Birds have developed similar tricks. Studies of ducks found that birds at the edge of a group keep the eye facing outward open while the matching half of the brain stays awake, so the group has a lookout without anyone staying fully awake. The most extreme case may be the frigatebird, a seabird that can spend days or even weeks flying over the ocean. In 2016, researchers fitted wild frigatebirds with small devices that recorded brain activity. The birds slept in the air for very short periods, adding up to about forty minutes a day, compared with more than twelve hours when they were on land.