I am not going to belittle IS, but I have found that one can do without for birds in flight and for reasonable-light perched birds. For some situations I like to have the monopod and tilt head, which allow one to stake out a nest or a hunting heron without having your arms drop off after 5 minutes. Monopod does a fine job of allowing 1/100 sec shots with the 400 f/5.6L no-IS. I am a fan of this lens and of carrying a monopod on a belt holster. The monopod also doubles as a hiking pole on stream crossings and steep bits where a little balance is convenient. On the other hand, I have seen people shoot from kayaks (in swamps and on creeks and rivers), and there's no question that one wants IS at 400mm, because even if your camera is rock-solid with relation to the kayak/canoe, the whole boat is moving slightly.
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