Lovely scene Layton, both birds look splendid.
The image looks a little dark on my screen and I feel the RHS bird is a little too close to the frame edge.
You could perhaps add a bit more room on the right and at frame bottom or try a 3:4 ratio crop (with a little more room on the RHS) instead of the 2:3 format presented.
I love this shot...terrific contrast, two good bird angles and shows what the rookery habitat is. I really like the fact that there is nothing between you and the left bird....and whatever there is in front of the second bird as near the feet and not very distracting.
I like the double poses, and the colours look great. A bit dark, as stated, but if you are going for late-day "after the sunset" feel then this wrroks well that way. The dry twig running above the heads and touching the left bird is unfortunate, but understandably hard to avoid in such rookeries. I agree with a bit more room at right if you have it.
The 5D II can handle high ISOs...don't be afraid to but it up in low light!
Nice work, Layton. That was such a difficult place to photograph and you made lemonade from lemons. I like the symmetry of the two birds and like the dark feel, gives the image a bit of mood.