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I see that CT says " IS was enabled"
I dont know about Canon but the Nikon suggestion is that when a camera is FIRMLY clamped on a tripod, the VR system looks for vibration, doesn't find any, and goes into a mental meltdown and then makes some.....:thumbsdown:
I CANNOT find this in writing at Nikon USA - might be one of those urban myths
like girls who wear red shoes........
When they refer to 'on a tripod' they mean with the head locked off and releasing the shutter with a remote release, when obviously there's no movement to be detected anyway and IS is pretty pointless. Earlier versions of Canon IS as still fitted in the 100-400L can actually cause corrupted images if used in this fashion, or even sustain damage to the IS system if IS isn't switched off.
The 2nd generation of Canon IS though, automatically detects the presence of a tripod by there being no detectable movement, and automatically disengages the IS system.
When we're talking about bird photography from a tripod though, we're invariably taking about one hand on the camera, one on the lens and firing the shutter with your finger in the usual fashion, with the camera mounted on a gimbal head.
Think of the tripod used in this way as being similar to a machine gun mount in which it's prime purpose is to relieve you of the weight and to enable smooth tracking of the target because that's exactly the benefit you get, and the IS system is detecting more than enough movement through your hands - no matter how still you think you are- to still be effective.
If anyone doubts that, try this simple test - mount your lens on a tripod and gimbal and focus on some static target holding the camera as still as you can. You'll still see a slight tremor of the image in the viewfinder which is more noticeable the longer the focal length of the lens. Now half press the shutter button and as you hear and feel the IS kick in, the viewfinder image becomes absolutely rock steady.
You can of course argue that if the shutter speed is fast enough it's benefit is questionable, but in the very nature of bird photography you'll be out there in all weathers and all kinds of light, often pushing the envelope with shutter speeds far less than desirable, but if there's movement to detect you can rest assured it will be working away for you and ensuring that at least it wont be your camera movement spoiling the shot.
I can't remember the last time I switched IS off on my long lenses and I don't intend to start any time soon.
Movement of those quick moving birdies is a completely different matter, and IS is never going to help you with that.