Evidence of movements from ringing and radiotracking

The implication from local fluctuations in the breeding densities of many species -that individual adults may nest in widely separated localities in different years -is supported by ring recoveries, although the proportions of birds that move can vary from year to year, depending on food conditions. As in the seed-eaters, evidence for movements comes partly from the high turnover among birds caught each year in the same localities. In all the species listed in Table 19.1 that were studied in this respect, return rates were extremely low, compared to what would be expected from their annual survival rates. Among Common Kestrels Falco tinnunculus, of 146 individual breeders trapped and ringed in a 63 km2 area in Finland over an 11-year period, only 13% of males and 3% of females were found back in the same area in a later year (Korpimaki & Norrdahl 1991). The implication is that a large proportion of breeders changed their nesting localities from year to year.

Other evidence for widescale movements stems from adults that were found in widely separated areas in different breeding seasons. Most information of this type relates to the Tengmalm's (Boreal) Owl Aegolius funereus, which nests readily in boxes and has been studied at many localities in Europe (Figure 19.5). In this species, the males in some regions are mainly resident and the females more dispersive. Both sexes tend to stay in the same localities if vole densities remain high, moving no more than about 5 km between the nest boxes used in successive years. But if vole densities crash, females move much longer distances, with many having shifted 100-600 km between breeding sites in different years (Figure 19.5). In contrast, fewer long movements were recorded from males, with only two at more than 100 km. The greater residency of males was attributed to their need to guard cavity nest-sites which are scarce in their conifer nesting habitat, while their smaller size makes them better able than females to catch small birds, and hence to survive (without breeding) through low vole conditions (Lundberg

Figure 19.5 Ringing and recovery sites of adult Tengmalm's Owls Aegolius funereus that were identified in different breeding seasons. Continuous lines -females; dashed lines - males. Only movements greater than 100 km are shown. From Newton (2003), compiled from information in Löfgren et al. (1986), Korpimäki et al. (1987) and Sonerud et al. (1988).

Figure 19.5 Ringing and recovery sites of adult Tengmalm's Owls Aegolius funereus that were identified in different breeding seasons. Continuous lines -females; dashed lines - males. Only movements greater than 100 km are shown. From Newton (2003), compiled from information in Löfgren et al. (1986), Korpimäki et al. (1987) and Sonerud et al. (1988).

1979, Korpimäki et al. 1987). In the following year, when voles become plentiful again, male owls still have their previous nest-sites, and are again able to attract females which move in to exploit the abundant prey. In the northern boreal zone, however, where small birds are scarce in winter, both sexes tend to vacate areas with low vole numbers.

Far fewer records are available for other nomadic owl species, because the chances of recording marked individuals at places far apart are low. However, in a study of Short-eared Owls Asio flammeus in south Scotland, 21 breeders were tagged in 1976. Vole numbers then crashed, and only one of the tagged birds remained to breed in the area in 1977. Two others were reported in spring 1977 in nesting habitat 420 km and 500 km to the northwest, and the latter, at least, was proved to breed there (Village 1987). Of seven breeders tagged in 1977, when vole numbers began to increase, three bred in the area in 1978. Hence, as in Tengmalm's Owl Aegolius funereus, individuals seemed more likely to remain to breed in successive years when voles were increasing than when they were declining.

Recoveries of Northern Hawk Owls Surnia ulula and Great Grey Owls Strix nebulosa ringed as breeding adults include examples of both males and females residing in an area from one nesting season to the next when microtine abundance remained high, and of both sexes leaving when microtine populations declined (Sonerud 1997, Duncan 1992). Adult radio-marked Great Grey Owls in Manitoba and northern Minnesota dispersed 41-684 km (mean 329 km, SD 185 km, N = 27) between breeding sites in response to prey population crashes. Eleven marked birds that did not disperse died (Duncan 1992, 1997). At least one male returned southward to its original home range in the following summer, but did not re-nest then, and two others were found breeding on their original range three years later. At least one female returned, and was found breeding on her original range two years later, and another, after breeding in the north, had set off south in the direction of her original range when her radio stopped working. These observations raise the possibility that these owls are not truly nomadic (in the sense that they may occupy a different area each year), but may shuttle back and forth between two or more regular breeding areas up to several hundred kilometres apart, but more work is needed to check this possibility.

More revealing information is available for four adult female Snowy Owls Nyctea scandiaca which were radio-tagged while nesting near Point Barrow in Alaska, and tracked by satellite over the next 1-2 years (Fuller et al. 2003). These birds mostly stayed in the arctic but dispersed widely in different directions from Point Barrow, reaching west as far as 147 °E and east as far as 116°W, a geographical spread encompassing nearly one-third of the species' Holarctic breeding range. Two birds that bred at Point Barrow in 1999 were present during the next breeding season in northern Siberia (147 °E and 157 °E respectively), up to 1928 km west of Point Barrow, and then in the following breeding season, they were on Victoria Island (116 °W) and Banks Island (122 °W) respectively, in northern Canada (having passed eastward through Point Barrow) (Table 19.2, Figure 19.6). The two birds that bred at Point Barrow in 2000 were present on Victoria and Banks Islands in the breeding season of 2001. The successive summering areas of these four birds were thus separated by distances of 628-1928 km (Table 19.2). From the dates they were present, some could have bred successfully, while others were unlikely to have done so, having arrived too late or left too early. None returned to the same breeding or wintering site used in a previous year, but three passed through Point Barrow in 2001. In winter these Snowy Owls made long-distance moves at various dates; one bird remained continually on the move, venturing as far south as 59 °, but spending no more than a fortnight at any one place, while another remained for 2.5 months in one place. Only two ventured south of the breeding range, one briefly but the other staying for more than two months at 60 °N in southern Alaska. Some spent time in winter on sea- ice far from land, presumably hunting seabirds.

Table 19.2 Locations of four adult female Snowy Owls Nyctea scandiaca in successive breeding seasons. All were radio-tagged at Point Barrow, Alaska in 1999 (numbers 54 and 57) or 2000 (numbers 80 and 81), and tracked by the Argos satellite system

Owl number

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