As Rick Sammon says “The name of the game is to fill the frame.” That is extra important in Avian photography, but really is a lot harder than it sounds. It does not matter whether you are using a 500mm lens, you always, always, always want to get as close as possible. Long lenses at or near minimum focal distance produce great compression and yield nice bokeh even at smaller apertures. Then there is the crop. If you ask me, cropping a bird image is to be avoided at all cost. One of the tips I have picked up along the journey to getting closer is to always look for some sort of separation. Birds usually feel more comfortable when they know there is a body of water, bushes, or even a fence between you and them. My favorite trick is to find a strip of water (this is easy to do at low tide around the coasts) and just ease up on the animal and shoot across the water. I often get as close as minimum focus distance (93″ on my Tamron 200-500mm).