Why do roosters crow in the morning?

For a long time, people assumed roosters crow because they see the sunrise. In 2013, researchers at Nagoya University in Japan kept roosters in constant dim light and found they still crowed at roughly 24-hour intervals, showing that crowing is driven by an internal circadian clock. Light and other sounds can also trigger crowing at other times of day. A later study by the same team found that the highest-ranking rooster in a group crows first each morning and lower-ranking males wait their turn. Crowing announces territory and status to other birds.