Info

Figure 18.6 Numbers and timing of Eurasian Siskins Carduelis spinus migrating through Ottenby each year in relation to the size of the Birch Betula seed crop further north. Drawn from data in Svardson (1957).

Tits Parus major from 2 to 7438, Blue Tits P. caeruleus from 7 to 3595, and Coal Tits P. ater from 0 to 18 785 (Ulfstrand et al. 1974). These annual fluctuations were far greater than those recorded in more regular long-distance migrants (counts of which typically varied by up to 3- to 4-fold, occasionally up to 10-fold, between years, Edelstam 1972, Ulfstrand et al. 1974, Roos 1991). Similar differences between irruptive and regular migrants have been recorded at other watch sites (e.g. Dorka 1966, Gatter 2000).

The amount of autumn emigration has been related to food supplies in almost all seed-eating and fruit-eating species discussed here, being greatest in years when high densities coincide with poor seed crops (Table 18.1, Figures 18.6 and 18.7). Different species that depend heavily on the same seed or fruit crops as one another

Pine seed crop

Figure 18.7 Irruptions of Red-breasted Nuthatches Sitta canadensis in relation to pine cone production in western North America. The pine seed index was calculated as the proportion of sites surveyed each year that had greater than average seed crops. The irruption index was calculated from the Christmas Bird Counts, as the proportion of sites south of 50°N at which greater than expected numbers of Red-breasted Nuthatches were recorded. Expected numbers for each year were calculated from a regression of the annual totals on year for the whole 21-year count period, 1968-1988. Based on data in Koenig & Knops (2001), in which the relationship was given as significant (P < 0.05) on a Spearman rank correlation test.

Pine seed crop

Figure 18.7 Irruptions of Red-breasted Nuthatches Sitta canadensis in relation to pine cone production in western North America. The pine seed index was calculated as the proportion of sites surveyed each year that had greater than average seed crops. The irruption index was calculated from the Christmas Bird Counts, as the proportion of sites south of 50°N at which greater than expected numbers of Red-breasted Nuthatches were recorded. Expected numbers for each year were calculated from a regression of the annual totals on year for the whole 21-year count period, 1968-1988. Based on data in Koenig & Knops (2001), in which the relationship was given as significant (P < 0.05) on a Spearman rank correlation test.

tend to irrupt in the same years. They include the Blue Tit Parus caeruleus and Great Tit P. major which both feed heavily on Beech Fagus sylvatica mast, and the Common Crossbill Loxia curvirostra and Great Spotted Woodpecker Dendrocopus major which both feed heavily on spruce seeds. Where different tree species fruit in phase with one another, the numbers of participating species is increased further.

Over much of the boreal region of North America, conifer and other tree crops tend to fluctuate biennially, and in the alternate years with poor crops several species that depend on them migrate to lower latitudes. The migrations of irruptive species are therefore much more regular in North America than in Europe and much more synchronised between species (Bock & Lepthien 1976, Kennard 1976, Larson & Bock 1986, Koenig & Knops 2001; Figure 18.8). At least eight species of boreal seed-eating birds tend to irrupt together, namely Common Redpoll Carduelis flammea, Pine Siskin C. pinus, Purple Finch Carpodacus purpureus, Evening Grosbeak Hesperiphona vespertina, Red-breasted Nuthatch Sitta canadensis and Black-capped Chickadee Parus atricapillus, as well as the Red Crossbill Loxia curvirostra and White-winged (Two-barred) Crossbill L. leucoptera discussed later. These species vary in the proportions of conifer and broad-leaved tree seeds in their diets (Table 18.1; Bock & Lepthien 1976, Kennard 1976, Koenig & Knops 2001). Over periods of years, different species of trees in the same area can drift in and out of synchrony with one another, affecting the movements of the birds that specialise on them. During the period 1921-1950, the biennial pattern and synchrony between the various North American seed-eaters was less marked than before or after this period (Larson & Bock 1986). In some years, irruptions were evident across North America, but in other years only in western or eastern regions (Koenig & Knops 2001). In addition, crop failures and irruptions from montane areas south of the boreal region were not well synchronised with those within the boreal region (Bock & Lepthien 1976). No consistent relationships were apparent between irruptions and environmental factors other than seed crops, such as warm summers or cold winters (e.g. Koenig & Knops 2001).

Was this article helpful?

0 0

Post a comment