Over much of Europe, Common Crossbills depend mainly on Norway Spruce Picea abies which has more fluctuating and sporadic seed crops than pine. Each year, new cones begin to form in May, providing food for Crossbills from about June into the winter, and particularly from January on, when the cones begin to open and shed their seeds. By late May, when most cones and seeds have fallen, they are lost to Crossbills, which then switch to alternative foods, and eventually to the new Norway Spruce crop (Marquiss & Rae 1994, 2002).
In most years, Common Crossbills remain within the boreal forest, each year concentrating in areas where spruce cones are plentiful. In particular areas, their breeding numbers tend to fluctuate on a roughly 2-4 year cycle, in line with the local cone crops (Figure 18.10; Reinikainen 1937, Formosov 1960, Thies 1996, Forschler et al. 2006). The birds have one major period of movement within the summer of each year, as they leave areas where the previous year's spruce crop was good but coming to an end, and concentrate in areas where the current
year's crop is good but forming. Between these times, some birds concentrate temporarily in Scots Pine areas. It is only at this period of transition, in summer, that Common Crossbills in Europe have been found with substantial fat reserves, presumably deposited as 'migratory fat' (Marquiss & Rae 2002, Newton 1972, 2006b). However, lesser movements occur at other times of year in relation to local changes in food availability, as seeds are shed or consumed, or as cones open and close.
In a good cone year for Norway Spruce, Crossbills may begin nesting in autumn, within weeks after arriving in a new area, and may continue for as long as the food holds out. A pair may start a second nest while still feeding young from the first, and young birds may begin breeding while they themselves are still in juvenile plumage (Berthold & Gwinner 1978, Jardine 1994, Hahn et al. 1997). The main breeding period, however, is in January-April, when the cones begin to open, making seeds more readily available. As seeds and cones fall through May, breeding in Norway Spruce areas comes to an end and the birds move on.
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