Did we ever hear back from the OP? Do we know their budget?
For that matter, do we know which Rebel-series camera they will be using the lens with? There have been ten different Rebel DSLRs, to date, with differing features and specifications.
The EF-S 55-250mm is one of the lowest cost, at about $200 US... Except that it lacks USM and 250mm isn't a whole lot of reach for some types of wildlife/bird photography.
Sigma 120-400 OS sells for just under $1000 US, which is a relative bargain among longer tele zooms. The Sigma 150-500mm OS sells for rounghy $1100. Compare to the Canon 100-400 IS, which sells for $1460 presently.
There are several Canon 75-300 or 70-300 lenses... some good, some not so good in terms of IQ and performance. These can cost very little (EF 75-300 III - $150) to quite a bit (EF 70-300L IS - $1400). I haven't used any of these personally... but the two or three least expensive 75-300s are generally panned here on POTN and elsewhere, the 55-250 is considered by most to give considerably better image qualiity. The more expensive 70-300L, 70-200 DO and other 70-300 models with both USM and IS also are considerably better... but all are more to a lot more expensive.
And all of them are 300mm max... which may not be "long enough" for wildlife. None of the 75/70-300s are practical to use with teleconverters.
The 70-200s are real workhorse zooms... Canon offers 4 versions... two f4 and two f2.8, two with IS and two without, selling for between $630 and $2100. Now these are a bit short for wildlife shooting, on their own. Any of them can be used with a 1.4X teleconverter, though. And the f2.8 versions can be used with a 2X. There's always some loss of image quality though, using teleconverters (more to a 2X than to a 1.4X). A TC also slows AF performance.
There are also some prime lenses to consider.... The more affordable include Canon EF 300/4 IS (about $1400 US), which works well with or without a 1.4X teleconverter. The Canon 400/5.6 ($1350) gives a bit more reach, but is less usable with teleconverters and lacks IS. I use the 300/4 IS a lot, with and without Canon 1.4X II teleconverter... it's very handholdable.
There are third party 70-200s, 70-300s too... I don't have a lot of experience with them, so you'll have to research a bit... But basically with the Sigma's for best AF performance look for HSM focus drive. With Tamrons, look for USD focus drive. These thir party lenses' focusing systems are roughly equivalent to Canon's USM.
Also on these longer lenses IS or image stabilization is a very valuable. Canon calls it "IS". Sigma calls it "OS". Tamron calls it "VC".