Added:
Se McGregor (one of the key moderators/zoomers) added the following as a reply to other comments on Facebook:
15's condition is not described as "grave". 50/50 are about the same odds as when we took him there last week. In my case the 50/50 was "my" assumption rather than a stated fact. They didn't tell us and we didn't ask, because the blood tests weren't back. The test results - hopefully - should indicate the cause of the feather loss.
The high WBC was also mentioned last week, probably in a response, rather than in a main post as was the anti fungal treatment (probably a precaution because they said that it is common for fish eating birds to have fungal bacteria in their gut.
I'm not an avian vet, but I do know of other rehab wild birds that have been given daily antibiotic injections for up to two weeks. Apart from the feather loss, he was outwardly calm, eyes were bright, he didn't "look" sick.
This interpretation of the same basic facts seemed a bit more hopeful, even though he is by no means out of danger, and it seems that there has to be some sort of underlying (and hopefully treatable) issue causing his primary feathers to be damaged and break or fall out as they grew to full size. And I think it's hopeful that it was only the primaries and a few secondaries that seemed to be affected and not the other 7000(?) feathers - though of course those feathers are essential for flight.
Keeping fingers crossed.
(There were many emotional comments by posters when the original post was written; this was in response.)