There are several custom function options available to the 7D which can be configured to assist with tracking moving objects:

• C.FnIII -1 AI Servo Tracking Sensitivity You want this set to "Slow" - this will stop the AF system trying to refocus on anything that briefly passes between you and the subject you are tracking - handy with birds where branches etc might flash by as you track

• C.FnIII -2 AI Servo 1st/2nd Image Priority This should be set to 0 so that the camera will give priority to focusing on the subject over taking shots - basically, if it thinks it hasn't got a lock, it won't shoot while it obtains one

• C.FnIII -3 AI Servo AF Tracking Method Set this to 1 - this further adds to the system ignoring obstacles by allowing the system to use AF points around the main focus point when something gets in the way of what you were tracking

Use single point AF with the AF Expansion custom function (C.FnIII -6) switched on - this gives you accuracy on the initial pick up but then allows the camera to help out as you track.

These settings should be correct from the camera side of things but the story doesn't end here.

Your 100-400mm L lens has a 2 mode IS system, with more predictable objects, you should use mode 2.

For tracking moving subjects it is important to get a good solid stance and to track the subject as it approaches some time before you want to start shooting (trickier with birds but achievable depending on the behaviour of the species). You should also follow through after you shoot (much like when shooting a shotgun).

You need your feet about shoulder width apart and need to be facing across the path of travel of the object you're tracking:

Object -------------------------------------------->

               ^ 
              You 

Hold the camera with your right hand as normal and with your hand a good way down the lens (similar to holding a rifle or shotgun). Keep your elbows tucked in, possibly resting against your torso to provide extra support and twist your torso from the hips upwards to meet what you want to track. Track it in the viewfinder, ideally if you can switch eyes so if you're tracking right to left, look through the viewfinder with your left eye so you can keep your right eye open to help keep track of the object (especially if it moves erratically) - do the opposite for left to right.

Start tracking the object, and smoothly pan with it. If you are shooting birds, turning off IS altogether is probably a good idea as the movement is likely to be too erratic for the IS to keep up with (even in panning mode 2) - aircraft are steady enough to use IS mode 2 with usually.

Squeeze the shutter release, don't press it (just like with a rifle) or it will jolt the camera and try to keep the object in the viewfinder as you get glances between shutter releases. Carry on tracking as the object moves away.

Getting a smooth torso twist is paramount and takes practice but getting the stance right gives you the foundations to build upon.