Altogether, about 250 landbird species from North America winter at least partly in the Neotropics, rather more than the Palaearctic migrants in Africa. Because these migrants are drawn from a smaller land area, however, their numbers are smaller than in the Palaearctic-Afrotropical system, perhaps around 3000 million (MacArthur 1972). Of the species that reach South America, the broad-winged raptors migrate mainly overland via Panama. Many other species that breed in eastern North America cross the Gulf of Mexico, or the Caribbean Sea (stopping on various islands), and yet other species take a longer overwater route, leaving the eastern North American coast and flying directly across the Atlantic to South America (Chapter 6). In autumn, different species take off at different points along the coast, so that the numbers seen on the coast diminish southward, and relatively few leave from Florida (Terborgh 1989). Hence, although shorter sea-crossings are available, many species in autumn make a substantial sea-crossing, which shortens their overall journey, compared with flying down the eastern seaboard, and crossing via Caribbean Islands. Species from western North America take an overland route to winter mainly in Mexico (although some first cross the northeast Pacific from Alaska to further south on the western seaboard, Chapter 6).
Most of the migrants from eastern North America winter in South America or on Caribbean Islands, while the ones from western North America are accommodated mainly in Central America. Moreover, because most of South America lies in longitudes to the east of North America, the prevailing direction of most intercontinental migration is southeast in autumn and northwest in spring. The latitudinal span that is occupied by Central America and the Caribbean Islands in the New World is occupied by the Sahara Desert in Africa, which accounts for why the New World migrants migrate less far, on average, than the Eurasian-Afrotropical ones.
North America south of the tree-line covers approximately 16 million km2, and within this area most migratory species have ranges extending over at least several million square kilometres. In winter, these migrants extend over a somewhat larger area, covering about 18.6 million km2 from Mexico southward to the tip of South America, and including the Caribbean Islands. However, as in sub-Saharan Africa, the migrants are not uniformly distributed over this whole area, but are concentrated in the northern tropics. In fact, the main wintering area for many migrant species extends from Mexico south to Panama, including the Caribbean
Islands, a region covering about 2.7 million km2, in which the main natural vegetation is forest. Such a concentration of migrants is known nowhere else on earth. The great disproportion between the breeding and wintering areas of the migrants may result partly from geographical circumstances, notably the progressively narrowing land area in Central America. Not only are the populations of many species highly compressed there, but more species are found together in the same area. For example, in deciduous forest in the eastern USA, some 4-6 warbler species are commonly found together in the same area in summer, but in homogeneous Neotropical forest, 8-10 species are commonly found together, and occasionally up to 15 species, densely packed in communities of extraordinarily high diversity. Some specialists do have extensive distributions in winter, but they are in the minority. They include the Black-and-white Warbler Mniotilta varia which feeds nuthatch-like on trunks and branches, and the Northern Waterthrush Seiurus noveboracencis and Louisiana Waterthrush S. motacilla both of which hunt insects along streams (Terborgh 1989). Unlike Africa, mid- to high-elevation areas (1000-2500 m) are heavily used by migrants in the Neotropics, presumably a consequence of the large proportion of the Neotropical land area that is over 1000 m and well forested.
In the Neotropics, then, the numbers of migrant species and individuals, and their proportion in the local avifauna, decline southwards from Mexico (Slud 1976), but with big variations between habitats (Terborgh 1980). This parallels the north-south decline in migrant numbers that occurs in sub-Saharan Africa. On both continents, it seems that species migrate no further than is needed to find suitable wintering habitat. If the survival chances on migration decrease as the journey lengthens, which may be true at least as a generalisation, this could provide the selection pressure to keep the migration as short as possible (Karr 1976). In addition, the decline in the numbers of migrant species towards the equator is more than offset by an increase in the numbers of resident species, so that competition may be another factor curtailing further southward spread (Keast 1980, Terborgh 1980). On various Caribbean islands, the proportion of migrants in winter bird populations declines with increasing distance from the mainland (Florida), from about 35% on Hispaniola to 10% on Puerto Rico, and 1% on the more distant islands of the Lesser Antilles, Trinidad and Tobago, as well as on the Venezuelan mainland. The effect of island size is difficult to separate from the effect of latitude and distance from the mainland, but comparison of adjacent large and small islands reveals some tendency for migrants to gravitate to larger islands (Terborgh & Faaborg 1980).
