Lack of Nest Site Limitation in a Cavity-Nesting Bird Community

Abstract

We examined the relationship between nest site availability and density of secondary cavitynesting birds by blocking cavities in an oak-pine (Quercus spp.-Pinus sp.) woodland. 1986 and 1987 we blocked 67 and 106 cavities, respectively, on a 37-ha plot. The combined density of secondary cavity-nesting birds did not decline in either year by a greater proportion on the treatment plot than on a control plot, indicating that cavities were not limiting. habitats where timber management has not substantially reduced availability of natural cavities, managers should not assume nest site limitation; natural nest site availability should be evaluated before implementing nestbox programs designed to increase populations of secondary cavity-nesting birds. J. WILDL. MANAGE. 54(2):239-245 Knowledge of factors that limit species' abundances is needed to formulate management guidelines and to predict effects of habitat alteration. Nest site limitation of secondary cavity-nesting birds (SCNB), i.e., cavity nesters that do not excavate their own cavity, is often assumed rather than hypothesized (e.g., von Haartman 1957, Hilden 1965, Thomas et al. 1979). his paper on the evolution of the cavity-nesting habit in birds, von Haartman (1957: 339) stated, In the case of hole-nesters it seems obvious that the number of holes, and not the amount of food, mostly acts as an ecological limiting factor, determining the maximum number of nesting pairs., Much of the evidence indicating nest site limitation has come from European nestbox studies (see von Haartman 1971 for review). Bruns (1960) reported that densities of some European SCNB species have been increased 5-20 times by adding nestboxes. Natural nest site availability throughout much of Europe, however, has been substantially reduced by long-term intensive forest management (Bruns 1960, Haapanen 1965, Slagsvold 1978). Many North American studies have documented nestbox use (e.g., Hamerstrom et al. 1973, McComb and Noble 1981, Savard 1988). However, we are aware of few controlled nestbox experiments, i.e., studies in which preand posttreatment densities were estimated from bo h treatment and control plots (Brush 1983, Brawn and Balda 1988). Dahlsten and Copper (1979) also used nestboxes to study population characteristics of SCNB's, but they did not obtain premanipulation estimates of abundance on nestbox and control plots. Nestbox use does not necessarily indicate nest site limitation; use can be merely compensatory rather than additive (see van Balen et al. 1982, Nilsson 1984a, Gauthier and Smith 1987). Our objective was to test the hypothesis that breeding densities of SCNB's are limited by nest site availability at a site where the number of cavities was essentially unaffected by man. Rather than test the hypothesis by increasing n t site availability with nestboxes, we reduced ne t site availability by blocking cavities. D. A. Duncan provided much support and assistance throughout this study. We thank J. C. Lovio and B. R. North who censused the plots in 1987. Thanks also to S. P. Duncan and W. A. Patrick at the San Joaquin Experimental Range and J. D. Stuart at Humboldt State University. Our study was partially funded by the U.S. Forest Service Pacific Southwest Forest and Range Experiment Station, the California Integrated Hardwood Range Management Program, the North American Bluebird Society, and Sigma