Do a little research and see what others are using for their photos, go to flickr and search for "seychelles Birds". There are lots of images, many of them taken from just a few feet away, and some flying terns where a bird more than fills the frame of a 400mm lens.


Not everyone uploads the exif data, and some of the images are not very good.


From what I see, a 300mm lens with a crop camera or a 400 with FF might be a good choice, but search for yourself.