Wildlife Damage Management, Internet Center for


Date of this Version

November 1976


Economic losses attributed to pests are usually estimated by visual assessments of the damage. In all cases, the amount of injury to plant parts is correlated with reduction in production, and any effects of plant response or compensation are ignored. Some recent experiments, using prairie grass grown with different degrees of grasshopper feeding activity, indicated that some plant processes were triggered by insect feeding (Dyer and Bokhari, 1976). Responses, such as the increase of net primary production on grasslands by livestock grazing, have been suggested in studies by Westlake (1963), Pearson (1965), and Hutchinson (1971); Vickery (1972) confirmed these findings by showing that maximum net primary production can be achieved on grasslands through optimum stocking densities. Studies by Kirby (1967) on the relationship of plant densities to yields show that plant compensation permits crops sown at low densities to produce yields as high as those sown at higher densities. Feare (1974) stated that any grain (barley and oat seeds) removed by Rooks (Corvus frugilegus) resulted in a reduced yield; but, owing to limited ability of the crops to tiller, the reduction in yield was not proportional to the number of grains (seeds) removed. Thus, to a certain extent, the crops compensated for the reduced plant densities caused by Rook feeding. Wright and Summers (1960) studied bullfinch (Pyrrhula pyrrhula) damage to buds of pear trees and found that the trees compensated for much of the damage by producing ancillary buds the following year. Hints of a similar compensation from bird damage in maturing grain crops have been made by Linehan (1967), Dawson (1970), and Dyer (1975). Blackbird (Icteridae) damage to maturing corn has long been considered a severe problem in certain areas of the United States (Stone, et al., 1972). Almost all estimates of damage to maturing corn are based on a comparison of the damaged and undamaged ears of corn converted to bushels per acre lost (Linehan, 1967; DeHaven, 1974). Dyer (1975) hypothesized that bird damage to maturing corn may in certain cases even increase the yield. Birds damaging kernels on the apical portion of ears could stimulate growth in other parts of the corn plant. At the time birds damage maturing corn, most kernels destroyed have not matured and therefore do not contain all of their potential biomass. Thus, surplus production may be channeled into the remaining undamaged kernels or other plant parts. A better understanding of this phenomenon is needed to truly evaluate the impact of blackbird depredation on maturing corn crops. This experiment was designed to permit a regression analysis of compensation (net loss in weight of shelled corn) as a function of two treatment factors: (1) level of simulated blackbird damage inflicted and (2) level of corn maturity at time of damage.