Superimposed on the overall latitudinal trend is another related to diet. Broadly speaking, those species that are resident year-round in a particular region exploit food supplies that are available there all year, whereas those that leave after breeding exploit foods that disappear then. In the northern coniferous forests, for example, residents include mainly species that feed directly from trees, on bark-dwelling arthropods (tits, tree-creepers), fruits and seeds (some corvids, finches, tits), buds or other dormant vegetation (grouse), or that eat mammals and other birds (some corvids, raptors and owls). Almost the entire resident landbird fauna at high northern latitudes falls into one or other of these dietary categories. In contrast, species that depart for the winter include those which eat active leaf-dwelling or aerial insects (warblers, hirundines) or which eat foods that become inaccessible under snow or ice (ground-feeding finches and thrushes, some raptors, waterfowl and waders). Towards the equator, as winters become less severe, the range of bird dietary types that remain for the winter increases, as a wider range of food types remains available year-round.
To illustrate in more detail the link between migration and seasonal changes in food availability, consider two examples. Most European songbirds feed either on: (1) insects or other invertebrates, (2) seeds and fruits, or (3) a mixture of both categories (Newton 1995a). Of these food types, seeds and fruits are clearly much more available in winter at high latitudes than are insects. Figure 13.4 shows the proportions of bird species in these different diet categories that are migratory at different latitudes. Within each group, the proportion of migrants increases with latitude, following the general trend in birds as a whole. But at each latitude from about 35°N, a larger proportion of insect-eaters than of seed-eaters leaves, while species with mixed insect-seed diets are intermediate. Furthermore, the insect-eaters generally move longer distances, many wintering in the tropics and some south of the equator (Figure 13.4). The result is that insect-eaters are concentrated at more southern latitudes in winter than are seed-eaters. The same holds for New World migrants, in which most small insectivorous warblers winter in Central America (30°N-10°S) and most seed-eaters further north (mostly 40°-15°N) (Keast 1995). Among these general relationships, specific exceptions occur, such as the seed-eating European Turtle Dove Streptopelia turtur which winters in Africa.
As a second example, consider the various European raptors, which also differ in the extents to which their foods remain available at high latitudes in winter (Newton 1998b). Species can be divided according to whether they feed primarily on warm-blooded prey (birds and mammals which remain active and available in winter at high latitudes) or on cold-blooded prey (reptiles, amphibia and insects, which become inactive and unavailable in winter). Within each raptor group, the proportion of migratory species again increases with latitude, but at any one latitude a larger proportion of the species that eat cold-blooded prey than of those that eat warm-blooded prey leaves for the winter, while species with mixed diets are intermediate (Table 13.1). The cold-blooded feeders also migrate furthest. The reasons for this difference are fairly obvious, in that species which eat cold-blooded prey and breed at high latitudes must winter in the tropics or in southern hemisphere temperate areas if they are to have access to the same types of prey year round. Of the 22 species of west Palaearctic raptors that eat mainly warm-blooded prey, most winter entirely within the Palaearctic and only one species (Booted ro 80 c
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