these are entry level telephoto zoom lens. Its about the same either u get Nikon or 3rd party ones, since the aperture is f4-5.6.
Lets say u shoot bird at 300mm, u'll be using f5.6, it will be slower than another using a pro range 300mm f2.8. The cost different is quite alot, approximately, price of 1 of the 300f2.8 is about 10x of 70-300 f4-5.6.
It can be used for birding on few conditions, iso at least 800, birds stationary, bright sunny day.