Other irruptive seed-eaters include the Spotted Nutcracker Nucifraga caryocatactes of Eurasia and the Clark's Nutcracker N. columbiana of western North America. These species feed primarily on the fruits of various large-seeded pines, which they cache in the ground in late summer to eat during the ensuing winter into the next breeding season. Their overwinter survival and breeding success vary from year to year in line with the size of seed crops (Lanner 1996). Like crossbills, they make one major annual movement, in July-September (as the new cones form), with occasional irruptions outside the usual range. Having settled in appropriate areas, their strategy of living off food stores ideally requires continued residence until the next crop is ready. Like Crossbills, Nutcrackers breed early in the year, and can rear their young entirely (or almost entirely) on seeds.
The Eurasian N. caryocatactes has several subspecies, each occupying a different part of the range and depending on different conifer species. The most widespread type is the slender-billed N. c. macrorhynchos, which occurs over most of Siberia, and depends on the seeds of the Siberian Stone Pine Pinus sibirica and, further east, the Korean Stone Pine P. koraiensis. In northwestern Europe, where large-seeded conifers are lacking, the thick-billed N. c. caryocatactes lives in mixed deciduous-coniferous areas, but depends in winter mainly on Hazel Corylus avellana nuts, which it caches in late summer in the same way that other subspecies cache pine seeds. Further south, in the Alps, it also uses the hard-shelled Arolla Pine Pinus cembra seeds.
All these nutcrackers adjust to annual variation in local cone crops by redistributing themselves each year within the regular range (mainly in July-September) and by exploiting alternative small-seeded conifers when their main food is scarce. However, in years when major food sources are scarce over wide areas, the birds leave their regular range in large numbers, and live as best they can off any suitable plant or animal material they can find.
The Siberian N. c. macrorhynchos performs the most spectacular irruptions, reaching in some years as far as western Europe, on journeys of several thousand kilometres. Thirty-one irruptions into Europe were recorded in the 250 years from 1750, at intervals of 1-33 (mean 8) years (Mayaud 1947, Cramp & Perrins 1994). In the twentieth century alone, the figures were 13 irruptions, at intervals of 2-16 years (mean 7.7). Only in the largest irruption of 1968, which started as far east as Lake Baikal, did large numbers of birds reach Britain, a journey exceeding 7000 km (Hollyer 1970), with smaller numbers occurring in at least five other years. Again, not all invasions necessarily originated from the same region. According to Formosov (1933), the invasion of 1911 came from southeast Siberia, that of 1931 from northwest Siberia, and that of 1933 from central and northern Siberia.
In such invasions, surviving birds normally stay until the next year, like crossbills, but in 1968 many nutcrackers that reached western Europe reversed their migration and returned homeward in the same autumn, but probably not as far as their region of origin. Recoveries of 14 migrants ringed in Finland were all between east and south-southeast of the ringing sites, including three at 22003300 km east. Another bird ringed on the southern Baltic coast was recovered 2500 km east two weeks later (Hilden 1969, Cramp & Perrins 1994). Other irrup-tive birds trapped on the island of Gotland in the Baltic were recovered in a later year at 70-85°E in west Siberia, some 4000 km to the east. A partial return eastward movement within weeks of the westward late-summer exodus has been recorded at Lake Ladoga in Russia (Noskov et al. 2005).
Like crossbills, nutcrackers can remain in invasion areas for up to a year, breeding if food supplies permit, and returning to the regular range when the next year's crop is ready. Small longer-term populations have been established after invasions in areas where suitable food trees had been planted where none occurred before (Lanner 1996). Following the invasion of 1968, colonies of Slender-billed Nutcrackers N. c. macrorhynchos from Siberia became established in Finland and Belgium in areas where Stone Pines Pinus sibirica and P. cembra had been planted (Cramp & Perrins 1994); and after the invasion of 1977, similar colonies were established in Sweden (Elmberg & Mo 1984).
Those Thick-billed Nutcrackers N. c. caryocatactes that live in northern Europe move shorter distances to the south, while those in the mountains of central Europe move mainly to adjacent lower ground in years of extreme food shortage. Big movements of central and northern European birds do not usually coincide with one another, nor with the bigger movements into Europe of Siberian birds (Mattes & Jenni 1984).
In North America, Clark's Nutcracker Nucifraga columbiana breeds in the western mountains, but not in the northern boreal forest which lacks large-seeded pines. Like the Eurasian species, it depends on different types of conifers in different parts of its range, moving around each summer and autumn to areas where new crops are good (Vander Wall et al. 1981, Lanner 1996). In years of widespread crop failure that follow good crops, the birds extend mainly to lower ground nearby, in the deserts, plains and coastal areas, but some have been recorded far to the east, more than halfway across the continent (Fisher & Myres 1980). Invasions into the lowlands of California and other southwestern States occurred in 1898, 1919, 1935, 1950, 1955 and 1961, at intervals of 5-21 years (mean 12.6 years) (Davis & Williams 1957, 1964). Another big one occurred in 1996, the largest for many years (National Audubon Field Notes). At the northern end of the range, in Alberta, invasions of low ground during 1904-1976 occurred in 1919, 1960, 1965, 1972 and 1976; that is, in mainly different years from those in the southwestern States (Fisher & Myres 1980). Like some other irruptive migrants, Clark's Nutcrackers sometimes breed in invasion areas before returning to their regular range and, like crossbills, they thereby move in opposite directions in successive years (Vander Wall et al. 1981, Tomback 1988).
In North America, other corvids also harvest and store pine seeds, namely the Pinyon Jay Gymnorhinus cyanocephalus, Steller's Jay Cyanocitta stelleri and Scrub Jay Aphelocoma californica, but none of these species is so heavily dependent on them, or so specialised as the nutcrackers, and none of them performs obvious irruptive migrations.
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