Is density an indicator of breeding success

-A new method of measuring reproductive success was used to test Van Horne's (1983) contention that reliance on population density as a measure of habitat quality can produce misleading results. None of the three emberizine sparrows in this study showed a clear correlation between high territory density and high reproductive success. Grasshopper Sparrows (Ammodramus savannarum) were most successful in medium-density plots, and did poorly when territory density was low. Savannah Sparrows (Passerculus sandwichensis) were significantly more successful at low density, and were least successful at high density. Vesper Sparrows (Pooecetes gramineus) were the only species whose reproductive success was unaffected by territory density and, thus, for which density would have been a reasonable measure of habitat quality. These results lend support to Van Horne's thesis that the singular use of density may be a poor indicator of habitat quality and that additional factors should be considered. Received 5 March 1991, accepted 10 February 1992. POPULATION DENSITY frequently is used as an indicator of a habitat's quality, because a greater number of individuals within a given area is generally thought to reflect larger amounts of the necessary resources to sustain a species (Van Horne 1983). By extension, it often is assumed that concentration of resources allowing a high population density also will be reflected in greater reproductive success, or other indices of fitness. Because density is easier to measure than fitness, Flood et al. (1977) and others who model habitat relationships have used density to make inferences about habitat quality, although not necessarily with a high degree of predictive success (Lancia et al. 1982, Bart et al. 1984, Maurer 1986). Van Horne (1983) questioned the singular reliance on density as an indicator of habitat quality and suggested that inferences about which habitats support individuals with the highest fitness often may be false. She suggested that, from a populationviability and management perspective, it is preferable to define habitat quality as a product of density and reproductive success. To test the hypothesis that density is a reasonable index of nesting habitat quality for three grassland sparrows, we examined the relationship between 3Present address: Conservation Department, Massachusetts Audubon Society, Lincoln, Massachusetts 01773, USA. 4Present address: Department of Natural Resources, Field of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853, USA. breeding densities and reproductive success for each species. METHODS The study site was located on a sandplain grassland in Kennebunk, York County, Maine (43024'N, 70? 59'W). We studied three emberizine sparrows: Vesper Sparrow (Pooecetes gramineus); Savannah Sparrow (Passerculus sandwichensis); Grasshopper Sparrow (Ammodramus savannarum). Vickery et al. (1992) provided additional information on site and methods. The site was managed for commercial blueberry production with one-half of the area mowed and burned on a biennial rotation. This management created profound year-to-year changes in the vegetation structure with a concomitant influence on habitat quality. These habitat manipulations allowed us to measure the avian response to these year-to-year changes in each plot. To examine the relationship between territory density and reproductive success, territories were "spot mapped" (International Bird Census Committee 1970) using eight replicate censuses on eight plots (8-24 ha) for three years (1984-1986). For each species, plots were assigned to a class depending on territory density: low, '2.5 territories per 10 ha; medium, >2.5 to <3.5 territories per 10 ha; high, 23.5 territories per 10 ha. There were at least four plots for each density category. The reproductive-index rank (Vickery et al. 1992) for each territory was assigned to the appropriate density category. We then used the KruskalWallis test (Conover 1980:229) to determine if there were differences in the reproductive-index rankings within each density classification. We measured percent cover for nine vegetation features for high-success and low-success Grasshopper, Savannah and Vesper sparrows, and also measured the same parameters on the unoccupied habitat in each plot. To evaluate whether Savannah Sparrows