Christinia, you really need to get almost eye level with them , to get good shots. To do it safely, you need a hunters tree stand and appropriate safety gear ,in case you slip. I've taken quite a few shots of herons in conditions like this, and it is the only way i've gotten good results. There is just too much clutter , shooting from the ground. If there are any trees near by (i don't recommend climbing the tree the nests are in), that you can climb, that is the answer. Then shoot either in the early morning or evening , which ever gives the best light. (A crossing light works well, as well as backlighting in the early morning.)
Another option, is to find a different roost, that gives you a higher position, or one that you can climb a tree.
A few examples of what climbing a tree can do for you...................................
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Another option , is to hit up the feeding areas and try and get some shots there.
If you climb, make shure you have the appropriate safety gear. It's not worth hurting yourself, for a photo.
As far as camera settings, i try and keep my shutter speeds up to 1/1000 or higher. I usually shoot in manual mode. Focus drive in AI servo. Shoot with an image stabilized lens, freehand. I turn the focus search to off in the camera menu. (this keeps the lens from hunting, when you loose focus of a moving target.) Always check your histograms and shutter speeds, and make adjustments as needed. Be safe, and have fun. Shooting a heron roost from a treestand is alot of fun, and a great experience. Give it a try, you will be glad you did.