I replied to Megan privately yesterday, but I figured I'd chime in publicly
as others are also now talking about the Canon EF 100-400mm f/4.5-5.6L IS
USM zoom. With a birding perspective, I wanted to expand on some of my
points, emphasizing mobility and image stabilization.

One lens you may want to consider is the Canon 100-400 zoom. It's around
$1,400, but you get up to 400mm ***with image stabilization***. It also
compacts nicely for carrying in a Lowe top-loader waist pack (although it's
a pretty big waist pack). (As others point out) a zoom doesn't get you the
level of quality of a fixed lens, but I've been reasonably happy with the
images I can get given the money I spent and the mobility I enjoy (weight
and size) with or without a tripod. When on a tripod, the IS on this lens
should be turned off. I also use the lens around the house taking pictures
of the kids out playing etc. so the lens is versatile. If you go with this
lens, avoid extending and retracting the lens often indoors/in dusty
conditions. While it is a professional quality lens, the seals still have
to allow some air to flow in and out. You can end up with dust inside the
lens which will progressively affect contrast. Canon was kind enough to
clean it for me once as a courtesy when I expressed my surprise at the need
for and the cost of the cleaning . . . but they can charge around $300 to do
a major cleaning. I'd be interested to hear from others ***privately***
about any dust issues they've had with the lens.

If you're going to take pictures handheld at all, I believe image
stabilization is *critical*. Going handheld will get you several quick and
adequate snapshots that you wouldn't get nearly as easily or at all with a
tripod. (quick flight shots, shots from a boat, birds like warblers that
move constantly) . . . and sometimes you just don't want to schlep a tripod
along. Tripods get you the creamier-of-the-crop shots, but fast
at-the-ready handheld shots are often the ones you need to prove/confirm
quick and unexpected sightings or just get those snapshots to serve as a
souvenirs of a memorable outing where windows of opportunity for some birds
few and short-lived.

Thinking longer-term, I think you will regret only having 300mm when you
would have rather had 400mm. I say this because with the zoom I have 400mm,
but I *MANIACALLY CRAVE* the quality and reach of a 500mm or 600mm prime.
Other photographers I know warned me about this, and they were right. I'll
get one of them one day. New, they are a humbling $5,500 and $7,200
respectively. Ouch. (but I digress and depress).

Examples of the kind of shots I get with the 100-400 zoom are at
http://www.pbase.com/nmcgarry/birds in a number of sub-galleries. All in
Recent Additions will be using the Canon zoom. At 400mm, extensive cropping
is a routine necessity.

Good luck with your decision.


______________________________________

Ned McGarry
Sammamish, WA