HI There:
I think you'll find bird photography a very rewarding pursuit. I just returned from Costa Rica and some of my bird pics are on:
http://mysite.verizon.net/vzer1cx2.
Your 70-300mm lens is a good start and the 2x converter will get you a good focal length, however the reduction in light caused by the converter (2stops) at times will be limiting. I use a Canon 100-400 Image stabilizing lens. It's a lot of money ($1300.00 US) but it's been worth every penny. I'm sure Nikon has an equivalent.
I started my bird photogaphy by shooting out the window at my bird feeder. If you can camoflage your feeder, they'll think you are shooting out in the wild. It would be a good way to start with the equipment you already have.
And a final note: As with any kind of photography, knowing the subject is really important. If you know their habits, you'll know when and how to shoot.
Good luck
Ed