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Evening Grosbeak

Hesperiphona vespertina

Crossbill

Loxia curvirostra

Crossbill

Loxia curvirostra

Figure 18.8 Year-to-year variation in the winter ranges of three species of seed-eating birds that breed in northern North America. Dots show large wintering populations in local regions. In years of low food supply in the north, all species tend to winter far to the south. From Bock & Lepthien (1976).

Figure 18.8 Year-to-year variation in the winter ranges of three species of seed-eating birds that breed in northern North America. Dots show large wintering populations in local regions. In years of low food supply in the north, all species tend to winter far to the south. From Bock & Lepthien (1976).

The role of food shortage in stimulating autumn emigration is shown in another way, namely that some species which take food from garden feeders have become less irruptive over the past 40 years, as increasing amounts of food have been provided. Winter feeding was held responsible for turning a previously migratory population of Great Tits Parus major in the city of Oulu in northern Finland into a resident one (Orell & Ojanen 1979), and reduced movements by various tit species, attributed to the same cause, have been recorded in other parts of western Europe over the same period (van Balen & Hage 1989, Wernham et al. 2002). No big invasions of Blue Tits and Great Tits have occurred in Britain since 1977, and no big invasions of Great Spotted Woodpeckers Dendrocopus major since 1974. All these species feed heavily from garden feeders.

Migration timing

In at least some irruptive migrants, the timing of autumn migration appears much more variable than among regular migrants, being markedly earlier in the big years. For example, the peak autumn days for Siskins Carduelis spinus at Falsterbo during 1949-1988 varied from mid-August to mid-November in different years (Roos 1991), and elsewhere heavy southward movements have been seen as late as December-January. Over a nine-year period, Svardson (1957) attempted to relate the numbers of Siskins passing through Ottenby Bird Observatory to the size of the birch crop further north (Figure 18.6). The birds tended to pass in largest numbers, and at the earliest dates, in years when the birch crop was poor. Similar relationships between numbers and migration dates have been noted in other species elsewhere (Berndt & Henss 1967, Gatter 2000), and the corresponding tendency for birds to arrive earlier than usual in their wintering areas in invasion years has been observed in various irruptive species. Annual variations in autumn migration dates are clearly much greater in irrup-tive than in other migrants, which travel at approximately the same dates each year (Chapter 12).

Most irruptive seed-eaters begin to take their main winter food in late summer or autumn before their migration begins, so the size of the crop can have a direct influence. Other species start migrating before the winter seed crop is ready, so numbers per se (and the resulting competition for whatever food is eaten then) may have a bigger influence on migration. Such species tend to migrate at more consistent dates from year to year. They include the Brambling Fringilla mon-tifringilla, Wood Nuthatch Sitta europaea and various tits, all of which eat Beech Fagus sylvatica mast in winter, but begin to leave before the seeds are ready (Berndt & Dancker 1960, Eriksson 1971).

Migration directions

In most European bird species, individuals from the western parts of the breeding range tend to use migration routes and wintering areas that lie to the west of birds from the eastern parts of the breeding range (Chapter 23). It is as though the birds from different sectors of the range migrate back and forth along roughly parallel routes. However, this tendency is less marked in irruptive than in regular migrants because irruptive migrants typically show a greater east-west component in their movements, and also greater directional spread, both from year to year and between individuals in the same year. In contrast to most migrants, moreover, irruptive species may end their journey at widely different places on the route each year. Thus, in any given year, birds from one part of the breeding range may stay in the north if food permits, while those from another part may extend far to the south, though not necessarily vacating the north completely (Figure 18.2). The following year, the pattern may differ. It is these behaviours that contribute to individuals turning up in widely separated areas in different winters. Most are at different points on the same migration axis (for Common Redpoll Carduelis flammea see Eriksson 1970b, for Siskin Carduelis spinus see Eriksson 1970c, for Brambling Fringilla montifringilla see Jenni & Neuschulz 1985), but some may end up in one winter far to the west or east of their position in a previous winter. Even within a winter, the birds may wander in various directions, and end up markedly to the east or west of their initial route (for Bohemian Waxwing Bombycilla garrulus see Cornwallis & Townsend 1968).

The greater directional spread of irruptive, compared to regular migrants, is evident from observations at migration watch sites, and also from subsequent ring recoveries of birds trapped on migration. Typically, the recoveries of irrup-tive migrants ringed in autumn and recovered in the following months show two or three times the angular spread as those of regular migrants. For example, from birds ringed at the Courish Spit in the southeast Baltic, winter recoveries of various non-irruptive species lay within an arc of 100° from the ringing site, but in irruptive species the spread was much greater: in the Eurasian Jay Garrulus glandarius 198°, in the Bohemian Waxwing Bombycilla garrulus 237°, in the Brambling Fringilla montifringilla 188°, and in the Eurasian Siskin Carduelis spinus 298° (Payevsky 1998). Similar differences between irruptive and non-irrup-tive migrants have been noted elsewhere, the recoveries of some non-irruptive species being mainly confined to a narrow corridor from the ringing site (for Eurasian Linnet Carduelis cannabina see Verheijen 1955, for Goldfinch C. carduelis see Newton 1972, for Chaffinch Fringilla coelebs see Bairlein 2001, for Pied Flycatcher Ficedula hypoleuca see Mouritsen 2001).

It is not yet clear to what extent the wide directional spread in irruptive migrants is apparent within years, or results from birds taking mainly different main directions in different years. From Common Redpolls Carduelis flammea ringed in Fennoscandia, most ring recoveries in 1965 came from directions east-southeast (mostly in Russia), whereas in 1972 and 1986 they came from directions to the southwest (mostly in western Europe) (Figure 18.9, Zink & Bairlein 1995, Lensink et al. 1986, Thies 1991). This difference could be explained if the birds were tracking seed crops, which took them in different directions from their breeding areas in different years, but this explanation remains untested. During the 50-year period 1951-2000, Common Redpolls appeared in large numbers in the mid-latitudes of Europe in at least 14 years, but concentrated mainly in the western, central or eastern parts of this region in different years. One of the biggest movements (in 1986) seems to have started mainly in northern Russia, and spread west-southwest across the continent; some 5.5 million birds are estimated to have entered the Netherlands then (Lensink et al. 1986), but very few extended further west to Britain.

Figure 18.9 Winter recoveries of Common Redpolls Carduelis flammea ringed in Finland. Circles - winter 1965/66, dots - winter 1972/73. After Zink & Bairlein (1995).

Another feature of irruptive migrants is that they often make long movements within a winter, staying in one area as long as food lasts, and moving further down the migration route when food runs out (for Pine Grosbeak Pinicola enu-cleator see Grenquist 1947; for Common Redpoll Carduelis flammea, Bohemian Waxwing Bombicilla garrullus and Fieldfare Turdus pilaris, see Haila et al. 1986). Even in big invasion years, Bohemian Waxwings Bombycilla garrulus seldom reach middle Europe before January (Glutz von Blotzheim 1966). In effect, the birds move progressively further from their breeding areas during a winter, stripping seed and fruit crops as they go. Because most species take their food from the trees, they are little affected by weather, but Bramblings Fringilla montifringilla, which feed on the ground, may be forced to leave areas of good seed crops when snow cover renders their food unavailable. Their winter distribution in particular years thus depends on both seed crops and snowfall (Jenni & Neuschulz 1985).

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