Birds in general are impressive creatures, but the Supurb lyrebird takes the cake for its ability to mimic sounds. Here's a video demonstrating its ability to mimic construction sounds. You can listen while you read the rest of this post:

The range of sounds that the lyrebrid mimics is amazing, ranging from other birds' songs to the sounds of human machinery such as cameras, chainsaws, and car alarms. For a more extensive display, I recommend either the BBC's recordings or David Attenborough's "Life of Birds" lyrebird video (the BBC wants it off YouTube, so I won't embed it).

This bird's repertoire is so extensive that some people find it to be literally incredible. A Google search for the lyrebird indicates that a number of people reflexively believe it to be a hoax. I saw no positive evidence that it is indeed a hoax--but it has spawned some spoofs. My first thought was that I trust the BBC on this type of thing (I first heard of the lyrebird when I saw Attenborough's video on YouTube). To follow up, Wikipedia provides links to plenty of documentation of the lyrebird's abilities.
p.s. While you're thinking about the validation of incredible phenomenon, why not consider
how we would validate spontanous generation, and how it would impact evolutionary theory. That's the topic on my submission to the Scientific Blogging writing contest. If you like it, please vote for it (once per day).