Whatbird.com

I forgot to add to this for between $20 to $50 you can get a monopod.

most of my images where done with a 70 - 300mm lens either Canon or Sigma But mine don't have IS  :(  wish they did so I use either a bean bag or a monopod  I have a tripod but by the time it is set up the shot is gone this is not the case with a monopod

here is the link to my gallery http://www.whatbird.com/forums/photos/the_kiwis_gallery/default.aspx

The answer depends on how much money you have to spend :-)    I absolutly love my 70-300 IS.  The focus is fast and sharp, the bokeh (background blur) @ 300mm is really nice, like in this photo http://www.flickr.com/photos/rdatkins/131047780/ and the cost isn't too bad.   I read a lot of reviews on this lens and haven;t heard of anyone getting a "bad copy" like some of the other lenses.  For example I'm on my third 28-135 IS the first 2 were horrible, the last on was factory refurb and perfect.

Just my $.02

You asked about comparable lenses, and you probably have already considered it, but you might want to think seriously about the 100-400mm f4/5.6 L.  All my birdshots are taken with that lens.  Even though it is about 3 times the cost of the 70-300 IS, it is well worth it, and I'd consider 400mm to be the least with which to do some serious bird photography.  The problem with wildlife shooting is that no matter the size lens you have, it's never big enough.  Perhaps the 70-300 will get you started and later you can consider something bigger.  After all, used lenses hold their value and selling them is pretty easy.

I have the Canon Digital Rebel XTi with Canon IS 100-400mm Lens.  You can see some of my photos on my blog.

I am no expert but I find the IS the most critical aspect of the lens. I wouldn't buy one without it for handheld photography. If buying again, I would look at the 400mm fixed IS lens. Almost all of my bird and wildlife photography is at the full zoom. The 400mm fixed has more clarity and can auto-focus with an extender (100-400 cannot). The 400mm fixed is also f4 at 400mm versus f5.6, which would be my second priority behind IS. 

Rob:
Actually, you're right.  With my 100-400, I shoot at 400mm about 99% of the time.  I also shoot the vast majority of my shots from a tripod, so IS is less important for me, but there have been times when it was essential. 

I don't know of a Canon 400mm f/4 IS prime lens, unless you're speaking of the 400mm DO, and that baby is expensive!  There is a 400mm f/5.6L, but it doesn't have IS.  If you've found an f/4, please let me know.  I sure would like to have something like that, assuming it won't cost me big bucks.

I was reading about a new lens been brought out by Canon and as you have a rebel it  may fit your camera as it is an EF-S mount it is a 55-250mm IS lens

here is a review of it http://www.beyondmegapixels.com/2008/01/canon-efs-55-250mm-lens-review.html

and ther is another review here

http://www.dpreview.com/news/0708/07082007canonefs18-55and55-250.asp

It may be worth looking to it it looks promising at a reasonable price

EF-S lenses are a very good and economical choice for cameras that accept them.  Before investing in them, however, one should realize the fact that a lens can be a very long-term investment, whereas a digital camera is most likely a short-term investment.  Given that, then, you run the risk of buying an EF-S lens that only fits EF-S mounts, which may not be available if you upgrade to a Canon that has the standard mount, whatever that's called.  Many folks may keep a camera, even a digital, for a very long time, and for that group maybe an EF-S is a good fit.  If, however, you think you may want to upgrade, you might want to consider investing in a lens that is most likely guaranteed (emphasize "most likely") to fit any Canon camera, current and future models.

I have lenses that are 4 and 5 years old that still work great, but I've upgraded my camera three times during that period.  I always keep my older cameras thinking I'll use them as back-ups or carry them in the field with a different lens connected to it.  Thing is, I almost never use them.  Point is, I guess, lenses are forever; cameras, sadly, are not.

2) 70-300/f.4 - 5.6 IS USM

I have this lens and LOVE it. I will never buy another lens without IS. I also have the 24-70L and the picture quality is nowhere as good. The 4.0 is nice, but really, 5.6 at 300m has a very blurry background.

One more vote, I have the 70-300mm 1:4-5.6 IS

An earlier comment was right on, your equipment will never be good enough.  I enjoy photography and enjoy my set up but wish I had better.  You can review my photos on this website.  Even with the IS, a monopod would be a good thing to have.  Biggest problem I have is depth of field for closer in work.  More depth of field requires more light which implies a bigger lens.  If you currently don't have a zoom you will be happy with the new toy whatever you choose.

Matt

zoutedrop:

One more vote, I have the 70-300mm 1:4-5.6 IS

An earlier comment was right on, your equipment will never be good enough.  I enjoy photography and enjoy my set up but wish I had better.  You can review my photos on this website.  Even with the IS, a monopod would be a good thing to have.  Biggest problem I have is depth of field for closer in work.  More depth of field requires more light which implies a bigger lens.  If you currently don't have a zoom you will be happy with the new toy whatever you choose.

 

Matt

Monopods are cheap enough and are easy to use

DOF (Depth of Field) this is obtained by using the zoom lenses and the larger you set the aperture opening (the smallest number ie 1.8 2.8 4  5.6 etc)  the shallower your DOF will be so and fstop of 2.8 or 5.6 is going to give you a blurry Background (OOF) the smaller you make the aperture the more narrow will be your DOF ie a F22 will show most of the details in your Background (They will be in focus) I am finding for my Camera at least f8 is giving me the best DOF and of couse it goes without saying the higher you can get your shutter speed the better