Years various kinds, but in other years bear almost none. Fruiting depends partly on the natural rhythm of the trees themselves and partly on the weather. Trees of most species require more than one year to accumulate the nutrient reserves necessary to produce a fruit crop. In addition, for a good crop, the weather must also be fine and warm in the preceding autumn when the fruit buds form, and again in the spring when the flowers set. Otherwise the crop is delayed for another year. In any one area most of the trees of a species fruit in phase with one another, and often those of different species also fruit in phase, partly because they come under the same weather. The result is an enormous profusion of tree fruits in some years, and practically none in others: good crops almost never occur in consecutive years, and are almost always followed by poor crops (Figure 18.1; Koenig & Knops 1998, 2000). Nevertheless, annual seed crops of some tree species fluctuate less than others, with Scots Pine Pinus sylvestris producing smaller, but more consistent crops than Norway Spruce Picea abies, for example, and Alder Alnus glutinosa more consistent crops than Birch Betula pendens and B. pubescens (for annual cropping patterns of different tree species, see Svardson 1957, Hagner 1965, Perrins 1966, Gotmark 1982, Knox 1992, Thies 1996, Koenig & Knops 1998, 2000).

The trees in widely separated areas may be on different fruiting regimes, partly because of regional variations in weather, so that good crops in some areas may coincide with poor crops in others. Nevertheless, good crops may occur in many more areas in some years than in others, so that the total continental seed production also varies greatly from year to year. An analysis of the fruiting patterns of various boreal conifer species at many localities in North America and Eurasia revealed high synchrony in seed production in localities 500-1000 km apart, depending on tree species. The synchrony declined at greater distances, and by 5000 km no correlation was apparent in the fruiting patterns of particular tree species (Koenig & Knops 1998). These figures give some idea of the range of distances that must separate the successive breeding and wintering areas of some boreal finches if the same individuals are to have access to good tree-seed crops every year of their lives. Besides total production, the timing of ripening and release of seeds is also important to seed-eating birds. Some tree species release most of their seeds within a short period in autumn, others more slowly over winter, and yet others in spring, or even more slowly over two or more years. This is true of some Pine Pinus and Larch Larix species which can thereby provide some food for seed-eaters even in non-cropping years. For most finches, the seeds are most available while they remain on the tree; once they fall to the ground they become rapidly removed by other animals or covered by snow. Among the European irruptive finches that depend on tree seeds, only the Brambling Fringilla montifringilla feeds for preference on the ground, though others do so occasionally; among the North American irruptive species, all seem to prefer the trees.

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