Scientists have long been fascinated with the physiological changes that birds undergo before and during migration. Some birds eat so much fat before their journeys that they double in body weight. In some species, their hearts are enlarged to pump more blood, or their digestive tracts grow and then shrink. But researchers have only recently started to explore at a fundamental level how migratory birds get the energy required to keep themselves aloft for days on end without eating.

Last year, two independent groups published research that explored migratory bird physiology in the lab and field to probe what happens at the subcellular level that allows birds to cover vast distances. They both found answers in biology’s most fundamental engine: mitochondria.

Their studies show how small changes in the number, shape, efficiency and interconnectedness of mitochondria can have huge physiological consequences that contribute to birds’ long-duration, continent-spanning flights.