EERDEN MR VAN & ZIJLSTRA M (1985) Cormorants Phalacrocorax carbo in the Oostvaardersplassen, the Netherlands, 1970 - 1985. LIMOSA 58 (4): 137-143.

Great Cormorant Phalacrocorax carbo The establishment of a new colony of Cormorants Phalacrocorax carbo sinensis in the IJsselmeer region, Netherlands, is described. Following embankment of Southern Flevoland in 1968, resting birds were seen regularly on bare ground near open water in the marshes of the Oostvaardersplassen (tab. 1). Raising the water level in the area during and after 1976 led to the loss of resting places and to decreasing numbers; but after willows had died, the dead trees served as resting places and numbers rose again. Greylag Geese Anser anser caused a marked change in vegetation structure. After 1975, intensive grazing during wing moult led to opening of the reeds and gave rise to open water adjacent to the willows (fig. 1). Cormorants were seen sleeping here in 1976 (peak 50) and 1977 (peak 250) after which breeding started in 1978. From 175 pairs in 1978, the colony expanded rapidly to 5100 pairs in 1985 (fig. 4). Because of the rate of increase, the new colony is thought to originate from a non-breeding cohort, possibly Naardermeer birds (fig. 5). Nest counts in winter provided an estimate of at least 80% of the number of breeding pairs in the summer before. Flight intensity of the colony birds was recorded in 1980-85 during one entire day in May when the majority of the breeding birds is present. The flight pattern of breeding birds was studied from a hide in the colony in 1982 (fig. 3). Age of the young appeared to determine the activity of the parents. To calculate the number of breeding pairs a correction of flight intensity was made based on the age of the young. Estimates were made of the proportion of non-breeders present (assuming two feeding trips per bird per day). From these data the overall number of birds present as well as the number of breeding pairs could be calculated from the number of flights recorded (tab. 2). Aerial photographs in 1984 revealed 4360-4550 nests compared to 4600 estimated by the method described before. It is argued that in recent years competition between adult breeders and immature nonbreeders might have led to the disappearance of the latter (tab. 2). Breeding space is by far not limiting, only 30% of the willow woods now being in use (1985) while many birds breed also on the ground (fig. 1). The mean height of nests was 2.49 m (n = 3490), only 32% breeding above 3.50 m.

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