Most long-distance migrants that breed in North America winter in Central America, where they are much more concentrated in than in their breeding areas, but others migrate further south, occurring to the southern tip of South America. Many both breed and winter in forest and, in some species, individuals tend to occur in the same breeding and wintering territories year after year. Some species have declined in recent decades. They include many forest species that breed mainly in eastern North America. These declines could be due to yet unstudied events in wintering areas, such as deforestation. However, considerable evidence has emerged that the declines may be due to events in North America, particularly forest fragmentation and its associated consequences of increased predation and parasitism of nests.
Many of the relevant predators, as well as the parasitic Brown-headed Cowbird Molothrus ater, have increased greatly in recent decades in response to agricultural and suburban developments. They can reduce the breeding success of migrants below sustainable levels, causing populations to decline, and leaving large areas of breeding habitat unused. Declines of Nearctic-Neotropical migrants are most marked in small woods in agricultural landscapes where predation and parasitism rates are greatest. Several life history features appear to make Neotropical migrants more sensitive than resident species to the direct effects of nest losses.
In addition, the abundance levels of some migrant species are also influenced by the abundance levels of caterpillars in their breeding areas, notably Spruce Budworm Choristoneura fumiferana, low levels of which during the 1980s may have contributed to declines in the numbers of several migratory species in eastern deciduous forests. Nonetheless, other migratory species have clearly declined in association with destruction of wintering habitat, which is likely to become increasingly important in the coming years. Recent population declines have also been noted in some non-forest migratory species, attributable to various causes, but often unknown.
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