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

Abstract

Abstract Henslow's Sparrows (Ammodramus henslowii) are reported to have irregular patterns of return to breeding areas. I present data supporting these reports at rangewide extents, while testing potential biases inherent in the North American Breeding Bird Survey data. Two measures of population variability were used to show that Henslow's Sparrows are less likely to use breeding areas predictably and consistently, but have similar variance in numbers at occupied sites relative to other sympatric grassland sparrow species. I illustrate how restricting analyses to single-observer-collected Breeding Bird Survey data results in subtle but significant effects not detected in data aggregated from multiple observers through the study period. The most conservative analysis (single-observer, restricted distribution) showed that Henslow's Sparrows exhibited lower prevalence of occurrence than Grasshopper (Ammodramus savannarum) (P < 0.001) and Savannah (Passerculus sandwichensis) (P < 0.001) sparrows but no diffe...