Spatial and temporal distribution of shorebirds: predicting the effects of habitat change on the Forth Estuary.
One of the many threats to coastal shorebirds globally is the loss or degradation of estuarine intertidal mudflats, a habitat that supports large concentrations of birds both on passage and throughout the winter months. British estuaries comprise 28% of the entire estuarine area of the Atlantic and North Sea coastal states (Atkinson et al. 2001), more than any other European country. Because of this, many UK estuaries are of great importance in both a European and international context for wintering birds (Pollitt et al. 2000). Furthermore, Britainâ€™s estuaries can be particularly important during periods of severe cold weather in continental Europe (Norman & Coffey 1994), when there may be influxes of waterfowl from other coastal regions or inland areas. Some sites also act as cold weather refuges where parts of the estuarine system freeze more slowly than other nearby coastal and inland wetlands and so can provide feeding habitat when other sites are unavailable. Habitat change may not always mean habitat loss, even though large intertidal areas have been removed via landclaim (Evans 1979, McLusky et al. 1992) and engineering works (Schekkerman et al. 1994) or are threatened by the gradual rise in sea level (Austin et al. 2001). Determining the effects of habitat deterioration on shorebirds is often more difficult to predict (Sutherland 1998b) as, although the habitat remains intact, it may reduced in quality due to pollution events (McLusky 1982, McLusky & Martins 1998) or disturbance (Burger 1994, Burton 1996, West et al. 2002).
The consequences of habitat change on estuaries are so potentially threatening to shorebird populations that programs of managed realignment (Burd 1995) have been introduced at some sites in order to mitigate such alteration (Dixon et al. 1998, French 1999, Hackney 2000). Such management activity involves the breaching of existing sea walls to allow the land behind to gradually return to estuarine habitat. It has been shown that invertebrates will colonise suitable intertidal habitats and that birds are quick to adapt to such new habitats (Evans et al. 1998).