Factors operating in wintering areas

Over the same period, much tropical forest, where many North American migrants spend the winter, has been destroyed. The highest rates of deforestation have occurred in Central America, the very regions where forest migrants are most concentrated (Myers 1980, Terborgh 1989). On present evidence, it is impossible to say to what degree tropical deforestation has contributed to the declines of forest migrants, but if it continues at the recent rate, it may soon overtake events in breeding areas as the major cause of declines. Not all species are likely to have been affected adversely, however, for some thrive in the secondary habitats that replace the forest.

One species already affected by tropical deforestation, according to Terborgh (1989), is Bachman's Warbler Vermivora bachmanii, probably now extinct. At the end of the nineteenth century, this species bred widely in damp, broad-leaved woodland across the southern USA, but by the 1950s it could be reliably seen only at a few places in coastal South Carolina. All wintering records came from Cuba, from the dense evergreen thicket that once covered large parts of the island, and which has since been largely replaced by sugar cane. Almost certainly, this warbler has succumbed primarily through destruction of its wintering habitat, for substantial areas of breeding habitat still remain. More generally, declines in breeding populations of Neotropical migrants have been linked to their winter habitat preferences - a pattern that is consistent with trends in forest cover in the tropics (Robbins et al. 1989).

All migrants are affected by conditions in both breeding and wintering areas, and in recent years species have presumably been adjusting to changes in one or other area. However, the habitats occupied by some species, in both breeding and wintering areas, are so vulnerable to human activity that such species seem destined to decline markedly in the coming years. The Cerulean Warbler Dendroica cerúlea has suffered extensive loss of breeding habitat in the past 200 years (Robbins et al. 1992). Its favoured nesting habitat, mature floodplain forest with tall trees, has become scarce over most of its original nesting range, and its apparent sensitivity to fragmentation of remaining tracts gives it an additional disadvantage. In winter the species is restricted to primary humid evergreen forest along a narrow elevation zone at the base of the Andes. This zone is among the most intensively logged and cultivated regions of the Neotropics. From 1966 to 1989, the Cerulean Warbler showed the most precipitous decline of any North American warbler (3.4% per year nationwide).

Surprisingly, few studies have examined annual fluctuations in the numbers of Nearctic-Neotropical species in relation to events in wintering areas. This contrasts with the situation in Palaearctic-Afrotropical migrants. However, the annual survival of Black-and-white Warblers Mniotilta varia, studied in a wintering area in Puerto Rico, was related to the local rainfall, with better survival in the wetter years associated with El Niño conditions (Dugger et al. 2004). Similar findings emerged in Black-throated Blue Warblers Dendoica caerulescens studied in New Hampshire and Yellow Warblers Dendoica petechia studied in Manitoba (Sillett et al. 2000, Mazerolle et al. 2005). In both these areas the wetter years were associated with El Niño conditions, which also affected climate in Central and South American wintering areas, leading to both improved survival and improved breeding success through effects on food supplies.

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