[Gwaihir the Windlord]

Interesting subject. It is true that Maiar and Children of Illuvitar can produce offspring, as in the case of Melian and Thingol. Therefore Radagast could theoretically have fathered Beorn. He was certainly in human form.

If I may make a few observations...

'Yes, I know Radagast. Not a bad fellow as wizards go.'

Unless Beorn didn't actually know that Radagast was his father, I don't think he'd speak of him in quite this way unless he said it in this way simply to cover up the fact. Which, of course, he could, I suppose, but certainly one would have thought that he would speakof his father somewhat differently to that.

'One day they shall perish and I shall go back!'

And they did perish, didn't they? After the BOFA, it was said that most of the Orcs of the Misty Mountains were scattered or slain, and those that stayed cowered in dark holes and didn't come out -- not yet at any rate, their numbers were two few. It is interesting to note that shortly after this, Beorn disappeared and his descendants took over the governing of his land. We have presumed that he died; actually, I think it very possible that he did 'go back'.

i sort of get the insinuation in the Silmarilion that one of the original gifts of man was to change shape can find the exact excerpt right now, but perhaps it is one gift that has remained only in the Pure line of man which Beorn and the Beornings are.

Well I've never come across this before. It sounds very, very unlikely and somewhat out of synch, too... I will be surprised if anyone can back that up with any sort of evidence. Of course though, if you can please do.
What do you mean by 'pure'? Most Men were in fact pure men, with the one exception of the Dunedain. The Rohirrim were just Men, the Breelanders were, the Haradrim and the Rhun were. Yet none of them could change shape. Beorn was unique in this respect, to be sure.

As the Istari, but it is well known that Gandalf as Olorin traveled around as an elf, and so on. Only during their battle against Sauron were they limited to their human shapes.

So on...? No, the Istari were stuck as men. Olorin, with his great love and understanding of the Elves, could perhaps summon some vague power so that he would appear as one, but apart from that kind of illusion-ish type magic no, I don't think so. The same illusionish trick was used by Felagund and a couple of others, but Beorn, when he chose, was obviously a real, physical great bear.