My original photo needed more contrast, as well as a good color boost. The background was also tilted and needed correcting.
I also put it through a little bit of sharpening...
When I'm working on an image just for basic correcting or optimizing, I err on the side of caution. I want the image to look as natural as possible.
Here are a couple of different versions...
I created two layers, a color blended version over a desaturated copy of the image. I used the erase tool to quickly knock out the black and white portion of the bird. Most of the time on this image was spent on color tweaking.
This is a combination of a shifting individual color hues and saturations, levels adjustment to get the high contrast, and running it through the unsharp mask to add the grainy texture.
The fun part about using GIMP for this kind of image processing is how much you can do with a few tools and a lot of experimenting!