Breeding Patterns of Henslow's Sparrow and Sympatric Grassland Sparrow Species

Spatial Scaling of Prevalence and Population Variation in Three Grassland Sparrows

“…1), which subsumes the site-to-site variation that is the subject of this paper. Although Grasshopper and Savannah sparrows have wider breeding distributions, restricting the area analyzed to the distribution of Henslow's Sparrows reduces variation resulting from irrelevant processes manifested beyond the range of the focal species (Dornak 2010). The reduced area covers 2.1 × 10 6 km 2 and is dominated by agriculture, both pasture and croplands, interspersed with patches of woodlands, shrub, wetlands, urban and developed areas, fallow pastures, and native prairie.…”

“…For example, for reasons unclear, this species may be present one year but not return to the area in subsequent years, even though the vegetation appears unchanged. Dornak (2010) found that on BBS routes, Henslow's Sparrow breeding sites were less predictable from year to year than those of the Grasshopper (A. savannarum) and Savannah (Passerculus sandwichensis) sparrows. That study concluded that Henslow's Sparrows were possibly nomadic because birds were not consistently resettling at the same sites for breeding.…”

“…Each year at a time appropriate for the region (~June in the range of Henslow's Sparrow), observers record all birds seen or heard at each stop for 3 min . We used 50 BBS routes per species, representing almost all routes (see below) on which Henslow's Sparrows have been detected and paralleling past analyses (Dornak 2010). Although the system is not without inherent bias, the BBS is the only standardized database for rangewide studies of North American birds, so BBS data are most appropriate as the basis for our analyses.…”

“…Although the system is not without inherent bias, the BBS is the only standardized database for rangewide studies of North American birds, so BBS data are most appropriate as the basis for our analyses. To control for biases in observer consistency (Dornak 2010), we used only data from BBS routes run by a single observer across the study period, which reduced the sample size of routes on which Henslow's Sparrow has been detected to 50. We used two metrics to describe the species' yearly resettlement patterns: prevalence of occurrence and coefficient of variation in abundance (Dornak 2010).…”

“…Nomadism can occur among individuals, groups of individuals, or entire populations (Andersson 1980), and across multiple spatial scales (Dean 1997). It is most commonly observed in species that live in highly variable environments, such as the North American grasslands (Bragg 1995), where the ability to track resources (e.g., food or suitable nesting sites) and colonize new breeding areas quickly in response to changing environmental conditions is advantageous (Cody 1985, Igl andJohnson 1999) Dornak (2010) analyzed nomadism only at one spatial scale, so it remains unclear if these grassland sparrows are nomadic over multiple scales and (most importantly) at what scales the differences in prevalence between them are manifested. In this study we investigate these questions by comparing the occurrence and variation in abundance patterns of Henslow's, Grasshopper, and Savannah sparrows at spatial resolutions spanning six orders of magnitude (0.5-511 360 km 2 ).…”