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Alaskaa (71°N, 156°W)

Banks islandb (73°N, 122°W)

aKnown to have bred; bprobably bred; cunlikely to have bred successfully.

Great circle distances between places where successful breeding was known or probable were: 628 km between Point Barrow and Banks Island (bird 81); 1548 km between Siberia and Banks island (bird 57); and 1928 km between Point Barrow and Siberia (bird 57).

aKnown to have bred; bprobably bred; cunlikely to have bred successfully.

Great circle distances between places where successful breeding was known or probable were: 628 km between Point Barrow and Banks Island (bird 81); 1548 km between Siberia and Banks island (bird 57); and 1928 km between Point Barrow and Siberia (bird 57).

The only other ringing-based records of breeding dispersal known to me involve three adult Long-eared Owls Asio otus in North America, which were found at localities more than 450 km apart in different breeding seasons (Marks et al. 1994), and two adult Great Grey Owls Strix nebulosa in northern Europe which were

Figure 19.6 Movements of a radio-tagged female Snowy Owl Nyctia scandiaca tracked by satellite between the summers of 1999 and 2001. Filled circles show known likely breeding sites in three consecutive years (see Table 19.2), and open circles show other sites where the bird spent more than four weeks at a time. Constructed from information in Fuller et al. (2003).

Figure 19.6 Movements of a radio-tagged female Snowy Owl Nyctia scandiaca tracked by satellite between the summers of 1999 and 2001. Filled circles show known likely breeding sites in three consecutive years (see Table 19.2), and open circles show other sites where the bird spent more than four weeks at a time. Constructed from information in Fuller et al. (2003).

found at localities 300 km and 430 km apart in different breeding seasons (Hilden & Solonen 1987). Regarding natal dispersal, movements exceeding 1000 km have been documented for Short-eared Owls Asio flammeus (up to about 4000 km), Long-eared Owls (up to about 2300 km) and Hawk Owls Surnia ulula (up to 2700 km), as well as for Rough-legged Buzzards Buteo lagopus (up to 2700 km) (Saurola 1983, 1997, 2002, Cramp & Simmons 1980, 1985). Some natal dispersal distances for young owls of various species ringed in Finland are summarised in Table 19.3. They indicate much longer median and maximum distances from irruptive than from 'sedentary' species. Among diurnal raptors, Galushin (1974) summarised data from Russian ring recoveries showing that irruptive (mostly vole-eating) species had much longer dispersal distances than non-irruptive ones. The mean natal dispersal distance of the Rough-legged Buzzard was given as 1955 km (but with no mention of sample size).

These various irruptive owls and raptors thus contrast greatly with more sedentary populations, which exploit more stable food supplies. In such species, adults usually remain in their territories year after year, with only small proportions moving to other territories, usually nearby (for Tawny Owl Strix aluco, Ural Owl S. uralensis and Barn Owl Tyto alba see Saurola 1989, 2002, Petty 1992, Taylor 1994). One consequence of such site-fidelity is strong mate-fidelity, as partners remain together year after year, so long as neither dies or changes territory. Natal dispersal distances are mostly less that 30 km, with few exceeding 100 km: for Tawny Owl see Petty 1992, Saurola 2002; for Ural Owl see Saurola 1987, 2002; for Barn Owl see Taylor 1994; also Table 19.3. In both nomadic and resident species, movements are generally longer in poor food years than in good ones (Taylor 1994, Saurola 2002). Typically, it is the young produced in peak prey years that move furthest, because it is they that experience the crash that follows the peak; they are also more numerous than young produced in other years,

Table 19.3 Average natal dispersal distances (km) of owls ringed as nestlings in Finland and found dead in a subsequent breeding season

Species

Number recovered

Median distances (km)

Maximum distance (km)

Sedentary species

Eurasian Eagle Owl Bubo bubo 563 (20)

Tawny Owl Strix aluco 1126 (1288)

Ural Owl Strix uralensis 538 (1036)

Pygmy Owl Glaucidium passerinum 10 (131)

Nomadic species

Northern Hawk Owl Surnia ululaa 2

Great Grey Owl Strix nebulosa 16 (6)

Long-eared Owl Asio otus 48

Short-eared Owl Asio flammeus 16

Tengmalm's Owl Aegolius funereus 96 (541)

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