Information from the sky
As the earth spins, the night sky appears to revolve (so-called celestial rotation). Stars closest to the South or North poles move the least, while stars closest to the equator move fastest and furthest. Young birds build up a picture of the position of the centre of rotation (over the nearest pole), which gives them the north–south axis. To use these celestial cues, young birds need to view the rotating sky prior to migration, for it to become imprinted.
During the day, the sun’s movement across the sky gives orientation signals that vary with time of day. Like many animals, birds have internal clocks that help interpret these signals. For example, they know that in the late afternoon the sun’s position is more to the west than it is at midday or in the morning. The height of the sun at midday indicates latitude if the time of year is known. However, knowing the time becomes complicated during the course of migration. Local time may change if the bird moves longitudinally on the east–west axis, and day length changes as birds change latitude.
Keep it in mind
Birds need a good memory to retain details about their route, and migratory birds do better on memory tests than non-migratory ones.
Migratory birds gain information from the sun better than humans can. They can detect the changing pattern of polarised light across the sky throughout the day, including the period before sunrise and after sunset. Birds need only a small area of blue sky to see light changes, so this is particularly useful when the sun is obscured by patchy cloud cover.
Magnetism and celestial rotation each have limitations, but used together they become more reliable. Birds constantly check one system against the other and adjust their direction. They may also rely on one system more than another depending on where they are.
Using sight, sound and smell
Several sources of information can help birds build a map as they fly. These include:
- visual landmarks
- sound, including infrasound beyond human hearing, and sounds signalling land, such as crashing waves on coasts and wind on ridge tops
- smells that characterise an air mass coming from a particular direction.