Although variations in autumn movements among seed-eating birds are most marked in those species that depend on the seeds of trees, they are also apparent to some extent in those that eat the seeds of herbaceous plants. In arid regions, the local production of such seeds can vary greatly from year to year, according to rainfall patterns, and in other regions their availability on the ground is affected by patterns of agriculture or snowfall. Many seed-eaters concentrate at different points on their migration routes in different years, depending on seed supply (Pulliam & Parker 1979, Dunning & Brown 1982), and ringed individuals have occurred at widely separated points on that route in different winters (for Goldfinch Carduelis carduelis and Eurasian Linnet Carduelis cannabinna see Newton 1972, for Dark-eyed Junco Junco hyemalis see Ketterson & Nolan 1982), or have moved further along the route during the course of winter (Haila et al. 1986, Terrill & Ohmart 1984). This is irruptive behaviour in less extreme form, with more directional consistency. Moreover, occasional eruptions are recorded from species that are normally sedentary, if they reach high numbers relative to available food supplies, as noted in Bearded Tits Panurus biarmicus and others (Axell 1966, Bjorkman & Tyrberg 1982).
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