Winged Beasts, a squandered resource?

[Lalwendë]

This:

whose kind, lingering in forgotten mountains cold beneath the Moon

means they are probably not reptilian creatures. I can't think of any reptiles which thrive on cold and moonlight. Even British snakes need to bask, and they hibernate in winter, as do Toads and Frogs, so I doubt it's Amphibian either.

If it's a mammal then it's one which doesn't need to be nursed on milk, or else has been weaned early. Being able to fly doesn't rule out it being a mammal of course.

Also, they were "lingering in forgotten mountains". Does that literally mean 'in' the mountain, as in a cave? Or does the use of 'eyrie' exclude that?