Nikon D90, D5000 are among the most popular digital SLRs with great image quality. But, Nikon high end lenses are extremely expensive.  For example:  Nikon AF-S Nikkor 500mm f/4G ED VR Lens ($7000 at amazon) is much more expensive than the corresponding Canon lens:  Canon EF 500mm f/4L IS USM $5800.

Normally  wildlife is most active in the early morning and late evening-when the light is dim.  Plus you have to stay far away so that the wildlife will not be disturbed. As a result, you  have to use long and fast  telephone lenses, as long as 500-600mm to shoot for flying birds.  Sometimes, a tripod can help when the animals stay still.

For beginners who just get into wildlife photography, the best bet is to get  a Sigma 150-500mm f/5-6.3 AF APO DG OS HSM ($860).

This lens has a Sigma original optical stabilizer function (OS) and has fast AF.  The addition of the (optional) 1.4x EX DG APO Converter  can produce a 210-700mm F7-9 MF ultra-telephoto zoom lens.  Before this lens was introduced, many people use Sigma 50-500mm.  But this lens does not have an optical stabilizer.  Both lenses will be long enough to shoot birds.

With some more money, you might want to invest on Nikon 200-400mm f/4G ED-IF AF-S VR $5600 and  Nikon TC-14E II (1.4x) Teleconverter AF-S. This lens comes with VR and, with 1.4 x converter, you can get 560mm, which is long enough to shoot birds.

Another more affordable option is Sigma 120-300mm f/2.8 EX DG IF HSM APO ($2900). you will need 1.4 x or 2 x teleconverters.

A good and light tripod is a good investment for wild life photography.  Read here for the best tripod for traveling.