Winter densities

Correlations between numbers of birds and size of fruit crops in wintering localities have been documented in several species of irruptive migrants, as shown in Table 18.1, but mostly in northern parts of the wintering range. Further south in the wintering range, such correlations would be expected to be weak or non-existent, because the birds reach the more distant localities only in exceptional years. In some years, seed crops are available, but no birds from further north turn up to use them, having settled in good feeding areas already encountered. Conversely, irruptive migrants may turn up in remote areas in some years only to find their favoured foods are lacking (as shown, for example, by the large numbers of Jays Garrulus glandarius which irrupted into western Europe in 1983, a year almost devoid of acorns, John & Roskell

1985). Other species (such as crossbills and nutcrackers) often migrate beyond the geographical range of their main food plants, feeding instead on whatever other seeds they can find. Mortality is almost certainly heavy in such years.

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