Three living bat species drink blood, all of them New World leaf-nosed bats. The common vampire bat feeds mostly on mammals; Diaemus youngi and Diphylla ecaudata concentrate on birds. Diphylla is the most specialised, feeding on roosting birds including wild species and, near settlements, domestic chickens.
The adaptations are structural rather than behavioural alone. The lower incisors are broad and fringed into a comb, the tongue is cupped for drawing blood along capillary grooves, and the hind feet grip well enough for the bat to hang beside a perched bird. Saliva contains anticoagulants that keep the small wound flowing.
A study in the Brazilian Caatinga found human blood in faecal samples from a Diphylla colony, which was widely reported as a switch to hunting people. The more careful reading is that in a landscape where native birds had become scarce, a specialist took what was available. Whether such feeding is regular or opportunistic is still not established.