Come closer....

Looking at a photo where a bird has been brought to focus with clarity and the background being blurred out has always got my attention. That is one of reasons why bird photography appeals to me more than any other form of photography. But this also brings us to the point how much can we zoom in so that we get the bird in the frame with sharp details and desired composition. When I was thinking on these lines I was trying to evaluate the type of lenses I would require to get what I want. I started analysing the possible options by which zoom can be achieved in a digital camera. It surprised me that so many combinations are possible.
Zooming methodologies :
1. Try to go as close to the bird as possible with out disturbing the bird. This is simple and straightforward. It also means this will give us the ability to use a normal zoom lens and still get a good picture. This though would require that we have a better knowledge of the types of birds and their behaviour. Understanding a bird better helps us prepare better on how the shot should be composed.
2. Use a good zoom lens like canon 100-400 lens f/4.5-5.6. This is a amazing lens at a very good price. But my feel is that with this lens at a focal length of 400 at 5.6 to get a good bokeh composition it would depend on how the much is the distance between the subject and the background object.
3. Use the crop factor of the camera to get the desired zoom. A 70-300 mm lens with a crop factor of 1.5 becomes a 105-450mm lens. Crop factor should not be confused with digital zoom since crop factor specifies the maximum zoom you can go during processing with out loss of details. Digital zoom is more by extrapolation once we have exhausted the details in the image.
4. Use the teleconverter to extend the range. A 2x teleconverter can convert a 70-200 mm lens to 140-400mm lens. A teleconverter is a magnifying glass. So if there is noise in the image in the form of dispersion it magnifies that also. So to use teleconverter and still get a good image it is important we have a lens that has low dispersion.

So looking at all the above it can be infered that there is nothing called as the correct method or the preferred method to achieve the desired zoom. As long as the approach gives us the results we want it should be fine. The zooming stratergies is not unique usually and is a mix of multiple stratergies mentioned above.

Based on the above analysis I came to a conclusion I will go with the following combo.
This is my personal choice and not necessarily the best choice.
Zoom Lens combo : 70-200 mm f/2.8 lens + 2x teleconverter = 140mm - 400mm.

My reasons are as follows.
1. In a picture bokeh is very important to me. Ability to seperate one subject from others in my opinion is what makes photography a art. At f/2.8 I can achieve this.
2. Dispersion increases with wider aperture. So at f/2.8 200m the dispersion can be very high. So a good quality low dispersion glass is required to control the dispersion. I am impressed about both Nikon and Canon 70-200mm glass in this regard. Since I am a nikon fan I am going with Nikon.
3. The picture quality of a 70-200 at f/2.8 200 mm will be far superior with a good bokeh than a 100-400 at f/5.0 at the same 200 mm.
4. With a teleconverter since the 70-200 mm would become soft if still used at 2.8 it would probably have to move to around f/4 but still the bokeh would be good and the clarity would be better than a 100-400 at 5.6 for 400mm.
5. A 70-200 mm without a teleconverter would still be a good choice if we plan to use the crop factor. This would also mean a camera should have a crop factor which is achieved by good number of pixels. This does not mean that the number of pixels is so high it comes at the cost of lowering the signal to noise ratio.
6. Can go for a 500mm f/4 lens but that turns out to be very expensive. So will be happy with a max focal length of 400mm.