Yup, SCraig is correct ... it's about knowing the target, and being able to be one step ahead to get as close as possible.
Most of the time you can't just walk up to a bird and snap a shot.
Hunters stalk prey, and so do photographers.
500mm fixed focal length (and higher) is a common size for most serious wildlife photographers, but many are not able to afford quality lenses of this size.
High quality teleconverters can also be used ... though in conjunction with a high quality lens ... better to get a larger focal length lens.
I use a Sigma 100-300mm f/4 APO (a little short for small birds) with a matching Sigma 1.4x ... but Blacksheep uses a Tamron 200-500mm LD.
I would like a Minolta 600mm f/4 APO G, but it costs $$$ and very large/heavy.