WIT AAN & SPAANS AL (1984) Changes in the breeding biology of the Herring Gull Larus argentatus as a result of increased numbers. LIMOSA 57 (3): 87-90.

European Herring Gull Larus argentatus Since the late 1960s the number of Dutch Herring Gulls has increased considerably (fig. 1). On Terschelling, where reproductive success was studied in 1966- 90 69, 6200 breeding pairs were counted in 1968 and 21 500 in 1983. In 1983 the effect of increased numbers on reproductive success was studied. The study area was the same 6-ha plot as in the 1960s. In 1983 599 pairs of Herring and Lesser Blackbacked Gulls L. fuscus bred in the study plot, more than three times as many as in 1967-69. The number of fledged Herring Gulls (0.43 young per pair) has dropped dramatically since 1967-69 (1.35 young per pair), mainly as a result of increased predation of eggs (29.8% now as against 8.2% in 1967-69) and chicks (21.3% now as against < 9.8% in 1967-69) by conspecifics and Lesser Black-Backs (tab. 1, fig. 2). At present, reproductive success is so low that less young fledge in the study area than in the 1960s, notwithstanding a three-fold increase of the numbers of breeding pairs there. Egg volumes have decreased since 1969 by an amount ranging from 2.6% for the first egg in a clutch to 4.2% for the third egg. Chicks now seem to grow less fast (21.8-32.3 g/day now as against 25.5-35.7 g/day in 1966-68) and to weigh less at fledging (698953 g now as against 772-1050 g in 1966-68) than in the 1960s. On average, eggs are now laid 4-4lf2 days earlier than in the 1960s. A comparison is made between the situation on Terschelling and on the Isle of May (Scotland) where in four years the density was reduced to a quarter of the original level (Coulson et al. 1982).

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