Twiceyearly Migrants

Twice-yearly migrants generally move towards more southerly latitudes in autumn and to more northerly ones in spring, but they may concentrate to breed and winter in different areas in different years, wherever their food is plentiful at the time. Their local numbers fluctuate greatly from year to year according to local food supplies, and in winter may range between total absence in years when appropriate tree seeds are lacking, to thousands of birds per square kilometre in years when such seeds are plentiful. The bird species involved seem to move each autumn only until they find areas rich in food, then settle there (Svärdson 1957, Newton 1972, Jenni 1987). In consequence, the distance travelled by the bulk of the migrants varies from year to year, according to where the crops are good, and only when the migrants are exceptionally numerous, or their food is generally scarce, do they reach the furthest parts of their wintering range, as an 'invasion' (Figure 18.2). The advantages of stopping in the first suitable feeding areas are presumably that birds travel no further than necessary, and do not pass over good feeding areas which might be the only ones available that year.

Other species also exploit the same foods as the irruptive species, but do not depend so heavily on them, so are less affected by the fluctuations in fruiting. Similarly, some of the species listed in Table 18.1 may be irruptive in some parts of their range but not in others, depending on the breadth of the diet and the level of fluctuation in the entire food supply. Populations that have access to a wide range of dietary items are less likely to experience a shortage of all types in the same year.

Table 18.1 Established year-to-year correlations between bird abundance and food supply in seed-eating and fruit-eating birds

Preferred winter Summer Winter Autumn References fooda densities densities emigration

Table 18.1 Established year-to-year correlations between bird abundance and food supply in seed-eating and fruit-eating birds

Great Spotted Woodpecker Dendrocopos major (P)

Spruce, Pine and other seeds

Pynnönen (1939), Formosov, (1960), Eriksson (1971)

Bohemian Waxwing Bombycilla garrulus (H)

Rowan and other berries

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