This would be a nice hobby, you have a good enough camera body,
now you need is a lens, you need about a 300 or 400mm lens.
I have something what goes from..70 to 200mm F2.8 and using a converter 2X, this gives me 400mm and F doubles, F5.6
I like this set up because I can use this prime lens 200mm for many
other things, when using for birds, I just add the converter.
Konrad

I have also been a serious amateur / semi-pro photographer since the early 1960's and like you, when I retired I decided to take up Wildlife & Nature photography as my retirement hobby primarily to gie myself an excuse to get out of the house regularly, but even more specifically because I suffer from C.O.P.D. and I find taking long nature hikes carrying a 30lb backpack of photo equipment very theraputic, as an example, one year ago I couldn't walk to the mailbox without carrying a portable oxygen tank but today I think nothing of 5 or 10 mile hikes and I have not even had the O2 equipemtn for the last 6 months.
According to my book, Birds of Ohio, there are 413 species of birds native to the state of Ohio. To date I have photo documented 119 species either in my yard or within walking distance of my house.
Bird photography presents a number of unique challenges and with experience you will find that your technique is every bit as important as your equipment.
Most birds fly between 20 and 30mph while some of the hawks and raptors are capable of speeds in excess of 70mph. This means that while in flight most birds are moving 30 to 45ft/sec which makes it nearly impossible to focus on birds while in flight, therefore if we have any aspriaton of getting flight action shots we have to anticipate where the bird will be, then set our equipment up in advance and await an opportunity to take the shot.
Obviously shooting birds when they are still is much easier, but keep in mind that birds are very elusive so we must provide some means to attract the bird to our viewing area. IN winter this is very easy, we simply put up bird feeders, then sit back and wait for the birds to come to us, but what do we do in summer?
Birds are opporunists and for basically the same reason that people frequent fast food restaraunts, birds can easily be attracted by bird feeders, bird baths, small decorative ponds in your landscaping and selected flowers and shrubs that attract birds. If your into capturing images of birds in their wild habitat you could do as I do, don full camoflage and stake out a spot where you have full view of wild fruits, berries, corn fields etc. Oh yes, I realize that this may seem gross to some, but don't overlook the photo opporunity of staking out a vantage point where you can watch some road kill. You will be amazed how many birds are scaavengers.
Although much more difficult to get a good viewing angle, you can get some very unique photos ir you can find a nesting site, but be careful here. If you repeatedly get too close to a nesting site the parents may abandon the young. Under no circumstances should you ever touch a nest becuse that would leave your scent and the parents will nearly always leave.
Now to answer your original question, what lenses do you need?
Let me begin by clarifying a point for you. Rgardless of what kind of camera we use, a normal lens has a focal length equal to the diagonal measurement of the negative format or the electronidc sensor. By example, a 35mm film camera produces a negative that is 24x36mm therefore the diagonal measurement is 43.2mm. (due to the physical space reqquirements of the multiple elements, aperture and focusing linkages the industry has accepted 43 to 55mm as the normal lens). Any lens which has a focal length longer than the Normal lens is said to be a Telephoto lens. Generally lenses up to 3x the normal focal length are defined as telephotos while lenses longer than 3x the normal focal length are defined as super telephotos.
Lenses with a focal length that is shorter than the normal focal length are called "Wide Angle" lenses.
Lenses with a fixed focal length are defined as "Prime lenses" while lenses with a variable focal lenght are said to be a "Zoom Lens" meaning that is can be zoomed from its minimum to its maximum focal lenght, thus you could have a zoom wide angle or a zoom telephoto.
On cameras that have a fixed lens they often define the focal ength by a number, by example, they may say the camera has a 10x zoom. In this case the numbrer is multiplying the focal length of the normal lens for that format, thus while the normal format for a 35mm film camera is 43mm, a 10x zoom would equal a 430mm lens.
For 35mm film cameras or DSLR's a 300mm lens is considered the longest lens that can consistantly be hand held and they should only be used in hand held mode when the shutter speed is 1/125 or greater. Lenses longer than 300mm or in situations where the shutter speed will be less than 1/125 it will require a monopod, tripod or other suitable means of support.
The actual diameter of the lens aperture can be mathematically computed by dividing the lens focal length by the index number, by example, if you have a 50mm lens set at f-2 the actual diameter of the aperture opening is 50/2= 25mm while a 300mm @ f-2 would have an aperture opening of 300/2= 150mm however due to the difference in angle of view both lenses would produce the same light level at the film plane. If we then examine the complete f-stop scale we would find that the the indexes are such that increasing the index one stop will reduce the cross sectional area of the aperture to 1/2 the cross sectional are of the previous aperture opening.
If you then examine the shutter speed scale you will note that each increment is approximately double the time of the next higher shutter speed.
Exposure is a combinastion of time and light intensity, thus if we change the f-stop one stop we either double or halve the amount of light striking the film or sensor, and if we change the shutter speed on index figure we also either double or halve the light to the film. In this manner we can change the overall exposure one f-stop by either changing the aperture setting or the shutter speed one index.
You could use a 200mm lense with a doubler as was siggested before, but when we attach a doubler the actaul apertue size remains constant while we double focal length so we instantly loose one f-stop. In addition, we generally loose another f-stop to the light loss through the glass lens in the doubler. As a result the actual insertion loss of a doubler is typically 2 f-stops and some cheaper after market doublers have an insertion loss of 3 f-stops.
When i first begani my interset in photography a built in light meter was still a new feature and in those days we had to learn how to eyeball an exposure. (When using my old manual focus pre-set lenses I still put my camera in manual mode and eyeball the exposure for practice)
ISO is the actual shutter speed required to produce a proper exposure at f-16 in mean daylight at noon at the equator. As you move north of shuth of the equator the angle of incidence of sunlight changes and we must compensate the exposure, thue in the USA in bright sunlight at noon in mid summer the proper expose shutter speed would be equal to the ISO number with the lens set at f-16 and we loose one f-stop for each two hours before or after noon.
Keep in mind that birds are most active in early morning or late afternoon. Using the aabove formula, if we are shooting a picture at 4pm on a bright summer day in mid summer our we could set the shutter speed to the ISO speed and set the lens to f 5.6. Keeping in mind that we want a shutter speed of at least 250 to stop action we must then use ISO 400 and if its a cloudy day or in shade the exposure would be f-4 but our lens will only open to f 5.6 so we must then increase the ISO to 800. If we were to attach a doubler we would need to push the ISO two more stops to 3200 and risk a lot of image loss to noise.
In my humble opinion the best economical lens for what you want to do would be a 70-300mm f 4-5.6
I would also strongly encourage you to get a good heavy duty tripod and a remote shutter release.
Your Canon Rebel uses the Canon EF mount and although I am not familiar with your camera it may also use the Canon EFs mount.
Out of curiosity I did a quick scan on Ebay and found 11 Canon 70-300mm f4-5.6 EF lenses listed with a "Buy Now" price ranging from $119 to $189 ( avg was $139). I also found a couple Sigma 70-300mm f4-5.6 with canon EF mounts for $129 and if you are willing to go through the bidding procedures I saw a couple that went as cheap as $65+S&H.
I am attaching some photos that i took with my Pentax ist DL (6.2mp) DSLR with a Sigma 70=300mm f 4-5.6 manual focus lens.

