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Significance of variation between categories (examined by Monte-Carlo randomisation test): = 35.9, P < 0.001.

Of 22 species that eat mainly warm-blooded prey, only one species (Booted Eagle Hieraaetus pennatus) winters entirely in Africa.

In contrast, seven of nine species that eat mainly cold-blooded prey winter entirely in Africa. They include six insectivores, four of which winter entirely (and two largely) south of the equator, where the seasons are reversed. The 12 species with mixed diets show intermediate patterns.

From Newton (1998a).

Significance of variation between categories (examined by Monte-Carlo randomisation test): = 35.9, P < 0.001.

Of 22 species that eat mainly warm-blooded prey, only one species (Booted Eagle Hieraaetus pennatus) winters entirely in Africa.

In contrast, seven of nine species that eat mainly cold-blooded prey winter entirely in Africa. They include six insectivores, four of which winter entirely (and two largely) south of the equator, where the seasons are reversed. The 12 species with mixed diets show intermediate patterns.

From Newton (1998a).

Eagle Hieraaetus pennatus) winters entirely in Africa (Table 13.1). Of the nine species that eat mainly cold-blooded prey, some winter partly in the Palaearctic and partly in Africa, but most winter entirely in Africa. Moreover, all six insectivorous species winter south of the Sahara, four of them entirely (and two largely) south of the equator, where the seasons are reversed. So most insectivorous species live in almost perpetual summer, in conditions in which they have easy access to their insect food supply year-round. The 12 species with mixed diets show intermediate patterns. Such patterns again underline the link between migration and the seasonal changes in specific food sources (Newton 1979).

That movements are related to diet is also apparent in tropical regions, even though many such movements are relatively short. In tropical forests, nectar-eaters and fruit-eaters move around more than other species, probably because flowers and fruit are much more seasonal and patchy in occurrence than are the insects and other small creatures eaten by other birds (Morton 1980, Levey & Stiles 1992). Because flowers and fruit are generally more abundant in the canopy and forest edge, it is these parts of forest environments that show the greatest seasonal variation in their bird populations, the understorey species maintaining the most sedentary lifestyles year-round (see above). Species of open habitats, such as savannah and grassland, tend to fluctuate strongly through the year because such habitats tend to be more seasonal than forest, notably in rainfall, which affects the food supplies of a wide range of species. In these grassy habitats, fires also attract many kinds of birds in the dry season to feed on the insects and other small animals disturbed and killed by the flames. Again, whether a species is resident or migratory depends mainly on its diet, and whether its particular foods remain locally available year-round. Its phylogenetic status seems largely irrelevant (Chapter 20).

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