How do migratory birds know where to go and how to get there? Is it learned or innate? In some species, young birds make the journey alone, weeks after the departure of their parents. In those cases at least, some form of genetic imprinting must be involved, but external navigational aids are still needed.
Navigation strategies
Some species migrate in large flocks, with experienced birds showing the way.
In other species, when birds migrate for the first time without a guide, a type of clock-and-compass orientation is used. This seems to be genetically programmed, with an instruction equivalent to: ‘Fly south-east for three days, then fly due south for one day.’ The birds may then build up an internal map as they go, using landmarks, which they remember for next time. Co-ordinates (equivalent to latitude and longitude) could be established once both ends of the route are known, and these would enable a bird to get back on course even if blown beyond their known landmarks. In reality, latitude can be determined more readily than longitude.
Migratory behaviour and navigational ability have evolved differently across groups of birds. Also, some navigation strategies work better in one part of the world than another. For example magnetic cues may be weaker or more confused, and constant cloud cover or fog can cause difficulties for techniques that depend on the sun or stars.
Detecting magnetism
Experiments show that many migratory birds can detect magnetism, but how they do this is not certain. One possibility is that it involves the cells with iron-rich magnetite crystals found in the brains of many birds and other animals (including humans). Another interesting idea has come from a study of silvereyes. It suggests that they see the earth’s magnetic field as a faint colour gradient, which indicates the field’s strength and direction.
While we think of magnetism as being useful for giving a compass direction, some species also use the variation in the strength and inclination (angle of dip) of the magnetic field to build parts of their map.
Magnetic field: an imperfect aid
The earth’s magnetic field is not altogether constant or reliable, and its information changes through time. The position of the magnetic poles in relation to the geographic poles changes year by year, so a bird may need to adjust its bearings within its lifetime. The strength of the field changes over centuries, and in the last 150 years has become weaker.
In addition, every so often (about 10 times over the last three million years) the magnetic poles reverse. This is accompanied by a great disruption to the magnetic field, which lasts for hundreds of years. So any inherited information about using magnetism needs to have inbuilt flexibility. Birds apparently achieve this by comparing magnetic field information with cues from the sky.