I'm not a huge fan of zoom lenses, but if you want to take pictures of wildlife on a budget they are the only option. For last year's trip to Shetland I took a borrowed Tamron 18-200mm lens, which had a convenient "one lens for everything" range, but didn't quite have enough reach. In anticipation of more trips like that I've been trying out a Tamron 70-300mm f/4-5.6 SP Di VC USD lens, which apart from being an impressive lens, gets you a lot of abbreviations for your money.
Shooting with a long lens like this takes a bit of practice, especially with moving subjects. I have some work to do on that front, but I took these pictures of flamingos at WWT Martin Mere, using a Canon 550D, and I think they speak for themselves as far as the quality of the lens is concerned (the quality of the photographer is another matter). I have quite tremulous hands, but these were all shot handheld at 300mm and while not super sharp, are impressively free of camera shake. The one above, for example, was shot at a relatively slow shutter speed of 1/250th of a second and I think I could have gone slower. The image stabilisation on this lens (the "VC" in that alphabet soup) is nothing short of miraculous.