Hmmm I dunno if what I am about to say has been said but I'm going to post it...as a clarification thing. Well strictly it's not what I said, but it's a good arguement put forth by someone else that sums up what I wanted to say without me typing it out Gandalf makes it clear during the council of Elrond that the way to get the Ring to Mount Doom has to be one of subterfuge. When Elrond suggests that Glorfindel, one of the most powerful elves in Middle-Earth, should be part of the Fellowship, Gandalf declines. He argues that Glorfindel's power is not sufficient to open a path to Mount Doom and that it would only draw Sauron's attention and lead to the recapture of the Ring. The same is true of the Eagles. If Gandalf and the Fellowship flew into Mordor on a giant squadron of Eagles, Sauron would see them coming miles away and squash them like bugs. The Eagles never enter the air in Mordor until a moment when Sauron's attention is totally devoted to the Ring and Mount Doom. As for them beign protected by Gandalf's magic, at no point does Gandalf claim to be Sauron's equal. And by the time Gandalf achieves his full stature (post-Balrog), Frodo is already on the way into Mordor, so the Eagles are not an option at that point. You may ask why no reasons is given as to why he didn't use to eagles, and that may frustrate you but, they don't need to be mentioned. Once he makes the point that travelling to Mount Doom openly is not an option, he doesn't have to mention every possible way in which they could travel openly, one of which is the Eagles.