Except in very arid areas, most grasslands require periodic vegetation removal to prevent colonization by scrub and trees, and in the case of some very wet grasslands, succession to fen. This can be done by grazing, cutting, or burning. The latter is likely to be applicable only on drier grasslands, particularly those formerly maintained by natural fires. The primary consideration when managing grasslands for birds is how to create the desired sward conditions at particular times of year, while minimizing the potential damaging effects of management activities on breeding birds. On wet grasslands hydrology is also important in influencing use by wildfowl and waders.
The most important factors influencing bird use on grasslands are usually the height and structure of the sward, and the quantity of litter and bare ground. All these aspects can be manipulated by management, as can vegetation composition. Vegetation composition can itself influence sward structure, and may also directly influence food supply, for example, by providing suitable seeds or palatable grass species.
Vegetation height and structure affect the suitability of nest-sites, abundance and accessibility of prey, and the ability of birds to detect predators (e.g. see review by Vickery etal. 2001). Structure can be difficult to define and measure, but generally refers to variation in density and height (see Chapter 11). It is useful to distinguish between fine-scale variation in structure over tens of centimeters (often referred to as "tussockiness") and coarse scale variation (over tens of metres or more). The availability of litter and bare ground can also influence conditions for some birds. Some species, such as Henslow's Sparrow Ammodramus henslowii, require dense litter for nesting, while others, such as Stone Curlew Burhinus oedicnemus, require bare or sparsely vegetated ground (Green et al. 2000). Bare ground may also increase access for birds to soil invertebrates (Perkins et al. 2000) and surface-living arthropod prey such as beetles, while a dense litter layer will reduce it. In general, variation in sward conditions will increase the likelihood of suitable conditions for nesting or feeding being present somewhere in the area.
Scattered scrub and trees can increase the numbers of bird species using a grassland, mainly by providing nest-sites and song posts for more generalist species, rather than grassland specialists. They may also provide nest-sites and look-out posts for predatory birds and thereby reduce the breeding success or survival of grassland species (e.g. Green et al. 1990a).
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