Grote Sterns Sterna sandvicensis op de Hompelvoet en Markenje, 1979-88

This paper provides a description of the breeding season of the Sandwich Terns in the ’Hompelvoet colony’. When the Brouwersdam was finished in 1971, Grevelingen turned into a salt water lake and the colony of Sandwich Terns was much better protected against flooding in spring. As a consequence, the population recovered gradually from the major crashes in the sixties (the Hompelvoet colony was abandoned from 1966-68) caused by contamination of the marine environment with chlorinated hydrocarbons. Today, the Hompelvoet colony is the second largest in the Netherlands (ca. 4,000 pairs). The terns breed either on the Hompelvoet, or on the smaller Markenje, both small, uninhabited islands in Lake Grevelingen (fig. 1). The number of breeding pairs (and the mean clutch size) are both presented in table 1. The first terns arrived mid-March in the Grevelingen area, and on average some 50% of the terns had arrived on 20 April and 50% had laid on 5 May. It was found that the terns arrived and laid earlier in favourable meteorological conditions (e.g. higher mean temperature, less precipitation; fig. 3, 4). The mean clutch size was positively correlated to mean temperatures in spring (not significant; fig. 5). The young terns usually left the colony mid-July. Breeding success (the number of fledged young) was rather constantly 0.75/pair, but this figure was notoriously difficult to assess precisely, because chicks and adults wandered around in the colony a few days after hatching. Many chicks died because of heavy rains in 1986 and the breeding success may well have been somewhat lower this year.