Competition among Bark-foraging Birds in Central Illinois: Experimental Evidence

Most studies on similar coexisting species enumerate ecological differences which presumably reduce the demand for common resources and allow coexistence (MacArthur 1958, Root 1967, Stallcup 1968). As Schoener (1974) pointed out, however, these differences alone do not demonstrate the role of competition in structuring avian communities. Even if niches were arranged randomly with respect to one another, differences would exist. Observational studies can reveal very little about how avian species influence one another except when direct aggressive interactions are observed. Experimental evidence is needed to more fully understand community organization as suggested by Reynoldson and Bellamy ( 1974), Colwell and Fuentes (1975) and Connell ( 1975). Thus, observation is useful as a first step to suggest hypotheses, which then need to be tested. Reynoldson and Bellamy (1974) suggested that manipulation of species in a community is the best way to discover if competitive interactions affect use of niche space. By removing birds during the breeding season, others have shown that conspecifics may limit breeding space (Stewart and Aldrich 1951, Orians 1961, Jenkins et al. 1964, Holmes 1966). Davis (1973) trapped birds for three winters and determined that Golden-crowned Sparrows (Zonotrichia atricapilla) used a willow thicket area more than Dark-eyed Juncos (Junco hyemalis) . During the winter of the fourth year, he removed the sparrows and found a significant shift in habitat use by the juncos. The guild of bark-foraging birds in deciduous forests of central Illinois during winter includes the Red-headed Woodpecker (Melanerpes erythrocephalus), Red-bellied Woodpecker ( Melanerpes carolinus) , Downy Woodpecker ( Picoides pubescens), Whitebreasted Nuthatch ( Sitta carolinensis), and Brown Creeper (Certhia familiaris). In another study (Williams and Batzli 1979b), we followed a natural experiment in which the dominant member of the guild (Redheaded Woodpecker) was absent during one winter but present in the next. When these woodpeckers occupied our upland study areas during winter, Red-bellied Woodpeckers used lowland areas where Red-headed Woodpeckers were much less abundant. When Red-headed Woodpeckers vacated our upland study areas during one winter because of a mast failure, Red-bellied Woodpeckers freely foraged over our entire upland study site. Similarly, in winters with Red-headed Woodpeckers present, we found Downy Woodpeckers foraging low in the canopy, but in winters when Red-headed Woodpeckers were absent, Downy Woodpeckers tended to forage higher. Here we report the results of a controlled experiment designed to test the following hypotheses generated from these earlier studies: ( 1) The aggressive actions of Red-headed Woodpeckers prevent Red-bellied Woodpeckers from using upland habitat during the winter. (2) Red-headed Woodpeckers constrain Downy Woodpeckers to forage lower in the canopy in upland areas. (3) Redheaded Woodpeckers also prevent Whitebreasted Nuthatches from using some upland habitat.