Nov. 6, 2024, an epidemiological study found that 56% of a large breeding colony of Caspian terns died from a 2023 outbreak of highly pathogenic avian influenza at Rat Island in Washington state. Since then, no birds have successfully bred on the island, raising concerns that the outbreak may have had a significant impact on an already declining Pacific-coast population.

As part of the study, a team including Washington Department of Fish and Wildlife (WDFW) as well as Washington State University researchers also documented that the avian flu virus H5N1 was transmitted to harbor seals for the first time in the northeastern Pacific. While there has not been another large coastal wildlife outbreak of H5N1 since, researchers estimated that about 10-14% of the Caspian tern population in the Pacific flyway have been lost to H5N1 infections.

Avian influenza has been killing many seals and sea lions in other parts of the world, particularly in South America, but so far that same spread has not yet occurred in the Pacific Northwest. The consequences for the terns in the region, however, has been more dire. There is no vaccine or treatment yet available in the U.S. for animals impacted by the avian flu