fluctuations must be movement. Some species, such as Gyrfalcon Falco rusticolus and Ferruginous Hawk Buteo regalis, fluctuate in breeding densities from year to year in some regions, but it is not known whether individuals change their breeding places over long distances from year to year.
Some species that exploit cyclically fluctuating vole populations move around mainly within the breeding range, as exemplified by the Tengmalm's Owl Aegolius funereus and Northern Hawk Owl Surnia ulula in forest. In other species, parts of the population migrate to lower latitudes in winter, thereby avoiding the worst effects of snow cover, and return to the breeding range each spring, settling in areas where voles are numerous at the time. This pattern is exemplified by the Short-eared Owl Asio flammeus, Long-eared Owl A. otus, Common Kestrel Falco tinnunculus and Northern Harrier Circus cyaneus over much of their breeding ranges (Hamerstrom 1969, Korpimaki & Norrdahl 1991). All these species hunt on the wing, a relatively high-energy method compared to the sit-and-wait methods of some other rodent-eaters (Sonerud 1984). Their expensive hunting methods may be why they tend to leave areas with prolonged winter snow cover. Such species may therefore re-distribute themselves in relation to vole densities at least twice each year; first in autumn when they search for and settle in good vole areas for the winter, and again in spring when they either stay where they are and breed if vole densities permit, or move on in search of another good area. In some regions, snowfall can modify this pattern because under deep snow voles can become unavailable to most northern owl species (the Great Grey Owl Strix nebulosa is exceptional in being able to penetrate up to 40 cm of snow). In their
Table 19.1 Established year-to-year correlations between bird abundance, emigration and food supply in rodent-eating birds
Hen (Northern) Harrier Circus cyaneus Rough-legged (Hawk) Buzzard Buteo lagopus
Eurasian (Common) Kestrel Falco tinnunculus
Black-shouldered Kite Elanus axillaris Nocturnal
Short-eared Owl Asio flammeus Long-eared Owl Asio otus Great Grey Owl Strix nebulosa
Snowy Owl Nyctea scandiaca
Northern Hawk Owl Surnia ulula Tengmalm's (Boreal) Owl Aegolius funereus
Summer Winter Autumn References emigration
Potapov (1997) Cavé (1968), Rockenbauch (1968), Hagen (1969), Korpimäki & Norrdahl (1991), Village (1990)
Malherbe (1963), Mendelsohn (1983)
Village (1987), Korpimäki & Norrdahl (1991) Village (1981), Korpimäki & Norrdahl (1991) Hildén & Helo (1981), Nero et al. (1984), Duncan
(1992, 1997), Bull & Duncan (1993) Shelford (1945), Chitty (1950), Parmelee (1992),
Newton 2002 Korpimäki (1994) Korpimäki & Norrdahl (1989)
movement patterns, the specialist rodent-eaters thus parallel the boreal seed-eaters discussed in the Chapter 18.
On the northern tundras, some species of skuas also eat microtine rodents. These birds spend most of their lives at sea, but return to the tundra to breed. They seem to return to the same breeding areas each year, but nest only if rodents are plentiful, remaining as non-breeders in other years. This pattern has been found in Pomarine Skuas Stercorarius pomarinus in Alaska (Pitelka et al. 1955, Maher 1970) and in Long-tailed Skuas S. longicaudus in northern Europe (Andersson 1976). In the latter, a similar number of pairs returned to the study area each spring, but the percentage that bred varied from 0% to 100%, according to rodent abundance, giving nest densities over nine years of 0-60 per 100 km2. In years of low rodent numbers the birds took a range of different foods, including birds' eggs, but in years of high rodent numbers they ate almost nothing else. At a number of localities across the Eurasian tundra, the numbers of nesting Pomarine and Long-tailed Skuas found in one year were correlated with local lemming numbers at the time, as were the local numbers of Snowy Owls Nyctea scandiaca and Rough-legged Buzzards Buteo lagopus, although the latter two also avoided one another (Wiklund et al. 1998).
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