Although in the last 30-40 years some tropical migrant birds have declined in western Europe and others in eastern North America, the causes seem to have differed. In Europe, declines have mainly involved species that winter in the arid savannas of tropical Africa, which have suffered from the effects of drought and increasing desertification. In several species, annual fluctuations in numbers and adult survival rates were correlated with annual fluctuations in winter rainfall, and by implication winter food supplies. Most species that were sufficiently studied showed no obvious changes in breeding success that could be linked with population changes (examples of exceptions: Corncrake Crex crex, Turtle Dove Streptopelia turtur, see Chapter 24).
In North America, by contrast, declines have affected many species which breed and winter in forest. In eastern forest regions, declines have been attributed to human activities on the breeding range, particularly forest fragmentation and associated agricultural and suburban developments, which have led not only to loss of forest, but to increases in the densities of nest predators and parasitic cowbirds. Declines in the numbers of some migrants are thought to result from declines in breeding success which is now too low to offset the usual adult mortality, though as yet convincing evidence is available for only a minority of species, such as Kirtland's Warbler Dendroica kirtlandii and Wood Thrush Catharus mustelinus in some areas. In other species, such as Bachman's Warbler Vermivora bachmanii, tropical deforestation seems to have played the major role in population decline, and is likely to affect an increasing range of species in the future. Whereas for the Palaearctic-Afrotropical system a considerable consensus exists on the causes of declines, for the North American system much of the evidence is as yet little more than suggestive, and no one explanation can account for all the facts (for discussion see James & McCulloch 1995).
In consequence, opinions remain divided on the causes of recent declines in Nearctic-Neotropical migrants. Those who favour events on North American breeding areas as major causes emphasise the following:
• Until recently, deforestation was far more extensive in North America than in countries to the south, which should have reduced migrant populations well below the limit set by wintering habitat.
• In North America, declines are much more marked in eastern than in central and western areas, and within eastern areas, are most marked in small woods, but are non-existent or less marked in remaining extensive forest tracts. This points to events on breeding areas as being most important, for if tropical deforestation had been the cause, the argument goes, declines would have been expected across the breeding range and (in some species only) in both small and large woods (Wilcove & Robinson 1990).
• Some migrant species were declining as early as the 1940s, well before deforestation in Central America reached anything like its current levels. Other species have declined in numbers in recent decades even in patches of unchanged wintering habitat (Faaborg & Arendt 1992).
• The declining species include some, such as Red-eyed Vireo Vireo olivaceus, Scarlet Tanager Piranga olivacea and Wood Pewee Contopus virens, which winter south of most other species, in the relatively undisturbed western Amazon basin.
• The numbers of some species are known to respond on a year-to-year basis to conditions in North American breeding areas, particularly food supplies, but also rates of nest predation and parasitism.
On the other hand, those who favour events on Central and South American wintering areas as causing declines point to the following:
• Enormous destruction and degradation of tropical forests has occurred during the last 50 years. Because the migrants are much more concentrated in wintering habitat, the destruction of each hectare of tropical forest could affect far more birds than the destruction of the same area of temperate forest.
• In the breeding areas, by contrast, total forest area has actually increased in recent decades, as abandoned farmland has become tree-covered (Sauer & Droege 1992). This and other factors have tipped the previously increasing cowbird population into decline in eastern districts. Many migrants are now scarce or absent from much apparently suitable nesting habitat, especially in the smaller woods.
• The declines in Neotropical migrants have not been paralleled by similar declines in residents or short-distance migrants that nest in the same woods, but remain year-round in North America.
The whole situation exemplifies the problems often facing ecologists, in understanding the primary cause of a problem when several likely factors change at the same time, when several may act together, and when different factors may affect different species. It also exemplifies the enormous geographical scale on which studies must be undertaken if reliable answers are to emerge. In reality, we are probably dealing with a continuum, with some species or populations declining primarily because of changing conditions in breeding areas (perhaps different conditions in different species), and others because of changing conditions in wintering areas. Also, because most species occupy large geographical areas, summer and winter, pressures and population trends may vary from one part of the range to another, and one period of time to another. It seems, therefore, that further progress is likely to come mainly from detailed studies of particular species, ideally in several different areas, summer and winter. Almost certainly, the division of populations into 'summer-limited' and 'winter-limited' is overly simplistic, because ecological conditions experienced by individuals at one season can influence their performance at another, as carry-over effects (Chaper 26). Bear in mind, too, that the causes of long-term trends may differ from the causes of year-to-year fluctuations about the long-term trends. In consequence, species can show different long-term trends but similar annual fluctuations, or vice versa.
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