raingarden
Crowing
It can transfer vertically in embryos and on the surface of eggs. But, as Stormcrow noted a few days ago, hatching temperatures will kill the virus.I read that AI cannot pass from mother to embryo. I also saw this on Meyer's website.
It's really difficult to know what to do about HPAI. You could get rid of the whole flock right now and find something else to do until it blows over. You could put up a fight and focus on improving biosecurity. You could ignore the threat, play the odds, take your chances, and just go about your business. All of these are justifiable positions. Keeping backyard chickens is different for everyone and everyone's HPAI response will be different.
Pesonally, I'd rather have my birds get HPAI than Marek's. HPAI is short and acutely painful for the owner. Marek's is prolonged and chronically painful. Remember, backyard chickens are supposed to be enjoyable.