Another vote for the DA-L 55-300, if you can possibly stretch to about $150. It's amazing value, compact and light weight. Unlike a lot of telephoto lenses, it produces good sharpness at the long end (300mm), even wide open (f5.8).
For birds and other wildlife 300mm is a signficant advantage over 200mm. If you really can't go beyond $100, hold out for the best 300mm lens that you can get within your budget. (Lots of helpful suggestions above - make sure you read the reviews carefully.) You could get a shorter lens with a teleconverter (e.g. 200mm lens with 1.4x TC, which would be the equivalent of 280mm), but such a lens and TC together for under $100 would not be likely to produce better results than a budget 300mm lens. (TCs, especially cheap ones, generally degrade image quality.)
As for auto or manual focus, I used manual focus lenses for 25 years, but I would much prefer an autofocus lens for moving objects like birds and animals.
Ideally you would buy from the PF marketplace or a reputable dealer, but if you are prepared to take your chances on eBay, try a sniping service like
www.ezsniper.com/ You might score something that would not be affordable from a dealer - perhaps even a 55-300!
Good luck.
Last edited by Des; 08-07-2014 at 05:18 PM.