Re: Bird in Flight
f/3.5
ISO 100
1/125 sec
50mm
The image on the right was heavily cropped.
The D90 is a very capable camera so you are fine there.
You need to use a long lens if you have one. Longer the better.
I will assume you are panning when you take the shot, if not, you need to be.
I would suggest using a faster shutter speed. At least 1/250 sec. Longer the focal length the higher the speed needed.
If needed use a higher ISO to get a faster shutter speed.
I personally use single point focus point when shooting.
This is what works for me and other responses my differ. There are many ways to achieve the same thing so others may give you different ideas. You need to experiment and see what works for you.