Soaring Eagle
This soaring eagle was captured with a very long lens. I use Nikon equipment and there are many options for Nikon long lenses.
For many birds, lenses in the range 300mm to 600mm are generally required.
When using long lenses it is good to use some means of long lens support, such as a tripod or monopod. Also in the Nikon lens range there are now manylenses with built in VR which can be a great advantage in stabilising the camera and getting a sharper image.
Zebra
I managed to fill the frame with the head of this zebra with a telephoto lens. In this case a 300mm lens was used with a Nikon D2H camera. DX cameras are good with the cropped area to make the image look larger.