Damage Identification and Assessment

The red-winged blackbird (Agelaius phoeniceus).

Blackbird damage to agricultural crops is often readily discernable because of the conspicuousness of the flocks of birds and the visible signs of the dam-age. However, correct identification of the species of birds in the agricultural field is important, along with evidence that the birds are actually feeding on the crop. For example, starlings superficially resemble blackbirds and some times feed in cornfields, yet they usually feed on concentrations of insects such as armyworms, doing little damage to corn. Also, red-winged blackbirds will often be attracted to agricultural fields, such as corn, initially to feed on rootworm beetles and other insect pests. They will not damage the crop itself until the grain has reached the milk stage. Blackbirds often forage in newly planted grain fields such as winter wheat, feeding on previous crop residue, weed seeds, and insects without bothering the sprouting grain.

Blackbird damage is also sometimes confused with other forms of loss. Raccoon and squirrel damage to corn can be mistaken for blackbird damage (Fig. 4). Also, seed shatter in sunflower caused by wind may resemble bird damage; however, the difference can usually be detected by examining heads for the presence or absence of bird droppings and by looking on the ground for hulls or whole seeds. Careful observation of the birds in the field and a little detective work will usually result in the correct identification of damage.

To estimate accurately the amount of blackbird damage in an agricultural field, examine at least 10 locations widely spaced throughout the field. For example, if a field has 100 rows and is 1,000 feet (300 m) long, walk staggered distances of 100 feet (30 m) along every 10th row (for example, 0 to 100 feet [0 to 30 m] in row 10, 101 to 200 feet [31 to 60 m] in row 20, and so on). In each of the 100-foot (30-m) lengths, randomly select 10 plants and visually estimate the damage on the head or ear of each plant to the nearest 1% (for instance, 2% destroyed, 20% destroyed). For corn, six kernels usually represent about 1% of the corn on an ear; for sunflower, it may be easiest to visually divide the head into four quarters and then estimate the percentage of seeds missing. When finished, simply determine the average damage for the 100 plants examined. This will give an approximation of the percent loss to the field. Multiplying the percent loss by expected yield can give a rough estimate of yield loss. In small grains, such as rice, estimates of loss are more difficult to obtain. One possibility is to simply compare the yields from plots in damaged and undamaged sections of a field

Legal Status

Blackbirds are native migratory birds, and thus come under the jurisdiction of the Federal Migratory Bird Treaty Act, a formal treaty with Canada and Mexico. Blackbirds are given federal protection in the United States. They may be killed only 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 as to constitute a health hazard or other nuisance,” as stated in federal laws regarding migratory birds (50 CFR 21). Some states have additional restrictions on the killing of blackbirds. Always consult federal, state, and local laws before initiating any control activities as regulations frequently change.

Economics of Damage and Control

Superficial surveys of agricultural fields often overestimate blackbird damage and thus exaggerate the overall severity of the economic threat for one of four reasons: (1) the conspicuousness of blackbird flocks tends to heighten the awareness of bird damage compared with other more subtle forms of loss caused by weeds, insects, other pests, and harvesting; (2) the eye naturally seeks out the conspicuously bird-damaged plants; (3) bird damage is often most severe along field edges where an observer is most likely to check; and (4) raccoon, other mammal, or wind damage is sometimes mistaken for bird damage. This is not to downgrade the problem of blackbird damage in agriculture; damage can be economically severe on occasion and quite frustrating to the farmer when relief is not readily available. It is important, however, to obtain objective estimates of damage levels likely to occur, for only then can intelligent decisions be made regarding the amount of money and effort to be invested on control. The final decision on control measures must take into account the value of the crop, cost of control, and the degree of effectiveness of the control measure in relation to the probable levels of damage.

Studies during the past two decades concerning blackbird damage to various crops such as corn and sunflower indicate that on statewide or regional bases, overall mean damage is low, generally less than 1% of the crop. If all farmers received less than 1% damage, there would be little concern; however, the damage is not equally distributed. While most farmers escape economically serious damage, a few farmers receive serious damage. For example, in North Dakota, South Dakota, and Minnesota in 1979 and 1980, overall loss of sunflower to blackbirds was estimated to be only 1.2% of the crop. Yet, 2% of the fields received more than 10% loss. Only in these relatively few fields that sustain high levels of damage can control measures generally be cost-effective.

While accurate prediction of damage is often impossible to obtain, knowledge of the location of a field in relation to traditional roosting sites often provides the basis for a sound estimate of potential damage. For example, studies of blackbird damage to ripening corn in Ohio have revealed that almost all losses exceeding 5% of the crop have occurred in fields within 5 miles (8 km) of marsh roosts.

Objective estimates of damage levels in previous years for the same or nearby fields are another means of predicting future damage levels, because bird damage is fairly consistent from year to year within a locality. This information also provides a good baseline for evaluating the effectiveness of management strategies. Of course, it is important that estimates be objective and applies to the entire field.