Austral migrants are species that breed in temperate areas of South America and migrate north, towards or into Amazonia, for the southern winter. Migrations among these species are the most extensive of Southern Hemisphere migrations, and the austral system represents a third major migration system, in the sense that the term has been applied to Northern Hemisphere temperate-tropical migration. The geography of South America greatly influences the austral system. Lack of east-west geographical barriers and the shape of the continent promote a pattern of partially overlapping breeding and wintering ranges. The suboscine family Tyrannidae, the tyrant-flycatchers, is the largest group of austral migrants, with other major families including Emberizidae, Anatidae, Furnariidae, Accipitridae and Hirundinidae. Tyrant-flycatchers constitute more than one-half of the passerine austral migrants and roughly one-third of total austral migrants, a taxonomic domination seen in no other global migration system. Parallels exist, however, between austral migration and the Nearctic and Palearctic systems. Many of the same families, including Hirundinidae, Anatidae and Charadriidae, exhibit similarly high degrees of migratory behavi-our in each system. Passerine migration in the austral system is similar in numbers to that of the Nearctic-Neotropical system, but species migrate shorter distances and breed in more open and scrubby habitats. Possible differences in year-round resource availability between South American and North American temperate forests, in addition to differing availability of these habitats, may contribute to the low numbers of forest-dwelling austral migrants.
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