. The Canadian field-naturalist. Natural history. The Canadian Field-Naturalist Volume 115, Number 4 October-December 2001 Great Horned Owl, Bubo virginianuSy Predation on Rj Ground Squirrels, Spermophilus richardsonii Gail R. Michener Department of Biological Sciences, University of Lethbridge, Lethbridge, a; email Michener, Gail R. 2001. Great Horned Owl, Bubo virginianus, predation on Richardson's Ground Squirrels, Spermo- philus richardsonii. Canadian Field-Naturalist 115 (4): 543-548. Although diurnal squirrels often form a minor component of the prey found in regurgita
. The Canadian field-naturalist. Natural history. The Canadian Field-Naturalist Volume 115, Number 4 October-December 2001 Great Horned Owl, Bubo virginianuSy Predation on Rj Ground Squirrels, Spermophilus richardsonii Gail R. Michener Department of Biological Sciences, University of Lethbridge, Lethbridge, a; email Michener, Gail R. 2001. Great Horned Owl, Bubo virginianus, predation on Richardson's Ground Squirrels, Spermo- philus richardsonii. Canadian Field-Naturalist 115 (4): 543-548. Although diurnal squirrels often form a minor component of the prey found in regurgitated pellets of Great Homed Owls, direct observation of the circumstances that result in a nomially diurnal species appearing in the diet of a normally noctur- nal predator is rare. During a long-term study of the behavioural ecology of Richardson's Ground Squirrels (Spermophilus richardsonii) in southern Alberta, I observed Great Homed Owls capture 14 adult ground squirrels in late February-early March and six juveniles in mid-summer. Temporal overlap between predator and prey occurred in two ways, through a shift toward twilight activity by estrous female and male ground squirrels during their mating season in spring and through a shift to daylight hunting by owls in summer. Twilight attacks were launched from the tops of objects such as grain augers and fence posts, whereas daylight attacks were launched from within the canopy of a tree. Key Words: Bubo virginianus, diurnal hunting. Great Horned Owl, owl pellets, predator-prey relations, Richardson's Ground Squirrel, Spermophilus richardsonii. Common mammalian prey items in the diet of Great Homed Owls (Bubo virginianus) range in size from mice (Peromyscus) and voles (Microtus) to rab- bits (Sylvilagus) and hares (Lepus) (Houston et al. 1998). Although almost all sciurids fall within this size range, squirrels generally account for a minor fraction (usually <2% of prey items) of the diet of Great Homed Owls, even in areas
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