbirds. The effectiveness of these devices is highly variable and depends on the persistence of the operator, the skill used in employing them, the attractiveness of the crop, and the number of birds and availability of alternative food sources. As mentioned with the use of propane exploders, birds tend to adjust or adapt to frightening devices. It is usually best to mix the use of two or more devices rather than to rely on a single device.
Harassing feeding blackbirds with an airplane can sometimes be an effective method of chasing flocks from sunflower fields. This technique appears to be most effective if combined with other mechanical methods on the ground, such as shotguns or pyrotechnics.
Airplanes can be used to scare blackbirds. (APHIS photo by David Bergman.)
Avitrol, the only chemical registered for control of blackbirds in corn and sunflower, is a cracked-corn bait in which 1 out of every 100 particles is treated with the active ingredient, 4-aminopyridine. The bait is applied by airplane along access lanes placed in the fields, at the rate of 3 pounds/acre. When a blackbird eats one or more treated particles, flies erratically and emits distress calls. This abnormal behavior often causes the remaining birds in the flock to leave the field.
Careful consideration must be given to the timing of initial and repeat battings. The first baiting should be when birds first initiate damage, and repeat battings should occur as necessary, about 5 to 7 days apart. Weeds that hide bait, ground insects (e.g., crickets) that eat bait, and excessive rainfall can contribute toward making the product less effective. Instructions on the label, especially the avoidance of baiting field edges, should be carefully followed to avoid killing nontarget birds..