Since the winter of 2022/23, the 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. Here, we investigate the spread, virus-related mortality, and population-level impact of genotype euBB in black-headed gulls (Chroicocephalus ridibundus) and common terns (Sterna hirundo) in Germany using phylogeographic analyses, ringing data, and information on spatiotemporal outbreak patterns. Transmission into German breeding colonies involved multiple independent incursions, likely associated with black-headed gulls returning from their wintering grounds in the Netherlands, Belgium, and France. It spilled over into common terns, 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 (-16% in black-headed gulls, -6% in common terns). Increased immunity, at least in common terns, may have contributed to the apparent subsequent 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.