We studied the effects of vegetation structure and nest density on territory size and aggression in the black-headed gull (Larus ridibundus). Vegetation structure influenced the types of aggressive behaviour, but not territory size. The proportion of overt aggression (attacks, fights) was highest in barren areas or those with short, sparse vegetation, and lowest in plots covered with dense, tall grass. We explain this by decreased visibility of conspecifics in habitats isolated by vegetation. In such areas, total aggression did not decrease, because the low proportion of attacks and fights was offset by an increase of more moderate (“upright”) displays. Only when nest density was high did the vegetation reduce the frequency of agonistic behaviour — but then it comprised almost exclusively attacks and fights. As expected, greater nest density was related to reduced territory size and an increased proportion of overt aggression. A high proportion of attacks and fights occurred in the plot with the lowest nest density, where at the same time there were also numerous first-year breeders. We conclude that, when analyzing the effects of habitat conditions on aggressive behaviour, it is important to consider the structure of behaviour, and not only the total frequency of all types of agonistic interactions.