In general, Central and South America, with their greater areas of high rainfall, provide greater areas of forest than Africa. Rainforest is again centred on the equator, but is much more extensive, reaching more than 20°N in Central America and more than 10°S in Amazonia, with an additional strip extending to nearly 30°S on the eastern coast. As in Africa, owing to decline in rainfall, habitats become progressively more open north and south of the rainforest, passing through woodland and scrub savannah and grassland (pampas) and in the south to shrubby desert. The driest desert occupies a strip between the western coast and the Andes. The mountains themselves support various types of montane forest and high-elevation grassland. Another area of high rainfall lies on the southwestern coast, supporting mixed evergreen forest.
Like Africa, South America falls under the climatic influence of the InterTropical Convergence Zone, but the effects are less clear-cut because of the more varied topography of South America, and the influence of the Andes range in the west. As in Africa, however, seasonal flooding greatly increases the area of wetland, especially south of the equator. The Amazon Basin receives more than 3 m of rain per year, which provides 57 000 km2 of seasonally flooded forest. The southern savannahs (llanos) support the greatest area of seasonal swamp on earth, the 60 000 km2 of flood, drought and fire that forms the Pantanal.
As in Africa, migrant species generally choose winter habitats that resemble their summer ones: species that breed in forest or scrub winter in forest or scrub, and most of those that breed in grassland winter in grassland. According to Hutto (1986), the similarity is even greater, and the majority of western migrants that winter in western Mexico occur in habitats closely resembling their breeding ones, whether desert scrub, riparian, oak or oak-pine woodland or conifer forest. However, some species show striking seasonal differences in feeding behaviour. Among passerines, the Worm-eating warbler Helmitheras vermivoras is a fairly typical foliage-gleaning bird during most of the breeding season, but becomes highly specialised in foraging from the leaf litter caught up in tropical forest trees in the non-breeding season (Willis 1980). The Yellow-rumped Warbler Dendroica coronata changes from mainly tree-feeding in the breeding season to mainly ground-feeding in the non-breeding season. In addition, among seven migratory species that winter in the Yucatan, four breed in forest and winter in scrub, while three breed in scrub and winter in forest (Askins et al. 1990). As elsewhere, many shore-birds breed on the tundra and winter on sea-coasts.
Also as in Africa, those migrants that remain north of the equator experience deteriorating conditions throughout their stay, as the dry season progresses, while the minority that move south of the equator experience improving conditions. The latter again include those species that depend on insects or other food items that reach peak supply in the southern rainy season. They include the Mississippi Kite Ictinia mississippiensis and Swainson's Hawk Buteo swain-soni which eat grasshoppers, the Common Nighthawk Chordeiles minor, Chimney Swift Chaetura pelagica, Purple Martin Progne subis, Barn Swallow Hirundo rustica and Eastern Wood Pewee Contopus virens, all of which eat flying insects, and cuckoos that eat caterpillars from young leaves. They also include some shore-birds, which winter on the coasts. Most long-distance raptors are nomadic during the dry season, and are often attracted to large-scale seasonal burns. Others, including Swainson's Hawks, follow migrating swarms of dragonflies, and still others, such as Swallow-tailed Kites (Elanoides forficatus), track populations of ephemerally swarming termites (Bildstein 2004).
Most forest species spend longer each year in their tropical wintering areas than in their northern breeding areas. They are clearly as much a part of the tropical bird communities in which they winter as of the temperate zone communities in which they breed, occupying different niches from one another and from resident species (Lack & Lack 1972, Rappole & Warner 1980, Terborgh & Faaborg 1980, Rappole et al. 1983, Rappole 1995). Like their Old World counterparts, many Neotropical migrants are insectivores, but whereas in Africa they are mainly in the families Turdidae (thrushes, 18 species) and Sylviidae (warblers, 29 species), in the Neotropics they are mainly in the Parulini (wood warblers, 46 species)
and Tyrannidae (tyrant flycatchers, 23 species) (Karr 1980). When the northern migrants leave in spring, their niches are mostly left vacant until the following autumn. Unlike the situation in Africa, few southern hemisphere birds migrate so far north in the austral winter that they could occupy these same niches while the Nearctic-Neotropical migrants are absent.
