Phylogenetic analysis of H5N1 virus isolates from elephant seals and terns demonstrated a distinct clade sharing similar mutations allowing infection of mammalian hosts.  It is presumed that migratory terns infected elephant seals along the coast of Argentina from 2022 to 2023 creating an epidemic involving marine mammals along the coasts of Brazil, Chile, Peru, Uruguay and Argentina.  The extent of mass mortality among marine mammals suggests contagion. This should serve as a warning in relation to possible emergence of a potentially zoonotic strain from poultry or dairy cows.