Community ecology and management of wintering grassland sparrows in Arizona

Abstract

This dissertation presents a four year field study on the movement patterns, community dynamics, and management of granivorous wintering grassland sparrows in Arizona. Chapter one focuses on within winter, local scale movement patterns. Recapture statistics and radiotelemetry both showed strong interspecific differences in movement, consistent with the idea that these species may partition niche space according to the regional coexistence mechanism. Both techniques ranked species from most to least sedentary as follows: Cassin's and Grasshopper sparrows, Baird's, Vesper, and Savannah and Brewer's sparrows. Data also indicated that fixed home range movements, and within-species constancy of movement behavior across years and study sites are generally the rule in this group. Correlations between bird abundance and summer rainfall suggest that movement may constrain large scale habitat selection processes. Chapter two presents larger scale movement data from grassland sparrows, along with a general discussion of facultative migration in birds. High between-year abundance fluctuations and low and variable rates of between-year recapture suggest that facultative migration strategies may be the rule in grassland sparrows. The use of alternative wintering sites by individual Grasshopper Sparrows provides direct evidence of limited facultative migration behavior. These patterns contrast with the largely non-facultative migration strategies that are the rule in birds. The evolution of facultative migration strategies is linked with unpredictable temporal variation in the spatial distribution of habitat conditions in the landscape. Chapter three presents three years of data on the effects of spring/summer burning and cattle grazing on wintering grassland sparrows. Vesper and Savannah sparrows responded positively to fire, while Cassin's Sparrows responded negatively. The ecologically and geographically restricted Baird's and Grasshopper sparrows utilized burned areas during the first post-bum winter and did not significantly respond to fire. Both Ammodramus sparrows also utilized the grazed pasture; they were more abundant there than in the ungrazed study area in one year. While field observations and a prior study suggest that heavy grazing can have a strong detrimental effect on Ammodramus sparrows, the results of this study suggest that moderate cattle grazing may be compatible with the conservation of these species.