A concentration of moulting Black-necked Grebes Podiceps nigricollis on the Grevelingen From July 1983 up till and including 1988 observations were made on a flock of Black-necked Grebes on the eastern part of the Grevelingen, Delta area. The first Black-necked Grebes arrive there in the course of July. They leave the location late in autumn. No Blacknecked Grebes were observed during winter on the location. A maximum of 116 birds were counted on 27 August 1988. Moulting of Black-necked Grebes on the place was confirmed in 1988. A considerable increase of fish-eating birds in the Delta area has been determined after the closure of the big estuaries. Probably the Black-necked Grebes gather in the eastern part of the Grevelingen for moulting and spread out over the Delta for wintering.