Experimental evidence on population limitation in migrants

Most of the evidence obtained on the causes of population changes, whether for Old World or for New World species, is based on correlation and inference. Experiments are almost impossible to perform at regional scales, but sometimes widespread changes in land-use or legislation have been followed by widespread changes in bird populations. Examples include the increases in many waterfowl and shorebird populations that followed the introduction of protective legislation or other conservation measures, or the increases of bird-of-prey populations that followed reductions in the use of organochlorine pesticides (Newton 1979, 1998b, Cade et al. 1988). In addition, local population increases have followed experimental manipulations of potential limiting factors or the introduction of specific conservation measures. Examples include increases in breeding success and breeding density that were associated with: (1) removal of predators (for various ducks see Duebbert & Kantrud 1974, Duebbert & Lokemoen 1980; for Sandhill Crane Grus canadensis see Littlefield 1995); (2) removal of Brown-headed Cowbirds Molothrus ater (for Bell's Vireo Vireo bellii see Griffith & Griffith 2000, for Black-capped Vireo V. atricapillus see Hayden et al. 2000); (3) provision of extra nest-sites for species with special needs (for many species of cavity-nesting and other birds see Newton 1994, 1998b); (4) removal or exclusion of competing species (for effects of tit Parus removal on Collared Flycatcher Ficedula albicollis densities (via access to nest-sites) see Gustafsson 1988); and (5) change or intervention in destructive agricultural procedures (for Corncrake Crex crex see Green et al. 1997, Green 1999; for Stone Curlew Burhinus oedicnemus see Aebischer et al. 2000). Similar measures have led to popul ation increases in some resident bird species too, as has the provision of additional winter food (Newton 1998b). Restrictions in hunting pressure and pesticide use have affected wintering populations, as well as breeding populations, but the planned experiments were restricted to local breeding populations. They confirm that many migratory species were at that time and location limited by conditions in breeding areas.

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