The two responses to change in food supply (delayed and simultaneous) found among rodent-eaters are not completely distinct, and different species of owls and raptors may be better described as forming a gradient in response, from the most sedentary at one end to the most mobile at the other. Moreover, the same species may show regional variation in movement behaviour depending on food supply, and the extent to which alternative prey are available when favoured prey are scarce. Examples of regional variation among diurnal raptors include Common Kestrel Falco tinnunculus and Hen (Northern) Harrier Circus cyaneus, in which the proportion of rodents in the diet differs from region to region. The more varied the diet, the less the chance of all prey types being scarce at the same time, the more stable are their local breeding densities and the greater the site-fidelity shown by individuals. Examples among owls include the Tengmalm's Owl Aegolius funereus, which has been described as a resident generalist predator of small mammals and birds in central Europe, as partially nomadic (with males mainly resident and females moving around) in south and west Finland, and as a highly nomadic microtine specialist in northern Fennoscandia, in areas with pronounced vole cycles and fewer alternative prey (Korpimaki 1986).
Interestingly, the return rates of adults to former nesting areas can vary regionally within species, according to the degree of year-to-year stability in food supply. In comparison with the return rates mentioned above for Kestrels in Finland of 13% males and 3% females, Village (1990) recorded rates of 29% and 18% for males and females in Scotland, and of 43% and 36% in southern England, commenting that the more sedentary nature of the English population was 'due to the greater stability of the food supply both within and between years'. In a study in the Netherlands, as many as 70% of adult Kestrels remained from year to year when vole numbers were on the increase, and as few as 10% when vole numbers crashed (Cavé 1968). Similarly, the Long-eared Owl Asio otus shows greater year-to-year site-fidelity in the Netherlands than in Finland (Wijnandts 1984, Korpimaki 1992), as does the Great Grey Owl Strix nebulosa in some parts of North America compared with others (Collister 1997, Duncan 1997), while the Barn Owl Tyto alba is highly sedentary in Britain (Taylor 1994) but more dispersive in parts of continental Europe (where movements up to 2000 km have been recorded) and in parts of North America (Bairlein 1985b, Marti 1999).
In addition to the owl species mentioned earlier, mean distances between birthplace and breeding place were greater for the Common Kestrel Falco tin-nunculus (277 ± 57 km) and Common Buzzard Buteo buteo (295 ± 105 km) in northern and eastern Europe than in western and central Europe (146 ± 39 and 60 ± 11 km) (Galushin 1974). Prey remains indicated that microtine rodents form a much greater proportion of the diets of these raptors in northern and eastern Europe than in western and central Europe. In their regional variation in movement behaviour, owls and raptors show parallels with some of the seed-eaters discussed in the previous chapter.
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