If you want longer than 300mm (which you need for birding) then you need to pay. I went with the Sigma 150-500mm and am happy with it. IQ is very good, and more than acceptable if you can keep it at f/8-11, which means you'll need to be boosting ISO if you want to shoot birds in flight at 1/1000sec. I opted for that as it was still under a grand, but in hindsight I sort of wish I'd saved for the 50-500mm Sigma as it has slightly better IQ and is a lot more flexible walking around. There's a Tokina or Tamron about to be released that's something like 150-600mm. No idea on price point, but there's a thread on it somewhere here.
For birds you want 400mm+, and unless you want to go 300mm with a TC (in which case you'll pay a lot for the 300mm f/2.8) then these are likely your best bang for the buck alternatives.