Seeing isn’t always believing: visual observations underestimate space use in the eastern Grasshopper Sparrow ( Ammodramus savannarum pratensis )

Abstract

Estimating space use in organisms is important for understanding their basic ecology and for effective conservation and management. Typically, areas that are guarded or defended are referred to as territories, whereas home ranges encompass space use for all activities. We estimated territory and home range size for the eastern Grasshopper Sparrow, a subspecies of conservation concern, at two sites near the northern limit of the breeding range near Ottawa, Ontario, Canada. The average territory size (1.05 ± 0.16 ha) was significantly smaller than the average home range size (2.69 ± 0.51 ha). Similarly, territories overlapped home ranges by an average of 55%, suggesting that visual observations alone may limit our understanding of space use in this species. Territories and home ranges averaged larger on a natural limestone alvar compared to a pasture study site. While space use averaged higher at the alvar site, birds at the pasture site compensated for smaller territories by overlapping more with neighboring conspecifics. Given high aggregation and overlap in space use in this study, we discuss our results in the context of improving survey count accuracy and conservation outcomes in this declining species.

Keywords

• conservation
• home range
• territory
• radio telemetry
• grassland birds