buying an extender

I'm new here and to photography in general. I have a 30d and really enjoy it. I also recently brought the 100-400L lens. My question is if an extender is needed to capture images of birds in flight, or other nature work, and if I do get the extender, what decrease in clarity should I expect?
Thanks in advance for your input.

Using an 1.4 extender on the 100-400 you will lose af unless you tape the pins or buy the cheap kenco converter. Either way the af will probably be too slow for birds in flight. With a 1.4 tc it will certainly degrade the image a bit whereas a 2x tc will degrade it a lot.

With the 1.6x crop the 400 already has an effective fov of 640mm. I'd think that would be long enough, at least for me it would. I have ordered the 1.4x for my 100-400 on a FF which will give me 560mm which should be plenty long for me.

400mm should be in most all cases long enough for bird in flight shots. much longer than that and unless they are very far away its going to be extremely difficult to do anyway.

400mm is good fro birds in flight, no T-con "needed"

The 1.4X T-con would just be a handy addition, but not very handy for fast moving subjects like flight shots.