Spatial Ecology and Survival of Swainson's Hawks (Buteo swainsoni) in the Northern Great Plains

Abstract

<p>In recent years, anthropogenic conversion of grassland habitat has raised concerns about the status of breeding Swainson's Hawks (<em><em>Buteo swainsoni</em></em>) in the northern Great Plains region of North America. During 2013–2014, we captured breeding Swainson's Hawks in north-central South Dakota and south-central North Dakota to estimate home-range size, determine adult survival rates during the breeding season, and evaluate habitat selection. We captured, radio-tagged, and monitored 13 Swainson's Hawks in 2013, and captured two additional Swainson's Hawks in 2014. In 2014, seven of 13 individuals initially captured in 2013 returned to the same breeding territory for the 2014 breeding season. Average 95% MCP home-range size in 2013 was 205.4 ha (SD = 135.3 ha, <em>n</em> = 10) and 211.1 ha (SD = 208.8 ha, <em>n</em> = 9) in 2014, and size did not differ between years (<em>t</em>13 = 0.07, <em>P</em> = 0.95), averaging 208.3 ha (SD = 244.9 ha, <em>n</em> = 19 home ranges measured for 12 birds) for the 2 yr of the study. Mean core home-range size (50% MCP) was 78.2 ha (SD =105.9 ha, <em>n</em> = 10) in 2013 and 59.7 ha (SD = 80.7 ha, <em>n</em> = 9) in 2014; core home-range areas also did not differ between years (<em>t</em>17 = −0.46, <em>P</em> = 0.65). Swainson's Hawks did not select habitats in proportion to their availability in 2013 (χ42 = 781.99, <em>P</em> < 0.001) and 2014 (χ40 > 999.99, <em>P</em> < 0.001). In 2013, breeding Swainson's Hawks selected against wetland and grassland habitats and selected for trees as foraging habitats. Similarly, Swainson's Hawks selected against grassland habitats for foraging in 2014. We used known-fate analysis in Program MARK to estimate adult survival during the breeding season. The top-ranked model indicated survival was constant at 0.94 (95% CI = 0.68–0.99) during the breeding season and did not differ between years. Our results suggest that Swainson's Hawks maintain a moderately high degree of breeding-site fidelity and have home ranges smaller than those documented elsewhere, and that their home-range size is influenced positively by the presence of grasslands and negatively by development.</p>