Vampire Finches.

Life on Wolf Island, a tiny speck of land 200 km north of the main Galapagos Archipelago, is so hard that birds there evolved to eat blood to survive.

But perhaps their most important source of food during the extended droughts is blood. The finches begin by landing on the tail of a seabird. They peck at the base of it's wing feathers breaking the skin and causing it to bleed. As the blood oozes out the finches sip it every few seconds. Other finches line up behind the booby like a queue at a blood bank and as soon as one leaves it's blood-sucking perch another takes its place.

http://www.abc.net.au/nature/vampire/finches.htm - via Scribal Terror