Breeding in migration and wintering areas

It is not only the autumn distances that vary from year to year in irruptive seed-eaters. The same holds for the spring distances, with many individuals settling in different areas in different years, depending on food supplies. The Common Redpoll Carduelis flammea provides a striking example, for this species curtails its migration by up to several hundred kilometres to breed in southern Fennoscandia in some of the years when the spruce crop there is good. Once the seeds have fallen, the birds in some years move north with their young to their usual birch-scrub breeding areas, where they raise another brood. Such movements have not been proved by ringing, but have been inferred from the simultaneous changes in the populations of the two regions and, in particular, from the late arrival in these years of birds in the birch Betula nana and B. tortuosa areas with their free-flying young. Such events have been documented in at least seven different years (Peiponen 1967, Hilden 1969, Antikainen et al. 1980, Gotmark 1982). In years with little or no spruce seed, the birds bred only in the birch, which shortens the overall breeding season.

A similar split migration may sometimes occur in Eurasian Siskins Carduelis spinus, in which adults and recently fledged juveniles were in several years seen migrating northeast in May-July over the Courland Spit in the southern Baltic (Payevsky 1994). Some of the adults were clearly paired at the time, and many trapped females had a well-developed brood patch, signifying a recent nesting attempt. The first females with brood patches usually appeared at this site towards the end of April. In the years 1984-1987, some 23-91% of adult females trapped in late April-July had brood patches, as did 35-86% of yearling females (total females caught = 1230). One juvenile caught in June 1959 had been ringed 25 days earlier, 760 km to the southwest, in Germany. While it could not be proved that the adults among these Siskins went on to breed elsewhere in the same year, they clearly had enough time to do so (for other movements of seed-eating birds within a breeding season see Chapter 16).

Some other irruptive species have been recorded breeding well to the south of their usual range in certain years, giving further evidence of variable spring settling patterns (for Brambling see Otterlind 1954; for Northern Bullfinch Pyrrhula pyrrhula see Svardson 1957; for Lesser Redpoll Carduelis f. cabaret see Newton 1972; for Bohemian Waxwing Bombycilla garrulus see Cornwallis 1961). They have also bred in their wintering areas for one or more years following a major influx, as noted in Lesser Redpoll (Newton 1972), Mealy (Common) Redpoll Carduelis flammea (Thom 1986), Northern Bullfinch (Svardson 1957), and Coal Tit Parus ater (Gantlet 1991), in addition to the crossbills and nutcrackers discussed later.

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