Highly pathogenic avian influenza (HPAI) H5N1 caused mass mortality of wildlife across the North Atlantic in 2022. Following European outbreaks, the first North American case was detected on the island of Newfoundland, Canada in November 2021, before spreading throughout North America. During the following summer, Newfoundland and surrounding islands (NFLD) experienced one of the most significant mortalities in Canadian provinces, with seabirds being the most affected taxa. From surveys and citizen reports, we estimate that 13?517 mortalities that can be attributed to HPAI occurred in NFLD from April to September 2022. Most estimated mortalities were among Northern Gannets (Morus bassanus (Linnaeus, 1758)) (6596), Common Murres (Uria aalge (Pontoppidan, 1763)) (5992), Atlantic Puffins (Fratercula arctica (Linnaeus, 1758)) (282), and Black-legged Kittiwakes (Rissa tridactyla (Linnaeus, 1758)) (217). Mortality reports moved from west to east along the southern, then eastern NFLD coast, and peaked in July and August. We formulated exploratory hypotheses regarding traits that could contribute to infection and mortality. Species differences in mortality most strongly associated with inter-nest distance, breeding phenology, and at-sea overlap with allospecifics from other colonies. Unprecedented seabird mortality and ongoing transmission within the circulating avian influenza viruses highlight the need for continued monitoring and development of conservation strategies.