That was exactly my thought process too :D There's pretty much nothing else in the price range of the Sport that does 600mm at less than f/8, and the Contemporary model has 90% of the performance of the Sport but at an even more digestible price. For birds the lens length can never be long enough, and I think the Sigma range is in a great price point where it's expensive enough to be good but you're not paying more for marginal gains. Plus some of their latest lenses can be hooked up to a USB dock to allow you to customise them a bit, such as making micro-focus adjustments, which is really cool.