UC Davis is your state lab. I would have the necropsy again.
@casportpony may be able to give you a contact number of someone to speak with prior to sending a bird in so you can get more detailed testing/information.
How "new" are the new birds? You mention a leak and using hay. Is the hay moldy? It would be odd that there's no symptoms if it were mold. I'm thinking while typing here. No mold in the nesting box where it leaked?
Any chance they ate something toxic - fertilizer, weed killer - those usually cause some visible symptoms prior to expiring as well. Even rat poisoning, birds would usually have noticeable symptoms.
Curious if there is a certain time of day? Do you find them in the morning like they have died inside the coop overnight or is it just random times of the day?