The impact of additional food provisioning on chick growth and breeding output in the Herring Gull Larus argentatus - a pilot experiment

By providing 15 Herring Gull Larus argentatus pairs on Schiermonnikoog, Dutch Frisian Islands, in 1987 with supplementary food I attempted to test the hypothesis that food availability is at present a limiting factor in the breeding success of this gull species in the Dutch Wadden Sea. Clutch mass of the fed pairs exceeded that of unfed pairs by 8%. The proportion of hatched eggs resulting in fledged young was also largest in the fed pairs (55% against 36% in the unfed pairs). This resulted in more pairs in which all chicks survived until fledging in the fed pairs (50% compared with 19% in the unfed pairs). The reproductive output in the fed pairs resembled that of Herring Gulls on nearby Terschelling in the 1960s when the number of breeding pairs was much smaller than at present. It seems likely that the negative impact of food shortage on chick survival under present conditions is mainly mediated through an effect of parental guarding behaviour.