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Am in the market for a lens upgrade also and some of the older photogs here in Canada have suggested the Canon 400 4-5.6 usm as very razor sharp lens.
My humble reasoning is that extra crisp glass allows closer cropping so the newer Canon 70 - 300 f4 will be my next lens. Surprisingly many have testified it is as sharp as the prime 400 Canon. Price here is about $1500 while the Canon 400 still holds out for about $1700 and that's without IS.
My current birding lens is the Tamron 70 - 300 and the AF is often not quick enough to keep up. Examples at; PhotoTonyG.blogspot.com. You can see that I get close enough to birds and ducks, it's the hit and miss auto-focus that costs me many good shots. If the 100 - 300 were not available, then the Canon 400 f4-5.6 would be my next lens.
Many will scold me for this, but I flick day-old [wholewheat] bread to the gulls and this attracts red breasted mergansers, ducks, gannets and grebes in. So far there is no need for more reach than 480mm...what 300 delivers on my T2i camera.
Hope this is helpful. Ps: Gulls eat too much protine so a little bread provides diet variety.