Nonforest species

Migrants dependent on natural grassland, such as the Sprague's Pipit Anthus spragueii and Upland Sandpiper Bartramia longicauda, declined greatly following the ploughing of the prairies, but species that can live in agricultural habitats, such as the Indigo Bunting Passerina cyanea, greatly expanded following the felling of the eastern deciduous forest. Of more recent changes, some have been attributed to changes in breeding areas and others to changes in wintering areas. Thus the Bobolink Dolichonyx oryzivorus has been declining in the eastern USA since the early 1900s. It nests mainly in hayfields, and decline has been attributed to procedural changes that make hayfields less suitable as habitat (Bollinger & Gavin 1992). In contrast, an earlier decline of the Dickcissel Spiza americana, which inhabits brushy pastures, was put down to destruction of wintering habitat, resulting from overgrazing by cattle (Fretwell 1980).

Most arctic-nesting shorebirds that winter on Neotropical coastlines have so far been spared the effects of large-scale habitat destruction. As in Eurasian species, their breeding habitats are still largely intact, while their wintering habitats may have expanded in recent centuries as a result of the soil erosion following deforestation, and the resulting expansion of coastal mud. During part of the twentieth century, some shorebird species increased in numbers in response to lessened shooting pressure, which had reduced their numbers in the past. Some raptors, including Swainson's Hawks Buteo swainsoni, suffered from pesticide-induced mortality on wintering areas in Argentina. Around 5000 individuals were picked up under communal roosts, following the spraying of their food organisms (grasshoppers) with the organophosphate compound monocrotophos, now banned in this region (Goldstein et al. 1996). Earlier declines in Peregrine Falcons Falco peregrinus were attributed to the use of organochlorine pesticides on both breeding and wintering areas, and recent recoveries in Peregrine numbers have followed reductions in organochlorine use (Cade et al. 1988). It is more difficult to explain recent declines in wetland and coastal species, such as the Least Tern Sterna antillarum, which in eastern North America declined by 14% per year during 1978-1988 (Sauer & Droege 1992).

0 0

Post a comment