There is an alpine legend involving a kingdom of eagles who could turn into men (or viceversa). In human form, they had only one arm, "so they could not use a shield, but they were so brave they did not need it".
These eagle-men, and in particular their ancestors, seem to match how ArM5 depicts faeries. They can sire children with normal humans, though, and at least some of these children are also eagle-men. Finally, when they shift shapes it seems that their accoutrements change with them -- including weapons: they arrive at a battle in eagle form, change into humans, and immediately start fighting.
I am tempted to mechanically represent being an eagle-man as a type of faerie blood:
Minor Virtue, Supernatural
Blood of the Eagles
You are one of the fabled eagle-men of the Alps. This is a variant of the Faerie Blood Virtue, but you gain no bonus to Aging rolls.
Instead, you can freely switch between the form of a very large eagle, and that of a human. When changing into an eagle any accoutrements of your human form, including weapons, that are truly yours change with you. Your eagle form is just as natural as your human one, so once assumed it cannot be dispelled, it is not blocked by magic resistance, and does not cause warping even over extended periods of time.
In human form you only have one arn. This affects you as if you had the Missing Hand Flaw. You receive no compensation for it, but you do not incur the usual -3 penalty in combat, and instead add your Brave personality trait to all defense rolls. You can freely spend starting experience on martial skills.
I think the Virtue is balanced, but I'm looking for feedback. The way I see it is this.
Being able to take the eagle form is slightly better than the "classic" skinchanger Virtue; I'd rank it as worth maybe 1.5 Virtue Points. One-Handed is a -1 Flaw, that is particularly serious for warriors -- so much that it is only partially offset by the bravery bonus (overall, I'd rank the two together as worth maybe -0.5 points). Sanity check: in combat, a Brave+3 character gets +3 to Def rolls, but using e.g. a longsword vs. a longsword-plus-heather shield costs +3 Def, while using a longsword vs. a greatsword costs +1 Atk, +1 Def and +3 Dam. And of course, a one-handed character incurs -3 penalty to other tasks, can't use a bow etc. Being able to take martial skills is a sufficiently small perk that it's included for free in Virtues such as Almogavar.