Originally posted by RioRico Snide remark: Any lens will do, as long as you can get close enough. Drug the birds, maybe.
there was actually a post once asking for the best lens for birding, my response was similar, the one you have on your camera at the time.
I have photographed a hawk with an FA28-105 F4-5.6, THis lens is neither long or fast, but the hawk was sitting on a fence post eating a mouse. He was more concerned about me trying to take his lunch than anything else.
Someone posted a grey jay (i think it was) with a 10-17mm fisheye. it is easy if the bird is eating seeds out of the lens hood at the time
but seriously, i think 300mm is a minimum, and if you are going to crop out of a 300mm lens, it better be a good lens. Otherwise, you get what I call record shots, enough to prove you saw what you claim, but not good enough to do anything else with
400mm at F5.6 is a reasoable starting point.
as zapp says big birds are easy, its the little buggers that are hard. for these you need technique as well, but if you can be patient, or stalk effectively cutting your distance down is always going to improve the image quality regardless of the lens you are using,