Interesting paper by Peacock et al.

https://www.biorxiv.org/content/10.1101/2025.01.06.631435v1

The H5N1 spillover into cattle from birds in the US was most likely a single introduction according to the phylogeny.

As the paper notes, the cattle virus has developed some mammalian generalist adaptations that allow it to more effectively jump to other mammals like humans and pigs. And there is little cost to viral fitness in birds, so the cattle virus could just as easily spread back to birds.

As an aside, just a nice picture into zoonotic spillover and you'd think it would make people pause about the coronavirus origins. Intermediate species can increase spillover risk to humans.

Pigs are a classic case for avian influenzas as they have avian and human preferred receptors, so are a great mixing vessel for influenza viruses.