So sorry you lost your rooster.
I read this comment:
Transmission is by inhalation exposure to an environment with a high spore count; there is usually little bird-to-bird transmission. Spores are highly resistant to disinfectants.
On this site:
http://www.thepoultrysite.com/diseaseinfo/7/aspergillosis/
I'm not sure moldy food is the culprit. I think you may be correct that the mulch pile may have been harboring the fungus.
Makes
me think I'm moving my compost bin out of the chicken pen and keeping the door closed so they can't get in there!
Thanks for sharing your necropsy results.