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Silvola (1967), Enemar et al. (1984), Nilsson (1984), Lindström (1987), Lindström et al. (2004), Hogstad (2000), Jenni & Neuschulz (1985), Jenni (1987), Eriksson (1970d), Mikkonen (1983)

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Preferred winter Summer food3 densities

Eurasian Siskin Carduelis Birch, Alder and spinus (P) conifer seeds

Pine Siskin Carduelis pinus (N)

Common Redpoll Carduelis flammea (H)

Arctic (Hoary) Redpoll Carduelis hornemanni

Eurasian Bullfinch Pyrrhula pyrrhula (P)

Pine Grosbeak Pinícola enucleator (H)

Evening Grosbeak Hesperiphona vespertina (N) Purple Finch Carpodacus purpureus (N)

Common (Red) Crossbill Lox/a curvirostra (H)

Conifer, Birch and Alder seeds Birch and Alder seeds

Various tree seeds and berries

Maple and other tree seeds Various tree seeds

Spruce and other conifer seeds

Two-barred (White-winged) Larch and other

Crossbill Lox/a leucoptera (P) conifer seeds

Parrot Crossbill Lox/a Scots Pine seeds pytyopsittacus (P)

Eurasian Jay Garrulus Oak seeds glandarius (P)

Winter Autumn References densities emigration

(1966), Hogstad (1967), Eriksson (1970c), Petty etal. (1995), Forschler et al. (2006) Bock & Lepthien (1976), Widlechner & Dragula (1984)

(1970b), Enemar etal. (1984) Bock & Lepthien (1976), Nystrom & Nystrom (1991) Svardson (1957)

Grenquist (1947), Bock & Lepthien (1976)

Parks & Parks (1965), Bock & Lepthien (1976)

Bock & Lepthien (1976), Koenig & Knops(2001)

(1960), Newton (1972), Bock & Lepthien (1976), Benkman (1987), Petty et al. (1995), Forschler et al. (2006) Newton (1972), Larson & Tombre (1989), Bock & Lepthien (1976)

Newton (1972)

Cramp & Perrins (1994)

Thick-billed Nutcracker Nucifraga c. macrorhynchos (P) Thin-billed Nutcracker Nucifraga c. caryocatactes (P)

Clark's Nutcracker Nucifraga columbiana

Hazel and Swiss Stone Pine seeds Siberian Stone Pine seeds, Brush Pine seeds Whitebark Pine and other conifer seeds

Schütz & Tischler (1941), Mattes & Jenni (1984) Formosov (1933)

Lanner (1996)

H, Holarctic; N, Nearctic; P, Palaearctic.

a Scientific names of trees: Alder Alnus, Beech Fagus sylvatica, birch Betula, Hazel Corylus avellana, larch Larix, maple Acer, oak Quercus, Rowan Sorbus aucuparia, Scots Pine Pinus sylvestris, Siberian Stone Pine Pinus sibirica, spruce Picea, Swiss Stone (Arolla) Pine Pinus cembra, Whitebark Pine Pinus albicaulis. Where several species in the same genus are involved, only the generic name is given.

b In this species, mass emigration has been more frequently linked with high numbers (which may cause food shortage) or with high spring temperatures which promote high breeding success (Markovets & Sokolov 2002).

Figure 18.2 Model of migration system in irruptive finches. The birds leave the breeding range across boreal Europe each autumn on a broad front, travelling roughly southwest, but only until they meet areas with abundant seed crops. The sporadic cropping pattern of trees means that each year birds from some sectors of the breeding range must travel further than others. The next year the pattern of tree cropping and migration may differ, so that birds from particular sectors of the breeding range travel different distances in different years. From Jenni & Neuschulz (1985).

Figure 18.2 Model of migration system in irruptive finches. The birds leave the breeding range across boreal Europe each autumn on a broad front, travelling roughly southwest, but only until they meet areas with abundant seed crops. The sporadic cropping pattern of trees means that each year birds from some sectors of the breeding range must travel further than others. The next year the pattern of tree cropping and migration may differ, so that birds from particular sectors of the breeding range travel different distances in different years. From Jenni & Neuschulz (1985).

Scenario 1 - good crops throughout the wintering range: all birds accommodated in northern areas, nearest the breeding areas, and crops further south remain unused; survival good.

Scenario 2 - good crops in the north of the wintering range, but not in the south: all birds accommodated in northern areas; survival good.

Scenario 3 - good crops in the south of the wintering range but not in the north: birds accommodated in the southern areas; survival moderate.

Scenario 4 - crop failure throughout the wintering range: birds spread widely, but at low densities; survival poor.

On this model, it is in the northern parts of the wintering range that correlations between seed crops and bird densities in different years would be expected to be most marked; further south, good crops can sometimes occur with few birds, or vice versa.

Most tree species, such as Birch Betula and Rowan Sorbus aucuparia, retain their seeds into the winter, so the finches that eat them take the seeds directly from the trees, regardless of snow. Other trees, such as Oak Quercus robur and Beech Fagus sylvatica, shed their seeds in autumn so that the birds that eat them must take them from the ground. Such birds are therefore also affected by snowfall, which can render abundant seed crops unavailable. Thus, the movements and winter distributions of Bramblings Fringilla montifringilla in different years are a function of both Beech crops and snowfall (Jenni & Neuschulz 1985, Jenni 1987).

