Plant-animal Interactions: Simulation of Bird Damage on Corn Ears

Within the past decade interactions among plants and animals have received increasing attention, mostly pertaining to selection of plants that produce toxic secondary compounds as a direct result of herbivory (Gilbert and Raven, 1975; Feeny, 1975; and Rhoades and Cates, 1976) and in turn selection of animals that detoxify these plant compounds (Freeland and Janzen,1974). Indeed, the plant-herbivore association has been regarded in the context of predator-prey relationships, especially for seed eaters also have been described. Thus it appears there are many associations which affect populations of plants and animals in evolutionary time (Janzen, 1976) and others which have short-term effects in "physiologi-cal time," i.e., periods ranging from minutes to a single growing season. It is this latter phenomenon that I am examining in this paper. A question has been raised about the overall effects of Redwinged Blackbirds (Agelaius phoeniceus) feeding on ripening ears of field corn in the east and middle west of North America (Dyer, 1973, 1975). In several instances, fields in which Redwings fed produced more corn than fields in which feeding levels were lower; this is true for plots within fields as well (ms. in preparation). On a regional basis the question is complex. Can birds recognize more productive corn fields, or even more productive patches within fields, or is there an interaction between the animal and the corn plant that triggers higher levels of production? Of the two alternatives, I have suggested that the more likely event is that avian herbivory stimulates grain production (Dyer, 1973, 1975). Thus it is of heuris-tic value to examine growing field corn ears in fields of the midwest in order to determine whether there is the possibility that one or more "herbivorous events" may trigger processes which lead to compensatory growth. The approach to identifying the possibility of compensatory growth has been through simulating bird damage on maturing corn ears by mechanical manipulation. The study was conducted during 1971 in hybrid field corn raised for seed. In August 1971 a field of field corn, a 3-way cross of A632 x B37-43 x 619, destined for seed production in Erie County, Ohio was selected for the study. It was planted May 10, 1971; fertilized at the rate of 169 kg N, 135 kg K, and 56 kg P ha-1 ; and had weeds controlled by 2.8 kg atrizine and 0.34 1 of 2,4-D ha-1 post emergence, plus one mechanical cultivation. …