Habitat Quality, Population Density and Habitat-Specific Productivity of Red-Winged Blackbirds (Agelaius Phoeniceus) in Boulder County, Colorado
Vierling, Kerri T., The American Midland Naturalist
ABSTRACT.-I monitored 382 red-4lnged blackbird (Agelaius phoeniceus) nests in natural (wetlands and tallgrass prairie) and human-disturbed (hayfields, roadside ditches) habitats in the vicinity of Boulder, Colorado during 1996 and 1997. I counted blackbirds in a total of 6216 ha and quantified habitat composition to determine both within-habitat population density and regional habitat availability. Population density mirrored habitat-specific productisin of red-winged blackbirds in habitats that had predictable sources of water (wetlands, roadside ditches). However, population density and habitat-specific productivity were decoupled in habitats that were unpredictable in the amount of water present during territory establishment (tallgrass prairie and hayfields, which were flood irrigated). Regional habitat productivity was estimated using data oil habitat-specific productivity; habitat-specific population density and regional habitat availability. Red-winged blackbirds breeding in wetlands in the absence of yellow-headed blackbirds (Xanthocphatus xanthocephahtos) contributed the most young to the regional population, whereas birds breeding in human-disturbed habitats (hayfields and roadside ditches) contributed relatively few young to the regional population. Due to the scarcity of high quality patches in the landscape (e.g., tallgrass prairie), the highest quality, habitats did not always contribute the most young to the regional population.
VanHorne (1983) noted that population density may be a misleading indicator of habitat quality. High population density frequently has been assumed to be correlated with high habitat quality. However, circumstances leading to the decoupling of this pattern may relate both to species characteristics and environmental conditions (VanHorne, 1983). Population density in poor quality habitats (i.e., sink habitats) may be higher than population density in high quality habitats (i.e., source habitats) due to intraspecific dominance interactions. For instance, territory acquisition by dominant males may displace less dominant males to marginal habitats, and this displacement may cause large numbers of individuals to occupy low quality habitats (VanHorne, 1983). In addition to species interactions, environmental conditions such as habitat patchiness in space and/or time may also lead to a decoupling of habitat quality and population density (VanHorne, 1983).
The relationship between habitat quality (e.g., reproductive success and/or productivity) and population density has been directly compared in two recent avian studies. Vickery et al., (1992) examined the population density-habitat quality relationships among three species of grassland birds: the vesper sparrow (Pooecetes gramineus), the savannah sparrow (Passerculus sandwichensis) and the grasshopper sparrow (Ammondramus savannarum). They failed to find any correlations between reproductive success and population density in these three species (Vickery et al., 1992). More recently, Brawn and Robinson (1996) examined the decoupling of population trends in source and sink habitats among neotropical migrants. Using source and sink status (after Pulliam, 1988) they found that long-term census data did not reliably indicate the habitat quality of a site; in particular, they found that sink habitats had higher population densities than expected, due to immigration from outside the study area (Brawn and Robinson 1996). The relationship between population density and habitat quality may have direct implications to the regional population. High quality patches may produce more young on a regional scale than low quality patches; however the importance of those high quality patches to the regional population will depend largely on the use of those patches and their availability in the landscape.
The first objective of this paper was to further examine the relationship between habitat quality and population density of red-winged blackbirds (Agelaius phoeniceus). …