DE is better as prevention. It definitely wont stop a infestation. I ended up with a group that were infested and lost a rooster to them before I realized he even had them. It was terrible- I was so new to all of it I didn't even know it was a thing to watch for. You have to part the feathers near the rump until you can see skin. they are tiny, like book mites, and sorta a tan color.
I was told to use adams flea spray. It worked. I dont know if that's really what you should use, i know there's significant cross-species issues with such medications and treatments. Though all other birds survived, the mites left (i used DE also with the adams) and life went on.
I dust my feathered birds myself periodically, just to make sure. That's just my two cents on my experiences. If I ever had it to do over I'd keep my birds enclosed to cut down on such things from the wildlife. It was not pleasant.