Artificial Trees as a Cavity Substrate for Woodpeckers

Abstract

Fifty 242-cm-high X 22-cm-diameter polystyrene cylinders were placed in a 65-ha woodlot in central Ohio. Free-ranging downy woodpeckers (Picoides pubescens) excavated 51 roost cavities in 42 cylinders. Cavity depth was negatively correlated with seasonal temperature, and entrance hole area was positively correlated with temperature. Entrances of cavities excavated in the winter were oriented away from the predominant wind direction, whereas entrances of cavities excavated in the summer were randomly oriented. Carolina chickadees (Parus carolinensis) and house wrens (Troglodytes aedon) used 2 and 28 cavities as nest sites, respectively. Laboratory results suggest that excavating in polystyrene is not deleterious to the health of the woodpeckers. Artificial substrates may provide a new method of population management for forest birds and could be useful for investigating cavity parameters, breeding biology, and territoriality of primary cavity nesters. J. WILDL. MANAGE. 47(3):790-798 A basic problem confronting forest managers is the damage done to natural populations of cavity-nesting animals by clear-cutting (Webb 1977, Thomas et al. 1979). In accordance with fire and safety regulations, and to increase yield, foresters often selectively cull dead and dying trees from commercial forests (Evans and Conner 1979, Mannan et al. 1980). Unfortunately, this procedure removes those trees that provide shelter for avian and mammalian cavity nesters (Scott 1979). The loss of these species represents more than a moral problem for silviculturists, because many of the avian species are important natural predators of forest pest insects (Bruns 1960, Bent 1964, Evans and Conner 1979, Holmes et al. 1979). In the intensively managed forests of Germany and Russia, nest boxes have been used for decades to increase populations of cavity nesters (Franz 1961, Thomas et al. 1979). This method has increased some species' densities 5to 10-fold in a given woodlot (Franz 1961). It also has kept woodlots green while neighboring tracts were completely defoliated during heavy insect outbreaks (Bruns 1960). Nest boxes attract secondary cavity nesters, those species that nest and/or roost in natural or artificial cavities but do not excavate their own. Birds of this group usually occupy cavities abandoned by primary cavity nesters. Primary cavity nesters include woodpeckers and a few species of chickadees and nuthatches (Thomas et al. 1979). Most woodpeckers rarely occupy a site they themselves have not excavated (Short 1979). Therefore, nest boxes should not increase woodpecker populations. Woodpeckers are a valuable asset not only because they provide nest and roost holes for secondary cavity nesters, but because they are significant predators of many forest insect pests (Donley and Acciavatti 1980). In 1 winter during a local spruce beetle outbreak in Colorado, woodpeckers eliminated almost half the entire beetle brood (MacCambridge and Knight 1972). Woodpeckers are a biological control against woodborers and are not ecologically disruptive, as pesticides may be. I Reprints may be requested from this author. 790 J. Wildl. Manage. 47(3):1983 This content downloaded from 207.46.13.122 on Fri, 01 Apr 2016 04:14:51 UTC All use subject to http://about.jstor.org/terms ARTIFICIAL TREES FOR WOODPECKERS * Peterson and Grubb 791 All North American woodpecker species fashion cavities in snags, those trees that foresters and woodcutters often remove from woodlands (Kilham 1971, Gysel et al. 1972). The interiors of these dead trees usually are softened with fungal heart rot, thus enhancing the excavation process while leaving a hard outer layer for protection against predators (Conner et al. 1976). In most woodlands, the number of available nest sites is the main factor limiting cavity-nester densities (Franz 1961, Mannan et al. 1980). If silviculturists continue to remove an excessive number of snags, populations of primary cavity nesters-and, therefore, numbers of secondary cavity nesters-will decrease. The fallen limbs left behind in clear-cut areas are prime beetle breeding grounds and often lead to local epidemics (MacCambridge and Knight 1972). The absence of entomophagous birds may exacerbate such outbreaks. Snag trees should be left standing in forest plantations and clear-cuts, or replaced with practical alternatives. From November 1979 through September 1980, we tested the efficacy of polystyrene bead board as an alternate roost and nest cavity substrate for woodpeckers. The study was conducted in a deciduous woodlot in central Ohio and was designed to determine to what extent woodpeckers would excavate cavities in polystyrene cylinders. We recorded the characteristics of cavities made in artifical trees and studied the selection of habitat surrounding cavity sites under conditions where the composition of all the cavity sites was controlled. We noted to what extent secondary cavity nesters occupied woodpecker excavations in the substrate snags and monitored reproductive performance. Finally, we investigated the effect, if any, of excavating in polystyrene on the health of woodpeckers. They carry excavated material in their bills and, perhaps, ingest a small quantity. This portion of the project was performed in the laboratory with downy woodpeckers. We thank T. Berner, W. D. Peters, C. Petrovic, G. Sattler, M. Stanley, and A. Thompson for field assistance; D. Aseltine for statistical consultation; and R. Gifford and the heirs of S. Finkbone for the use of their woodlots. D. A. Airola, T. H. Bookhout, T. J. Peterle, L. S. Putnam, and an anonymous referee commented on the manuscript. The research was supported by funds provided by the USDA Forest Service, Northeastern Forest Experiment Station.