I took the liberty to just use one of your silhouetted bird shots and try to adjust in RawTherapee. Keep in mind, I'm using your low-res exported JPG file.
I applied the D750 color profile, but to no immediate visible difference. I think it would have changed for the better with a Raw file.
I adjusted the white balance to "Sunny" and the sky changed to a blue. I enhanced this a bit by decreasing the color temperature a little more manually.
With a JPG file, this next step is very limited in results. I activated the Shadows/Highlights tool and brought the shadows nearly all the way up, decreased the highlights, also by a very large amount. It would not be so extreme an adjustment with a Raw file.
Moved the Lightness slider, Exposure, Black Level up a small amount.
Just to see how far I could pump that JPG, I also activated Noise Reduction and limited that to just a little bit of correction.
Again, only RawTherapee was used and I exported again as a JPG. GIMP does not make these adjustments as well similar to how Photoshop is not as good as Lightroom for those types of adjustments. At least that is what I have read online. I've never interfaced with either of those programs, way out of my price league. I use free and open-source software as much as I can. You really don't loose that much on features, and save so much cash.