In a new study published in the journal Proceedings of the Royal Society B, researchers have studied the efficiency of migratory bats—a species that weighs about eight grams and is found in almost all of Europe. Efficiency, in this case, is the ability to convert supplied energy into something we need. For bats and birds, it's the energy required to fly. Now, a research team in Lund states that the efficiency varies with the bats' flight speed. The faster the bats flew, the more energy they managed to convert into muscle power.