Spatiotemporal variability of populations and suitable breeding habitat in three grassland sparrows: a multiscale approach

Henslow's Sparrows (Ammodramus henslowii) are not only uncommon, but they are also unpredictable in grasslands that appear suitable. Furthermore, the extent, distribution, and year-to-year variability of their breeding habitat has not yet been characterized. In this dissertation, resettlement behavior was analyzed using two measures of population variability, prevalence of occurrence and variation in abundance, first at the Breeding Bird Survey (BBS) route level (Chapter 2), then across multiple spatial resolutions (Chapter 3). Variability of Henslow's Sparrow populations was compared to that of two other grassland sparrow species, Grasshopper Sparrows (A. savannarum) and Savannah Sparrows (Passerculus sandwichensis). In both analyses, Henslow's Sparrows showed lower prevalence and higher variation in abundance than the other two species at the BBS route level and across all but the broadest spatial resolutions. Henslow's Sparrows do not occur consistently at extents of less than 120,000 km2, suggesting nomadic behavior. To relate patterns of habitat turnover to the nomadic behaviors described in the first two research chapters, ecological niche models were used to identify the extent and distribution of suitable breeding area across three years (2008-2010), and then to characterize turnover in suitability between 2 sets of years (2008-2009 and 2009-2010; Chapter 4). Turnover varied across time and species, such that losses and gains fluctuated in dominance within species and between years. Turnover of both gains and losses was similar among species and relatively low in 2008-2009; however, in 2009-2010, Henslow's Sparrows showed much higher losses and lower gains, and more clustering of this turnover across broad regions compared to the other two species. Overall, conversion (i.e., state change) of grasslands was relatively low, accounted for a greater proportion of losses than gains of suitable area for all species, and varied within losses among species. In variable years when turnover is high, Henslow's Sparrows may be more nomadic compared to more static years, when turnover and movements are low. Results presented here provide a more complete understanding of the broad-scale dynamics of Henslow's Sparrow populations and their habitat, information that may be key to successful conservation of this and ecologically similar grassland species