[Elendur]

I would say that because the eagles were created by Manwe as his vassels (For lack of a better word, dont know how to put it.) he naturally made them learned in speech.

As you can tell, every animal can speek in its own way. (Like the fox passing the hobbits commented on how it was strange to see four hobbits camping in the wilderness.) Its only a matter of whether men or elves can comprehend the speech which is questionable.

So speaking animals is nothing special. They did not have to be Maiar.