Worldwide, the numbers of breeding landbird species found per unit area generally decreases progressively from equatorial to polar regions (but is also generally low in barren deserts). At the same time, the proportions of species that are migratory increase from equatorial to polar regions, as the contrast between summer and winter conditions widens (Newton & Dale 1996a, 1996b). Migration thus tends to steepen the latitudinal gradient in species numbers between summer and winter. Progressing northward up the western seaboard of North Africa and Europe, for example, the proportion of breeding bird species which move out totally to winter further south increases from 29% of species at 30°N (North Africa) to 83% of species at 80°N (Svalbard), a mean increase of 1.3% of breeding species for every degree of latitude (Figure 13.1).
The relationship between proportion of summer visitors and latitude holds, it has been suggested, because at high latitudes the numbers of resident birds are held at low level by winter severity. The flush of food in summer is greater than the small
Figure 13.1 Map of western Europe showing the proportions of breeding bird species at different latitudes that migrate south for the winter. From Newton & Dale (1996).
number of resident species can exploit, leaving a surplus available for summer migrants. The latter therefore increase in proportion with latitude, as the severity of the winters increases, and the numbers of year-round resident species decline. At lower latitudes, a high proportion of breeding species can remain year-round, leaving fewer openings for summer visitors (Herrera 1978, O'Connor 1985, Morse 1989).
A similar relationship between migration and latitude holds in the largely different avifauna of eastern North America, where the proportion of migrants among breeding species also increases with distance northwards from 12% at 25°N to 87% at 80°N, a mean increase of 1.4% per degree of latitude (Figure 13.2).
The difference between the two regions (Figure 13.3) reflects the climatic shift between east and west sides of the Atlantic: over most of the latitudinal range, at any given latitude winters are colder in eastern North America than in western Europe. Correspondingly, at any given latitude, a greater proportion (an average of about 17% more) of breeding species leaves eastern North America for the winter than leaves western Europe. The slopes of the two linear regression lines calculated from the data in Figure 13.3 do not differ significantly, but the intercepts do (F1/19=27.5, P<0.001), reflecting this climatic difference.
On both continents, this northward trend in migration is easily understood in terms of winter conditions. In Europe, over much of the twentieth century, mean January temperatures exceeded 10°C only in southern Spain and North Africa; they lay within the range 0-5°C in much of western Europe, but fell below freezing and as low as — 15°C in most of Fennoscandia, and to — 20°C in Novaya
Zemlya in the far north. Minimum winter daylengths were around 11 hours at 35°N in southern Europe but decreased to zero at the Arctic Circle. The season of plant growth lasted 6-9 months at 35-50°N, but shrank to less than three months in Svalbard, a mean decline in growing season of about one month for every 11° of latitude. In continental western Europe, most fresh waters north of 55°N froze in winter, although they mostly remained open in Britain and Ireland. Much of the Baltic and Barents Seas also iced over during the course of the winter, closing these areas for seabirds. In North America, similar latitudinal trends occurred, but were more marked because the continent spans a wider range of latitude than Europe. Throughout much of these areas, temperatures are now rising as part of global warming, and bird migration patterns are changing accordingly (Chapter 21), but they still relate to gradients in prevailing conditions.
The few species that remain to winter in the far north include the Common Raven Corvus corax, Rock Ptarmigan Lagopus mutus, Gyrfalcon Falco rusticolus and Snowy Owl Nyctea scandiaca among landbirds, and the Northern Fulmar Fulmarus glacialis, Ivory Gull Pagophila eburnea and Glaucous Gull Larus hyperboreus among seabirds. The most northerly seabirds depend in winter on the open water provided by polynyas, and some of the gulls also scavenge the remains of seals killed by Polar Bears Ursus maritimus. Some individuals of these species may move south to some extent in the weeks of complete darkness.
Figure 13.3 Proportions of breeding bird species (y-axis) at different latitudes (x-axis) in western Europe and eastern North America that migrate south for the winter (a) or north for the summer (b). For southward migration, on regression analysis for western Europe: y = 41.49-1.03 + 0.02x2, r2 = 0.97; for eastern North America: y = -75.05 + 4.33x-0.3x2, r2 = 0.98. For northward migration, on regression analysis for western Europe: y = 55.65-0.66x, r2 = 0.81; for eastern North America, y = 123.72-3.22x + 0.03x2, r2 = 0.87. For three of these relationships, a quadratic equation gave a significantly better fit than a linear one. From Newton & Dale (1996b).
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