We immediately did a thorough scrub down of the cage, bought all new food, etc., and added Ornacyn to Rosie's water just in case it might be an illness.

That was about a week ago. Rosie is eating and drinking normally, and her poop is completely normal also. The only unusual thing is that she is doing a lot of feather ruffling and stretching her wings up behind her--it's very odd behavior. Also her cere has changed and become somewhat translucent blue and smooth (it has always been rough and pink).

I'm hesitant to run her straight off the vet as she doesn't really seem ill (and we don't have any avian experts close), but on the other hand I am concerned about her. Could her behavior be a result of her being lonely since her mate is gone? We have another "family" (so to speak) of keets in another cage that she could join, but I don't want to put them at risk if she is carrying anything. Any ideas?

To reiterate, there aren't any avian experts near us and I'm hesitant to drop $100+ for a vet that will likely know less about keets than I do, plus the visit would be even more stressful for her. If she weren't eating, pooping normally, etc., of course I would do so, but for now I prefer to treat her at home if at all possible.

I suppose my questions in a nutshell are:

1. Is it likely that she is suffering from stress or an illness?
2. If an illness, am I taking the correct action at this time (with the understanding that of course I will take her to a vet if it becomes obvious that that is necessary)?
3. How long should she be isolated to rule out disease, before allowing her to join other keets?
4. Are there any other things I should be doing for her?

Thank you for your responses, I'm open to all ideas!

And when you actually begin to let Rosie interact with the other birds, don't just put her in their cage. Birds are territorial, and the others may fight her as an intruder.

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The sun may shine and the rain will clear out
Shine brighter then the sun and be the one that brightens up your day
Don't loose hope when the sun goes down
Because the night just brings another day ♥