'But why Bird Masks?' I hear you inevitibly cry. 'Don't Kenku already exist in D&D? Aren't the bird-demon-people-things cooler anyway? Is a man wearing a Beholder Mask better than an actual Beholder? What about the material nature of things? What about their physical presence in the imaginative space? Is this really the time or place to be playing silly conceptual games?'
Of course I understand your worries. Allow me to make three simple arguments in explanation.
If you follow Kenku long enough, you're going to see them take a shit.
Then you will see them clean up. Eventually no matter how strange the Demon bird-men were to begin with, the irrepressible force of Gygaxian naturalism will consume them and render them quotidian.
Men who happen to be Birds, no matter how strange they begin, must eventually develop a naturalism of their own.
But human beings are already known. There is no challenge to the discovery of the bare facts. The strange internal spaces of mankind however, are still beyond our reach. The darker, invisible reaches of the human soul are as distant to us now as they were a millennia ago. As far as the deeps of the sea or the cold reaches of space.
The deeps of mankind cannot be reached because they are a shadowed space, formed in the hinterland of our reasoning selves and expanding as fast as our power to comprehend them.
Men who happen to be birds can be, and must be explained. People who obsessively wear Bird masks cannot. The reasons for the mask can change as rapidly as our questions, always remaining a short distance out of reach.
Lines from 'A Canticle to the Waterbirds'
By Brother Antonius.
'For you hold the heart of his mighty fastness
And shape the life of his indeterminate realms.
And well may you sing his praises, birds, for your ways
Are verved with the secret skills of his inclinations,
And your habits plaited and rare with the subdue elaboration
of his intricate craft
Your days intent with the distinct astuteness needful for his
And your nights Alive with the dense repose of his infinite sleep
You are his secretive charges and you serve his secretive ends.
In His clouded mist-conditioned stations, in his murk,
Obscured in your matted nesting, immured in his limitless ranges.
He makes you penetrate through the dark interstatial joinings
of his thicketed kingdoms
And keep your concourse in the deeps of his shadowed world.'
I think that explains things fully.