The very fact that post-processing of bird images is already encouraged (to a degree) suggests that we are already condoning 'creative' images - so why not?

If you are using RAW, post processing is a necessary part of producing an image. It is like developing film. It is always best to try and get everything right in camera, but even if using JPG it may be necessary to lighten or darken, change the contrast or the saturation or the white balance. These are usually accepted.
How far an image is altered is the debatable point. Where is the cut off point. With my own images I aim not to alter the appearance of the bird, aside from the usual lightening/darkening etc. I don't drop in new backgrounds (too much work for me) but may soften the background slightly so that the bird stands out better, or clone out a twig or two. The surrounds though give viewers an idea of the habitat in which the bird is found. Sharpening is another item on the post processing list, but should be subtly done.
I believe there is TOO much emphasis on sharpness and on grain. An image though needs to be sharp enough to show good feather detail where possible. Grain is another thing. By pixel peeping at 100% many images are going to show some grain. If a 20mp image was printed at this magnification, it would be a giant mural suitable for billboard use. At a sensible viewing distance grain will not be seen.
One area I notice that would improve many photos is the lightening of shadows. Raising or lightening shadows to match surrounding bright areas is overdoing it, but showing a little detail instead of having a black blob is desirable.

I don't see any of the above techniques as being out of the ordinary if done subtly.

I like the idea as we not only should have a gallery for 'picture perfect' bird images a gallery for the artistic would be complimentary.

I should have mentioned previously that I like the idea of a creative gallery. Those who are not in favour of the idea do not have to contribute or even look at the images.

I agree, it's a great idea.

I am all for it. Here is a sample of mine.