Hi Mike,
I appreciate the reach of that Sigma lens. These photographs are able to take some sharpening and with a small contrast curve the pictures really pop.
Keep up the good work. Maybe consider allowing more space around the bird will create a milieu for the composition. So, for example, one can get the entire reflection of the bird in the water. The idea of cropping close is something which I consider is one of the easiest way of destroying a shot. I personally try to grab more of the surrounding world. My angle and position excludes what for sure I don't want, but otherwise I like generous framing so that we can recognize extra wonderful features at the time of editing.
Asher