The threat is thought to be so severe that it could wipe out the species entirely.
The discovery comes after the distinctive bird's genome was sequenced for the first time in 2021.
Ordinarily, this achievement would be something to celebrate from a scientific perspective – but a comparison with closely-related northern hemisphere white swans has revealed that certain key immune genes are missing from its DNA.
That's likely to be, at least partly, down to the way that the black swan is isolated geographically. These animals haven't had the same exposure to pathogens that are found outside of southeast and southwest Australia, the areas where it primarily lives and breeds.
"[B]lack swans are extremely sensitive to highly pathogenic avian influenza – HPAI which is often referred to as bird flu - and can die from it within three days," says microbiologist Kirsty Short from the University of Queensland in Australia.
"Our data suggests that the immune system of the black swan is such that, should any avian viral infection become established in its native habitat, their survival would be in peril."
The team used powerful computer software to compare the genes of the black swan with the closely-related mute swan (Cygnus olor), found in the Northern hemisphere. Tens of thousands of genes were compared in total in the search for differences.
It was discovered that a class of proteins in the toll-like receptor 7 (TLR7) family were not being activated as they should. These parts of the genome have been associated with protecting against bird flu and other pathogens.
In other words, the gene for guarding against the bird flu virus is there, it's just not being switched on when needed – and that puts the black swan under threat.
The team also identified an unregulated inflammatory response to infection that could be dangerous.
"We currently don't have HPAI in Australia, but it has spread from Asia to North America, Europe, North Africa, and South America. When it was introduced to new locations, such as Chile and Peru, thousands of wild seabirds perished," says Short.
During the course of their research, the study authors also identified another gene – SLC45A2 – that may be responsible for black swans being black rather than white.
In fact, as mutations of this gene leads to loss of pigment – the same gene has previously been linked to albinism in humans – it suggests the white swan is the newer variant, and that the ancestral swans of both species were black.
The good news is that knowing more about the vulnerability of these birds is going to help in efforts to protect them. Either through selective breeding, or through immunotherapy treatments, this TLR7 gap in the immune defenses could be filled.
Right now, the black swan is one of the species that conservationists are least worried about, with a population worldwide of up to a million. Those numbers could drastically change in a short period of time, however.
"The risk to one of Australia's most unique and beautiful birds is very real, and we need to be prepared if we hope to protect it," says Short.
The research has been published in Genome Biology.