No bird spends its life farther north than the Svalbard ptarmigan, which passes deepest winter in perpetual darkness and high summer bathed in 24-hour sunlight. But it seems that even the ptarmigan has a light-sensitive internal clock, which tells it when the breeding season has arrived.
Most birds have an inner clock that prompts them to perform specific tasks at specific times of day. But in summer, Svalbard ptarmigan (Lagopus muta hyperborea) live under a midnight sun, and their activity during a 24-hour period doesn’t follow a consistent pattern.
Nevertheless, David Hazlerigg, Alexander West and their colleagues at the University of Tromsø in Norway found that key genes for establishing 24-hour rhythms are active in the brain of the ptarmigan, which uses this daily ‘circadian’ clock to time seasonal events. In birds kept constantly in the light, genes linked to reproduction became active, and the birds increased their activity in preparation for mating. The researchers’ experiments suggest that 14 hours after sunrise, the birds’ internal clocks ‘check’ whether the Sun is still up.