Vampire finches have lived on Darwin Island and Wolf Island (both part of the Galapagos Islands - Ecuador) since 500,000 years ago.

In fact, the Galapagos Islands are home to 13 different species of finches that evolved from a common ancestor. Each finch species has adapted its diet to its environment. Some prefer to eat insects, seeds, and nectar, while others, like the “vampire” finches, prefer to drink blood from large seabirds.
Darwin Island and Wolf Island have harsh weather conditions and scarce food, even in the dry season there is no food. Therefore, the "vampire" finches have to eat parasites on the skin and feathers of large birds that live on the island such as some seabirds, red-legged boobies, etc. Over time, they start to suck blood while catching parasites. However, this behavior is only performed when other food sources such as seeds and insects are scarce, forcing them to drink blood to survive.

The vampire sparrow uses its sharp beak to peck at the wings of larger birds and drink their blood. Since larger birds are used to sparrows helping to “clean” parasites from their bodies, they do not fight back when their blood is sucked.
According to scientists, sparrows drinking blood is an unusual behavior and this is an "evolution" to adapt to the environment.

Blood contains few essential nutrients but too much salt and iron. However, because the digestive system of the "vampire" sparrow possesses a type of gut bacteria called Peptostreptococcaceae that can digest these two substances, when necessary, they are ready to use this "food" source.
Even if there is no food, they will attack seabirds and their young to drink their blood.
Not only do they drink blood, vampire sparrows also steal other species' eggs, then push them against rocks with their feet until they break open so they can eat them. Due to the scarcity of food, to survive, vampire sparrows also eat bird droppings and fish left over from other predators.