More than 13,000 southern elephant seal pups have died from the H5N1 strain of bird flu on Heard and McDonald Islands, remote Australian territories located about 4,000 kilometres south-west of the mainland and fewer than 2,000 kilometres north of Antarctica. New research estimates that 13,359 pups from a population of 17,364 on Heard Island perished since last August — a mortality rate exceeding 75% — with one area recording losses of up to 97%. Scientists warn the figures may even be an underestimate, as pups were still dying when the final surveys were conducted.

The findings, published as a preprint study in BioRxiv and not yet peer-reviewed, are based on drone surveys and ground visits carried out in October 2025 and January 2026 by scientists with the Australian Antarctic Program. Samples were collected from nine species, six of which tested positive for H5N1: southern elephant seals, king and gentoo penguins, Antarctic fur seals, and South Georgia diving petrels. Several hundred adult king penguins also died, a number above normal levels though representing a small fraction of the population. No unusual deaths were recorded among albatrosses or two endemic species, the Heard Island shag and the black-faced sheathbill. In a typical year, pup mortality among southern elephant seals remains below 5%.

Genetic analysis suggests the virus was most likely introduced in August 2025 by migrating wildlife from the French-administered Crozet Islands, roughly 1,800 kilometres away.