Even without a wristwatch, birds have a sense of time. Like many other simple and complex organisms, birds maintain an internal clock that can operate independently of the environment. In many species of birds, the clock appears to be regulated by the pineal gland and a specialized site (the suprachiasmatic nucleus) in the brain. The pineal gland is responsible for the cyclic release of melatonin, which helps modulate the circadian rhythm. The timing of daylight resets a bird's internal clock. But its internal clock keeps ticking even in constant darkness, and the period of one cycle of activity and rest is still 24 hours.
Some animals also possess a circannual rhythm. These seasonal rhythms may be triggered or controlled by the biological clock. Both circadian and circannual rhythms play a role in the migratory behavior of birds. The white-crowned sparrow migrates each year from its wintering grounds in the southwestern United States to the Arctic to breed. As the number of daylight hours increases, light reaches the brains of sparrows at a particularly sensitive time in their circadian cycle. As a result, hormones are released, and gonadal development is stimulated. Other hormonal changes cause the sparrows to increase their food intake and store fat before migration begins.
A biological clock is also essential to many birds as they navigate along their migratory routes. One way that pigeons sense direction is by using the position of the sun in the sky to guide their travels. The sun, however, does not remain stationary but moves from east to west across the sky during the day. To adjust for these changes, birds depend on their internal clocks to help them alter their direction relative to the moving sun. Birds also may use other means of navigation, including the position of stars, the earth's geomagnetic field, landmarks, and possibly even their sense of smell.