Birds, like most wild animals, get frightened quite easily, so, for this reason, you can’t get too close to them before they retreat for safety. The best way around this? Mount a telephoto lens on your camera.
A telephoto zoom lens is the most versatile, as you can zoom in and out to find the best composition. With the right lens on your camera, you can satisfy your need for the perfect cropped photograph, without your model flying off and you missing your shot altogether.
Lenses with brighter maximum apertures can be pricier, but the shallow depths of field that they provide are perfect for isolating birds from busy backgrounds.