In most circumstances, bird photographers require a telephoto lens of some description. Fixed focal length lens of 300mm or more are favoured and a 600mm f/4 lens is the mark of a serious individual. These monsters produce excellent results in the right circumstances but are very heavy, requiring a tripod for most situations. Shorter lenses can be handheld to a degree, depending on your strength. Another option is a telephoto zoom. The most appropriate models span focal lengths somewhere in the 100mm-500mm range. They are comparatively lightweight because fundamentally they contain less glass and have a resulting darker operating aperture (typically f/5.6). This has an adverse knock-on effect on the speed and ability of autofocus function.
A selection of Nikon lenses suitable for bird photography: 200-500mm f/5.6, 80-400mm f/4.5-5.6d, 1.4x converters, 600mm f/4