Northern Prairie Wildlife Research Center
Damage to sunflower and grain crops can be economically severe and quite frustrating to the farmer when relief is not easily available. During the past two decades, studies on blackbird damage to various crops such as corn and sunflower indicate that on statewide or regional level, overall damage is low, generally 1 to 2 percent of the crop. If all farmers lost less than 2 percent of their crops, there would be little concern; however, the damage is not equally distributed. While most farmers escape economically serious blackbird damage (that which affects more than 5 percent of their crops), profit margins for other farmers are significantly reduced. Some growers would like to plant sunflower in their crop rotation, but the threat of bird depredation precludes this option. Farmers whose crops are seriously damaged by blackbirds may find various damage control measures cost effective.
It is important to obtain objective estimates of potential damage levels in a field before investing large amounts of money on control. The person making the final decision on control measures must consider the value of the crop, the cost of control, and the degree of effectiveness of the control measure in relation to the probable levels of damage. Estimates of damage levels in previous years for the same or nearby fields are another means of predicting future damage levels because local bird damage is often fairly consistent from year to year. This information also provides a good baseline for evaluating the effectiveness of management strategies. Of course, it is important that estimates of damage be objective and reflect the entire field, not just a small portion of the field that has high damage.
To estimate the amount of blackbird damage in an agricultural field objectively, at least 10 locations widely spaced throughout the field should be examined. For example, if a field has 100 rows and is 1,000 feet long, the estimator should walk staggered distances of 100 feet along every 10th row (for example, 0-100 feet in row 10, 101-200 feet in row 20, and so on). In each of the 100-foot lengths, the estimator should randomly select 10 plants and visually estimate the damage on the head or ear of each plant to the nearest 1 percent (for example, 2 percent destroyed, 20 percent destroyed). For corn, six kernels usually represent about 1 percent of the corn on an ear; for sunflowers, it may be easiest to visually divide the head into four quarters and then estimate the percentage of seed missing. When finished, determine the average damage for the 100 plants examined, and this will give an approximation of the percentage loss to the field. Multiplying the percentage loss by expected yield can give a rough estimate of yield lost. In small grains, estimates of loss are more difficult to obtain. One possibility is to compare the yields from plots in damaged and undamaged sections of a field.