WINDEN J VAN DER, HAGEMEIJER W & TERLOUW R (1996) Is there a future for the Black Tern Chlidonias niger as a breeding bird in The Netherlands?. LIMOSA 69 (4): 149-164.

Black Tern Chlidonias niger The Black Tern is an endangered species included in the Dutch Red Data list. Study of information available on distribution and abundance of this species' breeding population in The Netherlands since 1930 resulted in the present review. The survey was based on periodical and regional quantitative and qualitative estimates and led to population estimates for several periods. Further, at least 311 sites were sampled for breeding Black Terns. A loglinear Poisson regression was used to establish nationwide long-term trends as well as annual indices. The regression was also used to establish regional differences in trends. The Black Tern population decreased from 13 00020000 pairs during 1930-50 to 1100-1500 pairs nowadays. Simultaneously, the spatial distribution decreased to 25% of that of the thirties. The Dutch population is more or less stable since 1985. At present, the Black Tern is restricted to just a few core areas in the Peat Moss district (grasslands and marshes) and Eastern River district. Trends during 1941-93 in six breeding bird districts were distinct. In Peat Moss grasslands and the Eastern River district, the decline was not as strong as in the other districts (Fig. 5). Vicissitudes in the six breeding bird districts since 1985 differed markedly. A sharp decline can be noted in the Fen and Peat Bog district, the Marine Clay district and the High Clay district. The Black Tern population increases in the Eastern River district, whereas it is more or less stable in the Peat Bog Marsh and Grasslands districts. Mean colony size is 530 pairs at present. More than 50 pairs in one area is exceptional. Large concentrations are generally subdivided in several smaller settlements. In most areas, Black Terns breed on floating vegetation (Stratiotes aloides, Nymphaea alba, Nuphar lutea and mats of algae) or on grasslands, mudbanks or floating objects. An increasing part of the population breeds on anchored rafts since the early sixties; currently, this concerns up to 48% of the population. Moreover, at least 75-80% of the total population breeds in nature reserves. Changes in water quality (eutrophication) resulted in a sharp decline of Stratiotes aloides and therefore a significant loss of suitable breeding habitat. Other plant species also used as a nesting site seem less suitable. Rafts are eagerly accepted in some parts of The Netherlands, especially in the Eastern Zwarte Stem (Paul Lodewijkx) Black Tern Chlidonias niger. River district and the Peat Moss Marshlands with eutrophic waters. Nesting habitat seems to be the limiting factor in these areas. In other districts rafts are less eagerly accepted; other limiting factors (e.g., food availability) seem to be more important there. Intensification of agriculture probably resulted in decreased food availability. A relatively small Black Tern breeding population will probably be maintained under the given circumstances. Improvement of water quality and development of marshlands as planned may result in better conditions for Black Terns in the near future.

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