The bill of Ensifera ensifera is 8 to 12 centimetres long, while the bird measures 13 to 14 centimetres from head to tail tip excluding the bill. The bill therefore exceeds the length of the body but not the body and tail together, and no other bird is known to reach even that proportion. The head is also held tilted upward at rest to balance the weight.
The shape is a coevolutionary result. Several Andean plants, notably long-corolla passionflowers, hold nectar at the base of tubes that shorter-billed hummingbirds cannot reach without robbing the flower from the side. The sword-bill reaches the nectar legitimately and carries pollen on its forehead, giving the plants a specialist pollinator.
The cost is that the bill cannot be used for ordinary feather care, so the bird scratches and preens with its feet, unusual among hummingbirds. How tightly the species depends on any single plant genus is debated: it visits a wider set of flowers than the classic textbook pairing with Passiflora mixta implies.