Warblers seemed everywhere in Central America in late November. Our North American breeders are spread out over a large area in the breeding season but in their wintering grounds in Central America, they are concentrated and very noticeable despite the lack of song and the muted colors of many species. Warblers are the most noticeable of our Neotropical breeding birds, but a closer look finds vireos, thrushes, flycatchers, and others among them. Central America is essential to these unsustainably declining birds.
Many of the birds seen in Central America seem familiar. Seventeen of the thirty-eight warbler species that overwinter in Central America breed in the Greater Sourland Important Bird Macrosite. Other central New Jersey breeding passerines that overwinter in Central America include two orioles, six flycatchers, five swallows, the blue-gray gnatcatcher, and two thrushes. Black and Turkey Vultures are abundant, Ruby-throated Hummingbirds mix with the local species, and familiar herons gather in wetlands and waterways. The list goes on.
It seems strange to see Hooded Warblers sitting on a fence post in the open and Kentucky Warblers sulking at the edge of the lawn, but birds don’t necessarily use the same habitat in winter that they use for breeding. Mature birds take the prime habitats in both the breeding and wintering grounds. Immature birds are left with the poorer habitats. Male and female birds of some species have differing winter habitats. In one study, Hooded Warbler males outnumbered females eight to one in primary forest, but females edged males out in secondary forest. A study of overwintering Redstarts in the West Indies showed adult males taking the best habitat, a forested area, and females and immature birds using substandard habitat, a garbage dump. This is an extreme case, but differential habitat quality is a factor in the higher survivability of adult birds and the high mortality of young birds. Most young birds do not survive their first year.
The Neotropical region is undergoing rapid deforestation. Primary growth forest is rare and becoming rarer. While many Neotropical migrants use second growth forest, new second growth forest is not keeping pace with loss of primary forest, resulting in a net loss of forest habitat. Land cleared for pasture is not suitable for forest birds but will support a variety of flycatchers and seed eaters. Land cleared for monocultures are strongly depauperate in bird species. Palm oil plantations are virtually devoid of bird life. Coffee has driven the rapid deforestation of the highlands. When sun grown coffee is converted to shade grown coffee, habitat is restored. When the understories of primary forests are destroyed and replanted in shade grown coffee, habitat quality decreases. The highest quality coffee is shade grown on the rich volcanic soils of Northern Central America and Europeans have developed an insatiable appetite for it, resulting in much habitat degradation. The factors driving habitat destruction and degradation are complex with the net result that major habitats are changing faster on the wintering grounds than on the breeding grounds.
According to ornithologist Douglas Morse, two-thirds of the breeding bird populations in many areas of North America winter in the tropics and half winter in a limited area in Mexico, northern Central America, and the West Indies that is one-seventh to one-eighth the size of the breeding grounds. The wintering ranges of many Neotropical migrants are entirely within this region. The density of migrating Neotropical birds drops rapidly south of northern Central America. Northern Central America, adjacent parts of Mexico, and the West Indies are essential for conserving these Neotropical migrant species.