ARTS F, GRAVELAND J & MEININGER PL (2000) Coastal breeding birds, vegetation succession and man-made habitats: implications for future management of coastal areas. LIMOSA 73 (1): 17-28.

Since 1950 the numbers of breeding plovers and terns in the Delta area of the SW Netherlands have shown strong declines. In this paper possible causes for this decline are discussed: loss of breeding areas, degradation of habitat quality due to succession of vegetation and increased recreational and predation pressure. Four hypotheses are tested: 1) the suitability of still-existing breeding sites has been declining in the Delta during the last decades; 2) more rapid vegetation succession has played an important role in the decrease of suitable breeding sites; 3) restoration measures are successful in the sense that they lead to rapid colonisation by breeding birds; 4) restoration measures in salt waters are more successful than those in fresh waters because succession of vegetation occurs less rapidly in salt water. The study is based on complete censuses of breeding pairs in 1979-98 and measurements of vegetation cover of 110 potential breeding sites in 1999.
      Since 1950 the total area of potential breeding sites has only slightly decreased. However, the proportion of areas influenced by salt tidal water decreased from 70 to 31%, the proportion of areas in saline lakes increased from 10 to 30%, and the proportion of areas in freshwater lakes increased from 20 to 39%. Almost all man-made breeding sites created since 1990 were colonised immediately alter construction. This suggests that the availability of breeding sites is an important factor limiting population sizes in the Delta area. In most cases bird numbers decreased again alter a few years. This decrease occurred much sooner in freshwater lakes than in saline lakes. The percentage of area without vegetation showed a strong negative correlation with the age of the site, and decreased much quicker in freshwater areas than in salt areas. Differences in age and recreational pressure between salt tidal water and the other areas and the limited number of salt tidal water sites does not permit firm conclusions on the relationship between tidal influence and vegetation succession.
      The results suggest that an increased rate of vegetation succession as a result of the Delta Project (damming ol several sea-arms) is the main cause of the decline in populations of coastal breeding birds since 1950. Habitat creation for coastal breeding birds has proven to be effective, but only temporarily, in particular in freshwater habitats. However, most man-made habitats suitable for coastal breeding birds were created in freshwater areas.
     To what extent is the relation between vegetation cover and bird numbers causal? Coastal breed ers like terns and plovers prefer sparsely vegetated or almost barren areas, but within these areas they generally select a nesting spot with at least some vegetation . So why do they prefer sparsely vegetated areas? There are two types of explanations, both related to predation: 1) the predation risk is smaller in sparsely vegetated areas, or 2) the birds infer that the risk is smaller. For management strategies this distinction is important: removal of vegetation may result in a smaller predation risk, but it might also lure the birds into an ecological trap.
     Conservation and restoration of areas influenced by natural, dynamic processes (salinity and tide) offer the best perspectives for coastal breeding birds. In the past, the effects of succession were partly compensated by the creation of new breeding sites as a result of the Delta Project. As a consequence of the vegetation succession and a lack of newly created breeding sites, the future of coastal breeders in the Delta area is bleak. Thus, changes in policy and management are necessary to maintain sizeable populations of coastal breeding birds in the Delta area. The emphasis should be on habitat creation or restoration of natural dynamics in salt tidal areas. Removal of vegetation in existing habitats should only be considered if additional research has proven that it does not create an ecological trap. #### Pied Avocet Recurvirostra avosetta Common Ringed Plover Charadrius hiaticula Kentish Plover Charadrius alexandrinus Mediterranean Gull Ichthyaetus melanocephalus = Larus melanocephalus Black-headed Gull Chroicocephalus ridibundus = Larus ridibundus European Herring Gull Larus argentatus Sandwich Tern Thalasseus sandvicensis = Sterna sandvicensis Common Tern Sterna hirundo Little Tern Sternula albifrons = Sterna albifrons

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