Thank you both so much for your great in depth answers. I will add the lenses you mentioned to my list and do a little more research and then go shopping! Armed with this information you've both shared, I will be much better prepared to get the lens I need.

Konrad, I love the one and only prime lens I have but it's just for closer shots. I would love to get one for birds!!

Lazypup, the photos you posted are amazing. I love the ones of the birds in flight. I've had great luck photographing hummingbirds at our feeders because they are very bold and don't seem to fear coming close to us humans. I even got a shot of a mama hummer feeding her demanding almost full-grown baby. And I've also been able to get some great shots of some goldfinches that feasted on a couple of volunteer sunflowers I left in my garden...apparently they were too engrossed in their meal to be bothered by my presence. In the wooded area near me, I have seen Baltimore Orioles and Indigo Buntings darting in and out of the trees as I drive by on the highway that runs right next to the woods. I am just determined to get photos of these beauties! They, of course, will be much more skittish and wary.

I have a lot of the equipment you mentioned (the remote shutter release, a good tripod, etc.) and now just to find the right zoom lens!

Thanks again for your prompt answers! I retire in 9 days!! :)

There is a heavy price to pay for a top lens F2.8, I started with a cheaper 70-300mm F5.6, junior is now using my old gear and I went all the way.
You can't beat a high quality lens when it comes to crisp shots and good detail.

{{gwi:108230}}

Konrad

Konrad, those are beautiful! I love the cedar waxwing in the pine! What detail!!!

If I've learned one thing from hanging out at the DPReview website, its that not all lenses are created equal. Experienced Canon owners can advise you about the best lens for your camera.

Here's a link to the Canon DSLR Lens Talk forum.

Here is a link that might be useful: Canon Lens Advice

PS: In case no one else has already mentioned it, not all telephoto lenses are zooms, and vice versa. :-)