Since winter 2022/23, high pathogenicity avian influenza virus (HPAIV) H5N1 clade 2.3.4.4b, genotype euBB, has caused extensive mortality among wild birds. This genotype emerged in France in spring 2022 through reassortment between a gull-adapted low-pathogenicity virus and HPAIV H5N1. Phylogeographic and spatiotemporal analyses show that transmission into German breeding colonies of black-headed gulls (Chroicocephalus ridibundus) and common terns (Sterna hirundo) involved multiple independent incursions, likely via black-headed gulls returning from wintering grounds in the Netherlands, Belgium, and France. It spilled over into common terns, which breed in shared colonies with black-headed gulls, and led to high adult mortality in both species in 2023 (at least 8,137 black-headed gulls and 614 common terns; >3% of the breeding population), followed by significant breeding pair declines in 2024 (-15.9% in black-headed gulls, -5.8% in common terns). Increased immunity, at least in common terns, may have contributed to the apparent fade-out of genotype euBB. These findings highlight how integrating ornithological, epidemiological, and virological data can aid our understanding of viral transmission routes and population-level impacts, while also stressing that HPAIV should be added to the growing list of pressures on seabirds, a group that was already the most threatened among all bird taxa globally.