Many migratory species maintain the same feeding territories throughout their stay and return to them year after year. Examples include the Northern Waterthrush Seiurus noveboracensis, Northern Oriole Icterus galbula, Summer Tanager Piranga rubra and various warblers (Schwartz 1964, Stiles & Skutch 1989, Rappole 1995). Other species, such as the Eastern Kingbird Tyrannus tyrannus, defend short-term territories, wherever food is temporarily available. The various hummingbirds provide extreme examples, as individuals defend particular flower patches only for the short periods they produce nectar, and then move on to other more productive patches. In some territorial species, the sexes are found in mainly different habitats (Lynch et al. 1985, Wunderle 1992), either because of different preferences, as in Hooded Warbler Wilsonia citrine (Morton et al. 1987), or because male dominance causes females to occupy largely poorer habitats, as in American Redstart Setophaga ruticilla (Marra et al. 1993, 1998). The effects of male dominance, in enabling males to occupy the better wintering habitats, may partly account for the great preponderance of males seen in many species of Neotropical migrant passerines in their North American breeding areas (Stewart & Aldrich 1951, Marra & Holmes 1997).
Some wintering forest migrants join group territories containing individuals of several species. Such groups consist predominantly of Neotropical residents, such as ant-wrens and ant-vireos, but for half the year they also contain Nearctic migrants such as Blue-winged Warbler Vermivora pinus and Canada Warbler Wilsonia canadensis (Greenberg & Gradwohl 1980, Greenberg 1985). Each group normally contains only one pair (or family) of each species. Every morning the birds assemble to forage together. All group members recognise the same territorial boundaries so that in effect the group, rather than its component pairs, holds the territory. This is a social system which is also known from the forests of Southeast Asia (Harrison 1962), but is unknown among the few Palaearctic breeding species that spend the non-breeding season in the Afrotropical forests.
Other forest migrants join single species or mixed species flocks which move around to exploit sporadically available food supplies (Rappole & Warner 1980, Greenberg 1985, Terborgh 1989). Some species, such as Swainson's Thrush Catharus ustulatus, join groups of local species which follow columns of army ants to feed on the other insects disturbed (Willis 1966). Like the African migrant insectivores, many also eat fruit in winter (Parrish 2000), and some, such as the Yellow-rumped Warbler Dendroica coronata, may make long movements within a winter (Lack & Lack 1972, Terrill & Crawford 1988). The same is true for various thrushes which move slowly southward, consuming fruit crops as they go, until the time comes for them to return to their breeding areas (Karr 1980). One kind of fruit is especially abundant in the Neotropics, namely the aril, in which soft oil-rich tissue surrounds a hard, indigestible seed coat. When the dry inedible pods that contain arillate seeds dehisce in spring to expose the red, yellow or white arils, migratory birds of many kinds eat them, including flycatchers, thrushes, vireos and warblers. Individuals of some species show fidelity to particular sites, even though they may move between several sites during the season (for Indigo Bunting Passerina cyanea, see Johnston & Downer 1968). It seems, however, that itinerancy is much less common among Nearctic-Neo tropical migrants than among Palaearctic-Afro tropical ones. This difference is associated with the drier, more seasonal habitats occupied by most species in Africa, which in general are hospitable for only a part of the non-breeding period.
As in Africa, many migrants of grasslands and savannahs feed at fires on the animals disturbed or killed. Such fires start up toward the end of the dry season, and in due course range over much of the non-forested parts of the South American continent. A wide range of bird species, including raptors, flycatchers, swallows and many others drawn from among both intra-tropical and Nearctic-Neotropical migrants, accompany the fires as they work their way across country, and feast upon the various animals that are flushed or injured.
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