Big glass does not always give you the best results. What does is knowing how to approach a bird without scaring it away and learning to be quiet and do talk or make sure you don't step on branches or leaves that will spoke the bird. What has worked for me is hiding behind trees listen to bird noses and then stake out an area for awhile and wait for the birds to come to me. Find out what plants and trees the birds like and stake out the area and wait for the birds to show up. This i never tried but I heard guys that do this for a living, would craw on the ground where the bird will not see them and wear dark hunting clothing where the bird would not notice you as fast. I would not recommend fake bird callers as this is unethical and is bad for the bird and is very harmful for the bird. Some parks will fine you if they caught you with one. Also, use a lens coat. This does make a difference in not scaring birds away with big white lens. I have found using Canon 300 f/4 is better for flight shots since is not as heavy to shoot with than a Canon 500/f4 (too heavy to shot birds in flight).