Date of this Version
Blackbird (Icteridae) damage to maturing corn (Zea mays) in the milk and dough stages has long been considered a severe problem in localized areas of the United States (Stone et al. 1972). Most estimates of primary loss to corn yields are based on visual or measured surface area estimates of damage to individual ears (Linehan 1967; De Grazio et al. 1969; DeHaven 1974; Granett et al. 1974). These estimates of loss ignore any compensating growth in the undamaged kernels and secondary losses from insects and disease that may have occurred following damage. Numerous studies of fruit, small grain, and hay have shown that moderate injury or loss to stems, leaves, roots, or fruit can occur before yield is actually affected (reviewed by Stern 1973). Compensation in maturing corn has been suggested by Linehan (1967), Dawson (1970), Dyer (1975; 1976), and Woronecki et al. (1976). The present study was conducted to quantify possible compensatory growth responses and to estimate the effects of secondary losses associated with bird damage to maturing corn. If compensatory growth is a significant factor, visual assessments of bird damage should be adjusted to reflect more accurately the impact of bird depredations. Our previous study (Woronecki et al. 1976) suggested that net loss of shelled corn weight was a function of both the level of damage and the maturity of the ear at time of damage. The study also showed that low levels of bird damage could be partially compensated by increased kernel weight. Regardless of maturity, damage of less than six kernels per ear resulted in no net loss and in some instances resulted in increased yield. The results from that experiment were considered preliminary, because certain disparities left a doubt whether or not the data were confounded by plant responses to other primary and secondary damage. The present experiment, a modification of our previous study (Woronecki et al. 1976), was designed to provide a more sensitive test of the direct effects of bird damage on maturing field corn. Modifications were made to improve procedures, eliminate some of the disparities contributing to weight loss of shelled corn, and reduce sampling errors.