Other striking examples of the link between widespread winter emigration and food supply are provided by the Northern Goshawk Accipiter gentilis and the Great Horned Owl Bubo virginianus, which feed on Snowshoe Hares Lepus americanus. The fluctuations in Snowshoe Hare numbers across North America have been well recorded for more than 200 years, initially from the number of hare pelts provided each year to the fur market, and latterly from detailed studies (Elton 1942, Keith 1963, Krebs et al. 2001). However, unlike the situation in rodents, the hare cycle appears synchronised over much of boreal North America, with populations across the continent peaking in the same years, at roughly 10-year intervals (Keith & Rusch 1988, Krebs et al. 2001). In particular localities, peak densities can exceed troughs by more than 100-fold (Adamcik et al. 1978). In addition to the Northern Goshawk and the Great Horned Owl, other predators of Snowshoe Hares include Northern Hawk Owls which eat young hares, as well as rodents.
The Great Horned Owl seems to show much greater numerical fluctuation in the north of its range, where it depends primarily on Snowshoe Hares, than further south where it has a wider range of prey, but I know of no detailed studies in the southern parts. Because Snowshoe Hares seem to fluctuate in synchrony over their whole range, there would be little advantage in Great Horned Owls in northern regions breeding in widely separated localities in different years. Any that left one area because of a shortage of hares would be unlikely to find many more elsewhere (thus contrasting with the situation in rodent-eaters). If they moved southwards, and attempted to breed outside the range of the Snowshoe Hare, they would come up against other Great Horned Owls, already breeding there. Nevertheless, the hare-eaters do perform massive southward irruptions, abandoning their northern breeding areas at least for the winter, and returning there in summer when alternative prey are available.
Because Goshawks fly by day and are more readily seen, their invasions have been better documented than those of owls. They occur for 1-3 years at a time, but at about 10-year intervals (Table 19.5), coinciding with known lows in the Snowshoe Hare cycle (Keith 1963, Mueller et al. 1977, Keith & Rusch 1988). It is usual for juveniles to predominate among migrants in most years when relatively few birds leave the breeding range, but for adults to predominate in invasion years, which follow poor breeding seasons. Thus, among more than 12 000 Goshawks trapped over a number of years at Cedar Grove in Wisconsin, juveniles formed more than 80% of the total in most years, but less than 50% in the invasion years of 1962-1963 and 1972-1973 (Mueller et al. 1977).
Migrating Great Horned Owls fly by night, and because they move into more southern regions already populated by resident Great Horned Owls, their irruptions have been less well documented. However, all those that I could find recorded coincided with Goshawk invasions, and hence with low hare numbers, again providing indications that food shortage stimulated large-scale emigration in this mainly resident species (Table 19.5). In a study at Kluan, Yukon Territory, annual emigration rates of radio-marked Great Horned Owls increased over a period of years from 0 to 33% for territory holders (N = 2-54 in different years) and from 0 to 40% for non-territorial floaters (N = 2 — 18 in different years), as hares declined (Rohner 1996). To judge from ring recoveries, migration of Great Horned Owls from Saskatchewan is mainly to the southeast, but over greater distances in years with low hare numbers, with some individuals travelling more than 1000 km from their breeding sites (Houston & Francis 1995, Houston 1999).
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