Opvallend en aanhoudend agressief gedrag bij Zilvermeeuwen Larus argentatus na jongenverlies

Descriptions are provided of contrasting behavior of one and the same pair of Herring Gulls in the IJmuiden colony at Forteiland. Extremely aggressive interactions with adjacent breeding pairs were recorded in 2010, after the offspring had disappeared and the birds were in fact off duty. In contrast, in 2011, when the pair reared chicks successfully, abnormal aggression was not recorded. The report has been provided as a case study demonstrating how different individual gulls may behave in subsequent seasons (whether or not in response to the chick loss in one year) and demonstrates what valuable behavioural observations can be obtained by observing colour-ringed (known individual) birds.