With birding, there are three main things that you need to get the perfect shot:
1. Fast glass. When birding, you want to catch the bird directly in flight. In lower light conditions, especially in the late afternoon or on a cloudy day, you'll need faster glass to catch the bird in flight without motion blur. F8 may be too slow, and your shot would be ruined. Although some IS features compensate for camera shake, they only reduce hand vibrations, they have no affect on capturing action.
2. Fast autofocus. A few days ago I was out birding with my Nikon FM, and a 200mm F4 AI lens, with MF only. I must have missed many, many shots, simply because I couldn't get the focus right in time. Now I must admit that on some shots I didn't fire simply in fear that I may be "wasting" film, in case the focus isn't exactly on or it's a tad bit overexposed. This is digital, where you can shoot as much as you want. Although, you'll still want a fast AF system to catch the bird right in flight!
3. Telephoto capabilities. Telephoto capabilities are obviously a key component to birding. While a normal lens may be good for a group of birds, or a W/A lens may be good perhaps for a group of birds flying in the sky, telephoto capabilities are very important for getting close up to a bird, on a tree or in flight. Also note that telephoto zooms can be convenient when birding, a zoom to change the focal length is a lot better than cropping it afterwards in photoshop.
So, what lens could I recommend to you? I'd consider the Nikkor 70-300mm VR lens, it's pretty fast, comes at a fair price and has a wonderful AF system. Remember that when you're birding, the longer the better! The Tamron 200-400 f5.6 also would be a decent choice, although you must note that the AF system is slower and it's not very fast, but it's good if you're after a tele zoom on a budget.
Gear: 7 Nikon Nikkor AI-S and AF-S lenses, SB-700 flash, Nikon D7000, Nikon FM, variety of accessories
"There are no rules for good photographs, there are only good photographs."
- Ansel Adams