Examples of species that behave as irruptive migrants in only parts of their range include Eurasian Bullfinch Pyrrhula pyrrhula, Wood Nuthatch Sitta europaea and various titmice Parus spp. In other parts they are either residents or regular migrants.

Breeding densities

Among irruptive seed-eaters, local breeding densities can vary from nil or few in poor food years to dozens of pairs per square kilometre in good food years (for references see Table 18.1). For example, local densities of Common Redpolls Carduelis flammea fluctuated about 39-fold over a 20-year period, with the highest densities coinciding with exceptionally good Dwarf Birch Betula nana crops, the low bushes and their seed-catkins protected under snow since the previous year (Enemar et al. 1984). Densities of Eurasian Siskins Carduelis spinus fluctuated 6- to 50-fold in four different areas in parallel with the spruce cone crop (Haapanen 1966, Hogstad 1967, Shaw 1990, Forschler et al. 2006); and densities of Bramblings Fringilla montifringilla fluctuated by 5- to 26-fold in three different areas in parallel with the abundance of moth larvae Epirrita autumnata (Box 18.1; Silvola

Box 18.1 Bramblings and caterpillars

In the Brambling Fringilla montifringilla, unlike most other seed-eaters, high breeding densities in northern Europe seem particularly linked to outbreaks of a single insect, the autumnal moth Epirrita autumnata. The caterpillars of this small geometrid moth are found mainly on birch trees, and in the alpine and arctic regions of northernmost Europe outbreaks occur on average once per decade (variation 5-15 years, Ruchomaki et al. 2000, Selas et al. 2001), when large areas of forest may be defoliated. Each outbreak lasts about 3-4 years and in the intervening years larval densities are hardly measurable. In addition, the outbreaks seem well synchronised over large areas of northern Europe, hundreds of kilometres across. In such years, Bramblings nest at much higher densities than usual (Silvola 1967, Hogstad 2000, Ytreberg 1972, Enemar et al. 1984, Lindstrom 1987, Lindstrom et al. 2005). In a 19-year study, breeding densities of Bramblings varied by three-fold (density index 45.1 to 126.2), in association with densities of Epirrita larvae which varied by more than four orders of magnitude (between 0.1 and 235.1 larvae per 1000 short shoots). Bramblings produced more young per pair in the outbreak years, and 49% of the annual variation in adult to juvenile ratios in late summer was explained by annual variation in Epirrita densities. However, Bramblings still produced some young in the lowest Epirrita years (Lindstrom et al. 2005).

Another predominantly seed-eating species that concentrates in areas of abundant caterpillars in the breeding season is the Evening Grosbeak Hesperiphona vespertinus in North America. It is especially attracted to outbreak areas of the Spruce Budworm Choristoneura fumiferana (Parks & Parks 1965).

Year

Figure 18.3 Densities of Common Redpolls Carduelis flammea in 9 km2 of birch scrub in Swedish Lapland, 1963-1982. Over this period, the fluctuations in numbers were mostly moderate, but with exceptionally large numbers in 1968 and 1971, which could only have resulted from massive immigration. In these two years, the birch crop was unusually good, the seeds having remained on the trees from the previous summer when they were formed. In these years, Redpolls overwintered in the area, started breeding earlier than usual, and produced larger clutches, some raising more than one brood. In 1978, few Redpolls occurred in the area, despite good numbers in the preceding and following years. From Enemar et al. (1984).

Year

Figure 18.3 Densities of Common Redpolls Carduelis flammea in 9 km2 of birch scrub in Swedish Lapland, 1963-1982. Over this period, the fluctuations in numbers were mostly moderate, but with exceptionally large numbers in 1968 and 1971, which could only have resulted from massive immigration. In these two years, the birch crop was unusually good, the seeds having remained on the trees from the previous summer when they were formed. In these years, Redpolls overwintered in the area, started breeding earlier than usual, and produced larger clutches, some raising more than one brood. In 1978, few Redpolls occurred in the area, despite good numbers in the preceding and following years. From Enemar et al. (1984).

1967, Lindstrom 1987, Hogstad 2000). In all these species, local densities sometimes increased so much from one year to the next that the increase could not be explained by good reproduction and survival from the previous year, but must have involved mass immigration (Figures 18.3 and 18.4). This in turn implies that some birds changed their breeding areas from one year to the next, or that young moved in from distant as well as local natal areas.

Breeding dispersal

Among populations of regular migrants, if the birds occupying a particular study area are trapped and ringed in the breeding season, large proportions of the same individuals are usually found breeding in the same area next year. Return rates are usually within the range 30-60% for passerines, and 60-90% for non-passerines (Chapter 17). Allowing for expected mortality, such high figures imply that most surviving individuals return to breed in the same limited area year after year. In some regular migrants, the same also holds for wintering areas (Chapter 17).

The situation differs in irruptive migrants, whose return rates are normally much lower (Table 18.2). For example, among Bramblings Fringilla montifring-illa trapped in the breeding season in various areas, individuals were seldom or never caught in the same locality in a later year, so that each year's occupants were different from those the year before (Mikkonen 1983, Lindstrom 1987, Lindstrom et al. 2005, Hogstad 2000). In one such study, only seven (0.6%) of 1238 adults were retrapped in the same area in a later year, and none of 1806 juveniles, despite an annual trapping programme over many years (Lindstrom et al. 2005). In another area where Bramblings were studied, the closely related non-irruptive Chaffinches Fringilla coelebs showed much more stable densities and greater

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