Thorondor as a Maia???

[Sharkû]

Well, Tolkien pondered this difficulty in one place in Myths Transformed, and seemed to have come to the satisfying conclusion later:

<sub>These have been rather lightly adopted from 'serious' mythologies, but play a part which cannot be excised. They are certainly 'exceptions' and not much used, but sufficiently to show that they are a recognized feature of the world. All other creatures accept them as natural if not common. But true 'rational' creatures, 'speaking peoples', are all of human / 'humanoid' form. Only the Valar and Maiar are intelligences that can assume forms of Arda at will. Huan and Sorontar [Thorondor] could be Maiar - emissaries of Manwë. But unfortunately in The Lord of the Rings Gwaehir and Landroval are said to be descendents of Sorontar.

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In summary: I think it must be assumed that [orcs] 'talking' is not necessarily the sign of the possession of a 'rational soul' or fëa...talking was largely echoic (cf. parrots), in The Lord of the Rings Sauron is said to have devised a language for them.... The same sort of thing may be said of Húan and the Eagles: they were taught language by the Valar, and raised to a higher level - but they still had no fëar.</sub>

The criterion for what is a truly rational creature is therefore possession of a fea. Mating with beings without a fea, and as stated, only humanoids have fear in addition to the higher beings, would be off the table for a Maia. We cannot assume that there were non-maiarin eagles, possessing a fea.