The Swainson's Hawk (Buteo swainsoni) is a highly gregarious species that can migrate in large flocks of up to 5000–10,000 individuals (Bechard et al. 2020). Flocks of hundreds of birds have been observed feeding on grasshoppers in late August and early September en route to their wintering areas (Bechard et al. 2020). Smaller flocks may form in late summer, prior to the onset of migration. Such pre-migration aggregations of Swainson's Hawks have been reported in several studies (Fisher 1893, Bent 1937, Woffinden 1986, Johnson et al. 1987). Here we document an early aggregation of Swainson's Hawks in southwestern Idaho that occurred from early July through early August 2021.
In the first week of July 2021, we sighted flocks of Swainson's Hawks of up to 20–50 individuals soaring over an area approximately 24 km southwest of Boise, Idaho, USA (43°29.876′...