How was Gwaihir so well informed ?

[Tuor in Gondolin]

However, Tolkien recognized the limitations of using the Eagles.
In his critique of a suggested film synopsis by Morton Grady Zimmerman.

Here we meet the first intrusion of the Eagles. I think they
are a major mistake of Z., and without warrant.
The Eagles are a dangerous 'machine'. I have used them
sparingly, and that bis the absolute limit of their credibility or
usefulness. The alighting of a Great Eagle in the Shire is absurd;
it also makes the later capture of G. by Saruman incredible, and
spoils the account of his escape......[when leaving Rivendell] the
Eagles are again introduced. I feel this to be an altogether
unacceptable tampering with the tale. 'Nine Walkers' and they
immediately go into the air! The intrusion achieves nothing but
incredibility, and the staling of the device of the Eagles when at
last they are really needed.

And, of course, it can (and has in this forum) been argued that the
Great Eagles may be descendants of maia, among other possibilities
of their origin, and so have their own agenda and interests. (Think
of Treebeard's observation about being on someone's side).