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Figure 18.4 (a) Numbers of breeding Eurasian Siskins Carduelis spinus in relation to the spruce crop in Finland. Siskins shown in pairs per km2 and cone crops classified in four categories. From Haapanen (1966). Similar data were also given for a Norwegian area by Hogstad (1967). (b) Numbers of breeding Bramblings Fringilla montifringilla in relation to the abundance of geometrid Autumnal Moth Epirrita autumnata caterpillars in northern Norway. Bramblings measured as number of singing males, and Epirrita as the number of caterpillars per 100 net sweeps in birch trees during 1972-1998. Redrawn from Hogstad (2000). Similar data were given for other areas by Silvola (1967) and Lindstrom (1987).

site-fidelity from year to year, with most surviving individuals returning in successive years (Mikkonen 1983). In all the irruptive species listed in Table 18.2, return rates were extremely low, compared with what would be expected from their annual survival rates. The implication is that large proportions of individuals changed their nesting locations from year to year.

The small numbers of relevant ring recoveries available for irruptive species confirm that some individuals have indeed occurred in widely separated localities in

Table 18.2 Annual return of individual birds to the same area -

irruptive migrants

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