Croaking while flying hundreds of feet up in the air might cause some blunt force trauma. I haven't heard of any toxicology reports being released just yet, so we'll have to wait and see.
http://www.ktsm.com/arkansas/days-after-arkansas-louisiana-reports-hundreds-of-birds-falling-dead-from-sky
Autopsies conducted on 17 birds revealed the birds died due to multiple blunt trauma to their vital organs. Poison has been ruled out, and state biologists believe stress or weather might have played a vital role.
This doesn't really say that avian flu has been ruled out, but it's not mentioned. In another article I read:
Health officials have said that there was no sign of disease in the birds, but test results won't be in for at least a week.
That article describes what a chicken looks like in necropsy if it had bird flu and what these birds looked like. You would have to read the details and decide, but the health officials said they saw no sign of it. That's all I can tell you, I'm not an expert. http://www.postchronicle.com/news/original/article_212341561.shtml