There are many models of nature photography lenses that you can get and use, some are better for particular types of shots and therefore it is a good idea to know what you will mostly be using it for before you buy one.
An excellent Nikon model is the Nikon 600mm f/4D ED-IF II AF-S Nikkor.
Here are some of its features:
This model is very lightweight and has a tripod mounting foot, it is very easy to handle under demanding conditions and is a very good lens all round.
An excellent Canon model is the Canon EF 400mm f5.6L USM
Here are some of its features:
This is a very high performance lens and was designed to be specifically easy to handle and very portable, you will get sharp, crisp pictures using this model.
These are both excellent lens models and are both great options when it comes to choosing a nature photograph lens. You should know what you will be using them for most of the time before making your decision as that will make it clearer which one is best for you.