WIND-DEPENDENT IMPACT OF GULLS (LARUS SPP) ON THE BREEDING SUCCESS OFCOMMON TERNS (STERNA-HIRUNDO)

The breeding populations of the black-headed gull (Larus ridibundus) a nd the herring gull (Larus argentatus) on the Wadden Sea island of Sch arhorn (Germany) increased until 1985 to about 320 and 280 breeding pa irs, respectively. The aim of our study was to investigate whether the breeding success of the common tern (Sterna hirundo) was affected by chick removal due to gull predation. Attacks by flying gulls started s hortly after the first tern chicks hatched. More than 360 successful c ases of tern chick removal by black-headed gulls and about 770 cases b y herring gulls were observed. For the herring but not for the black-h eaded gull, time of day had an effect on the rate of successful chick removals. No significant effect of the tidal stage was discernible. Re gardless of time of day and stage of tide, chick removal by black-head ed and herring gulls was positively correlated with wind velocities. D uring stormy days the smaller terns were not able to attack intruding gulls successfully. Gull attacks affect the breeding success of terns directly (by chick predation) and presumably indirectly (by enhancing chick mortality due to underweight caused by frequent interruptions of the breeding process). As gull attacks are largely restricted to stor my days, we conclude that terns do not suffer from gull attacks during breeding seasons with good weather conditions. We recommend to refrai n from regulating gull stocks in the breeding colonies, and focus inst ead on other, more effective management procedures such as closure of waste disposals and reducing the by-catch of fisheries.