Highly pathogenic avian influenza (HPAI) H5N1 viruses of clade 2.3.4.4b have caused widespread mortality in wild and domestic birds globally. Colonial breeding seabirds have been severely affected across Europe, the Americas and Antarctica since 2021. Despite extensive outbreaks, most reports have been limited to morbidity and mortality, while less is known on behavioural effects of infection. We monitored six colonies of black-headed gulls (Chroicocephalus ridibundus) throughout an outbreak during the 2023 breeding season (confirmed as H5N1 clade 2.3.4.4b, EA-2022-BB genotype), combining field observations, global positioning system (GPS)-tracking, virological and serological analyses to assess the epidemiological and behavioural impacts of the disease. Die-offs were observed in five of the colonies, resulting in more than 90% chick mortality and substantial adult losses. Serological analyses of samples taken 1- and 2-years post-outbreak revealed high neutralizing antibody titres against HPAI viruses in surviving individuals. Movement data from GPS-tagged birds confirmed a high adult mortality in affected colonies, reduced daily speed and earlier onset of migration. Our findings emphasize the severity of HPAI in seabirds with effects not only on direct mortality but also on behaviour. Integrating mortality, behavioural and immunological information is therefore crucial to improve surveillance and mitigation strategies for future wildlife disease events.