Despite an ongoing debate about the number of species involved, and the significance of regional variation, most studies in North America have attributed declines in tropical migrants to events in breeding areas (Terborgh 1989, Askins et al. 1990, Hagen & Johnston 1992, Sherry & Holmes 1992, Martin & Finch 1995, Latta & Baltz 1997). For some eastern forest species, problems are thought to stem from forest loss and fragmentation, the latter probably causing population declines much greater than expected from the areas of habitat lost. The evidence consists partly of an apparent contrast in population trends between birds in small forest fragments and those in extensive forest areas, and partly from current species distributions, which show that many species are now almost absent from small woods. Relevant studies span a wide area from the Atlantic to the Mississippi (Bond 1957, Robbins 1979, 1980, Whitcomb et al. 1981, Ambuel & Temple 1983, Blake & Karr 1987, Howe 1984, Lynch & Whigham 1984, Hayden et al. 1985, Freemark & Merriam 1986, Robbins et al. 1989, Askins et al. 1990).
Many studies have entailed assessment of the numbers and kinds of species breeding in forest fragments of different sizes in the same region. The main finding was as expected, namely that total species numbers increased with increasing size of forest. While certain species were found in both small and large forest fragments, others were found entirely or almost entirely in large ones, mainly in the forest
2ln the USA, the total area of forest reached a low of 200 million ha in 1910-1920, since which time the area has increased to more than 250 million ha, or approximately 60% of the original area. Much of this forest is relatively young, however, and in parts highly fragmented, so is not suitable for all species (Terborgh 1989).
interior. More importantly, however, species numbers per unit area also increased with increasing size of forest fragment. This was especially true of long-distance migrants that winter in the tropics. In most studies, forest area accounted for most of the variation in species richness and total density of birds, and especially of Neotropical migrants (Askins et al. 1990, Lynch & Whigham 1984, Blake & Karr 1987, Robbins et al. 1989). Other variables, such as the degree of isolation from other forests, or vegetation structure and heterogeneity, explained relatively little additional variance in species numbers, as judged from the findings of multiple regression analyses. However, vegetation variables influenced the kinds of species that were present, and their local densities (Ambuel & Temple 1983, Lynch & Whigham 1984). As expected, edge species occurred at greatest density in small forests. Similar relationships between species numbers and area have been found in forest and other habitat areas in other parts of the world, but European woods seem to hold few species that could be considered as forest-interior specialists (Newton 1998b).
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