Have you seen those birds that climb around the rhinos, picking away parasites, and thought, "Ah, nature can provide inspirational stories of peaceful cooperation"? Get ready to have your illusions shattered. Those birds are on their way to being vampires.
A Symbiotic Relationship
The creation of a vampire starts innocuously. A bird notices a tasty-looking bug on a large animal, and down it goes to eat the bug. Over time, the animal adjusts to the bird's diet, and ignores the bird's light pecks and tweaks at its body. This relationship plays out everywhere on land and sea. (Although in the sea the birds' place is taken by small fish.)
And then something disturbing happens. The bird notices that if it pecks at a wound, or even pecks too hard on healthy flesh and breaks the skin of an animal, it will still get nutrition. Sometimes it might get shooed away, or even killed. Much of the time, though, it suddenly has access to delicious blood.
A Tale of Two Vampires
This is what scientists think happened to two types of birds half-a-world apart from each other. Parasite-eating birds found out that they could make a better living if they hacked away at their fellow symbiote's body and drank the blood that poured out. The prey of these birds had been conditioned to stay quiet.
The birds are so sly about it that it even fooled scientists for a while. Oxpeckers in Africa were thought to be medical workers, digging into the skin of rhinos and impalas in order to extract parasites and worms. It was only after scientists started really keeping an eye on them that they discovered that the oxpeckers simply re-opened and widened existing wounds to get at the blood. These birds have their choice of a buffet that a lion would literally kill for, eating giraffes, buffalo, antelopes, and local cattle.
Meanwhile in the Galapagos, the vampire finch gets its daily supply of liquids by sidling up to red-footed boobies and masked boobies and pecking at their tails until they draw blood. They drink the blood to replenish themselves. Their prey, for the most part, is entirely passive.
[Via Why Dogs Eat Poop]