By changing breeding and wintering areas between years, irruptive seed-eaters lessen the effects of the massive food shortages they would experience if they occupied the same areas every year. Nevertheless, they may still be exposed to a hugely fluctuating food supply, as reflected in their reproductive rates. For example, in an area of northern Sweden, Bramblings Fringilla montifringilla bred every year over a 19-year period, but in greatly varying numbers, depending on food supply (Lindström et al. 2005). Post-breeding juvenile-to-adult ratios varied more than 10-fold over this period, from 3.54 in good food years to 0.33 in poor ones. The Redpolls Carduelis flammea and Siskins Carduelis spinus mentioned earlier can breed for more than twice as long in good spruce years than in other years (giving time for 2-3 broods instead of 1-2), and could thereby double their production of young (Peiponen 1967, Shaw 1990). In good seed years, both species begin nesting as early as March, when conifer cones open. After these seeds have fallen, the birds raise another one or two broods on the fresh seeds of herbaceous plants which form in May-July, depending on area, or (in the case of Redpolls on the tundra) on the seeds of Dwarf Birch Betula nana left from the previous year. In poor spruce years, only the later broods are reared. Among Common Crossbills Loxia curvirostra, the annual variations may be even greater, for in mixed conifer areas in which different tree species release their seeds in widely different months, individual Crossbills could in theory breed for more than nine months each year, raising brood after brood (Newton 1972). But in areas containing only Norway Spruce Picea abies, individuals breed for no more than half this time or, in poor cone years, not at all.
The fact that juveniles often predominate among irruptive species caught on migration has been taken as evidence that irruptions follow good breeding seasons (Lack 1954). Care is needed, however, because in such facultative migrants juveni les often leave the breeding areas in greater proportion and earlier than adults, and move greater distances. Nevertheless, when most of the migrants in particular years are adults, this probably gives a reliable indication of a poor breeding season. This situation was recorded, for example, among Common Crossbills in 1963, when young formed only 6%, 8%, 31% and 37% of the birds caught at four localities in western Europe (Newton 1972). This compares with up to 88% recorded in other irruption years. Similarly, study of Nutcracker Nucifraga caryocatactes skins in museums suggested that the movements of 1864, 1911 and 1968 consisted entirely of adults, and about half the adult females collected in the 1968 irruption had never laid eggs, evidence that they had not bred that year. In contrast, from the irruptions of 1885, 1913 and 1954 only first-year birds were collected in western Europe, while in other irruptions both age-groups were represented (Cramp & Perrins 1994).
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