All birds need to keep their weight down. A lightweight beak means that a bird can not chew. It can only pluck, crush, tear and rip. A bird also has fewer bones than a mammal - no tail bone, one wing bone, and a slim pelvis that is fused to the backbone. The bones are hollow, though they have a lattice of cross struts to keep them strong. The most remarkable weight-saving feature is its feathers, which have a very complex structure. They have hollow quills and filaments on either side of the quills that latch together like a zip and are easily repaired.