Tamron makes very good lenses, and is very well respected - they are one of the 'big two' third-party lens manufacturers, the other being Sigma...that make the most third-party lenses for various camera mounts.
What you purchase will depend on how much you're willing to spend, what weight and size you're willing to carry, and how much reach you really need. Going by the assumption that you need as much reach as you can get, which tends to be true of most bird photographers, you'd probably start off by considering the Tamron 150-600mm lens, which covers the greatest range and has the most overall reach, for a still-reasonable price - it's a large and long lens, and a bit heavy, if you're not accustomed to birding lenses...but actually fairly compact and light for the reach and range compared to other 600mm lenses out there for those experienced with birding lenses. Another good option to consider if you don't need as much reach and also want a smaller, lighter lens would be the brand new 100-400mm lens - this can work if you live in areas where you can get reasonably close to birds, and it's also a good focal range for shooting birds in flight. It's significantly smaller and lighter than the 150-600mm, so it will fit better in your camera bag and not be as much to lug around all day if you're not into the big lenses.
You can also consider the Sigma 150-600mm lenses, as well as Nikon's own 200-500mm lens, which falls int he same price range (a little higher) as the Tamron 150-600 and between the price of the Sigma 150-600mm sport and Sigma 150-600 Contemporary.