SPECIES-AREA RELATIONS OF BIRDS ON SMALL ISLANDS IN A MINNESOTA LAKE.

Although the publication of MacArthur and Wilson's theory of island biogeography (1963, 1967) stimulated a great deal of interest in island biogeography, relatively few studies have concentrated on islands that are small in relation to the size or home range of the species being studied. In this paper we discuss the speciesarea relations of breeding bird populations on 52 small islands each supporting two to 21 species. The islands are located on Burntside Lake in northeastern Minnesota and range in size from 0.04 ha to 17.2 ha. Most of the smaller islands are dominated by stands of red pine and white pine, while some of the larger islands also support aspen-birch-fir associations. All islands are located within 0.4 km of the mainland or other islands, and many are much closer. Forty-nine islands were censused once during May and June, 1972 (Rusterholz, 1973; Appendix 1). From late May to July, 1975, seven islands (including three islands not previously censused) were each visited nine to fourteen times as part of a more intensive study of species foraging patterns (Howe, 1977). Summaries of results from the latter study are presented in Tables 1 and 2 and in Appendix 1. Because it was found that a large proportion of the species observed on the smallest islands were transients or irregular visitors (Table 2), only those species that were recorded during at least 40% of the 1975 censuses were used in determining speciesarea relations. Large islands were visited proportionally longer than small ones, and in both studies it is unlikely that many species were undetected. It has long been known that the number of species (S) on islands is closely correlated with area, but this relationship is commonly expressed in two different ways. One approximation is the power function, S = cAz, where S is the number of species in a particular group, A is area, c is a constant, and z is the slope of the regression line when plotted on a log-log scale (Preston, 1962; MacArthur and Wilson, 1967). Higher z values coincide with increased dependence of species number on area. The relation between species and area has also been expressed as an exponential function (Gleason, 1922; Kilburn, 1966). This function yields a straight line when species number is plotted against the log of area. Although it has been shown that species-area data for birds on a number of archipelagos more closely approximate an exponential function (Diamond and Mayr, 1976; Schoener, 1976), our data provide a slightly closer fit to the power function. (Standard errors of estimate for the exponential and power functions are 5.73 and 5.53 respectively.) Reviews of z values for a variety of island groups suggest that most z values range between 0.20 and 0.35, regardless of the species group considered (MacArthur and Wilson, 1967; May, 1975). Larger values have been reported only for land bridge islands and mountaintops where species numbers are thought not to be at equilibrium (Brown, 1971; Diamond and Mayr, 1976), for birds on small islands along the Maine coast, and for ants on small islands with considerable areas of bare rock (Goldstein, 1975). In the latter case Goldstein (1975) found that if only the vegetated area of the islands is considered, a much lower z value is obtained. A z value lower than those usually reported was calculated for New Hebridean