The diversity of life really is amazing. Just about every possible niche you can think of has been exploited at some point - if there is a way to make a living out there, you can bet natural selection has shaped some creature to utilize it. Take sanguinivores for example - better known as "blood-suckers" (Latin: sanguinis = blood; vorare = to eat).
Blood is a protein-rich material so it is not so surprising that some creatures utilize it as a food-source. In fact, sanguinivory is not so uncommon - we all know about mosquitoes, ticks, biting flies (deer/horse flies) and have some exposure to those "mysterious" creatures of the dark - vampire bats (though I bet you'd love to know more). But did you know that there are also vampire birds? Yup, blood-sucking birds.
You know those sweet, helpful, caring birds that eat the awful, ugly ticks and bugs off the backs of rhinos and giraffes, thereby preventing diseases and alleviating discomfort? Well, it turns out that they might not be so sweet and innocent after all. And on our favorite playground of natural selection, the Galapagos Islands, one of 'Darwin's finches' is a blood-sucker - this one doesn't even pretend to be helpful.
I'll let you read the bloody details on your own. Below are some links to some info on sanguinivory, blood-sucking birds, and vampire bats (I've left out mosquitoes, ticks, and flies - they're just too icky). The posts actually follow an order and are from one of the new blogs I am enjoying (yes, I know I'm a geek) - Tetrapod Zoology. There's a lot of info in the following posts, so take them slow. Enjoy!
Check this video out first:
Video: Vampire finches
Then read these in order:
Evolution of vampires
Evolution of vampires, part II
Evolution of vampires, part III