Ardea
Official journal of the Netherlands Ornithologists' Union

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Mörzer Bruijns M.F. & Tanis J. (1955) De Rotganzen, Branta bernicla (L.), op Terschelling. ARDEA 43 (4): 261-271
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The Brent Goose (Branta bernicla) on Terschelling (Netherlands). Since 1932 the Brent has noticeably decreased in the Netherlands as the result of a disease of its staple food, the eelgrass (Zostera marina), which died out completely along the Dutch coast. Although no accurate counts were made before 1932, Brent-shooting inhabitants and ornithologists agree that on Terschelling the number of birds was formally at least three times greater than at the present time. Table I gives maximum numbers observed per month during the period September 1951 until June 1955 on the collective forage grounds of Terschelling. Counts were usually made about four times a month, sometimes more frequently. During April and May almost daily counts made by the bird-wardens of the nature reserve 'Boschplaat' are available. The Brent geese normally arrive in Terschelling at the end of September and the beginning of October, numbers increasing gradually until December and the beginning of January. Many birds leave the island when cold weather and frost set in, often at the end of January. During the second half of March the bird returns in fairly large numbers and during April and May the flocks increase to a maximum of about 2000 individuals. All geese depart during the last week of May. Records for June are rare. Five areas of tidal flats and salt marshes along the south coast of Terschelling are important for the Brent as forage grounds. These areas are (see map): tidal mud-flat Dellewal, tidal flats and salt marshes near Strijp, tidal flat and saltings of the 'Ans', the tidal flats south of Oosterend and the common 'Grie', tidal mud-flats in the area where the tide turns (wantij) and the sandy tidal flats farther east. The tidal flats and the saltings in the nature reserve 'Boschplaat' are the most important among the forage grounds mentioned, even being the most outstanding in the Netherlands. In autumn, winter and spring the geese concentrate in different biotopes of the area. These concentrations can be correlated with the growth of the plants which form the staple food. In September, October, November, and even December the Brent often-perhaps mainly feeds on leaves and roots of Zostera nana in the Zosteretum nanae (see map). During these months they also feed in the saltings of the Salicornietum and the Puccinellietum, eating Salicornia and Puccinellia and Enteromorpha on the mud flats. In December and January the birds move increasingly to the tidal flats where they feed on algae (Enteromorpha sp.). From the end of January until mid-March the birds forage principally on these algae, probably because the other forage plants have disappeared, or have become withered and inedible (Puccinellia). In March, when plant life on the salt marsh begins anew, the geese return to the saltings. On Terschelling during April and May they feed almost exclusively on the vegetation of the Salicornietum and the Puccinellietum. The taking of Salicornia herbacea seedlings has actually been observed. Occasionally the Brent is seen on the highest salt marsh with vegetation of Armeria maritima, Festuca rubra, etc. Isolated groups of Brent geese are seen moving from one Wadden-island to another (e.g. from Terschelling to Vlieland and vice versa), but in most cases the geese remain constant to one vicinity, not spending one day on this island and the next on another or on the mainland.


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