Couple other thoughts from somneone who recently went through the process of getting started.
Alot will depend on your budget and your anticipated wants.
On bodies.
Probably the 2 most important aspects are the AF capabilities, fps ( and buffering capacity). I find the XT just fine in most situations but it can struggle getting focus on small, fast movers.
Example: Swallows. In order to get swallows in flight nothing short of a pro body will do.
On FPS
Im not as familiar with other bodies so wont comment, but mention that as something to keep in mind as some PnS camera's may have the fps but no buffering capacity so your max burst at high res may only be a couple shots, then you have to wait a few seconds. With the XT and at the highest res Ive yet to have trouble with 3fps and max burst.
On Lenses.
I would say the minimum focal length you want is 400mm if its important to you. If not 300mm can do the trick. Other things to consider are their repective AF speeds, aperatures, IS, limiters. Theres no perfect lens and for the most part its all about trade offs. Budget vs IQ vs focal length vs max aperature vs IS etc.
Primes are typically regarded as the best "bird" lenses but the trade off is limited application.
Zooms have more flexibility but trade off a tiny bit of IQ and AF speed.
Zoom options tend to be the Canon 100-400L, the Sigma 50-500 and the Tamron 200-500. In thi group there are differences in weight, AF speed ( thats one of the knocks on the T 200-500) IQ and budget.
So it really depends on what you *want* and there are no one or two answers.
Example: I wanted a bird/wildlife set-up, so I was leaning towards the 400mmprime. My wife who knows nothing about photography insisted on a zoom so I can take pics of the kids at the zoo/park etc also wanted IS once I explained that. Guess which lens I got
TCs and Tubes are also accessories to consider.
In closing:
Look for fps/buffer/af capabilities in your body
Decide on max focal length you want be it 300, 400 or 500
Decide on prime (dedicated lens good for birds only) vs zoom ( less IQ, less AF speed dependant on maker but more flexibility)
Find one that comes closest to budget and requirements.
Some lenses may also require a monopod.
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