Grasshopper Consumption and Summer Flocks of Nonbreeding Swainson's Hawks
Over the past 100 years, several large flocks of Swainson's Hawks (Buteo swainsoni) have been reported in western North America. The accounts describe what appear to have been premigratory aggregations of up to 200 hawks gathered in late summer in locations experiencing grasshopper infestations (Fisher 1893, Bent 1937, Taylor 1946, Woffinden 1986). In June 1985, five flocks (X = 76 birds) of immature and adult Swainson's Hawks were observed in southern Saskatchewan and Idaho in areas undergoing grasshopper outbreaks, but, in this case, flocking occurred a full 2 months earlier than previously reported.
Johnson, C. G.; Nickerson, L. A.; and Bechard, Marc J.. (1987). "Grasshopper Consumption and Summer Flocks of Nonbreeding Swainson's Hawks". The Condor, 89(3), 676-678. http://dx.doi.org/10.2307/1368663