Birds have built-in compasses – thanks to the presence of iron rich magnetite at various places in the head – and they make use of the earth’s magnetic field and can easily start out in the correct direction when beginning to migrate (called orientation or direction-finding). Thereafter, they make use of a number of different techniques to find and maintain their way to the end destination (called navigation or route-finding). The most common of these is use of the sun by day and stars and the moon by night (called astrocues). Other cues maybe prevailing winds, sounds (many birds can hear infrasound), smells (the nostril nerves are rich in magnetite and assist with orientation) and the shape of the land (mountain ranges, rivers, coastlines, etc). Experience of undertaking the migratory route is also beneficial as young birds learn where to go from older birds. Young birds learn how to deal with differing conditions and how to counter them. In other words, they know when to turn and how long to fly in one direction.
Birds spend the last few weeks prior to migration eating excessively (called hyperphagy), fattening themselves up for the trip by a process known as hyperlipogenesis. They spend more time each day feeding and become less active. This enables them to build up fat reserves that will be burned as fuel when travelling hundreds of kilometres without food or stops. The duration or number of rest stops varies considerably between species, resulting in the overall trip being long, although total flying time is only a few days.
Route selection is based upon food requirements, resting places, landmasses and prevailing winds. Big birds that prefer to soar (to save energy) will rely on thermals. These only occur over land, and these birds will therefore avoid big water masses, even if it means taking an indirect route. Those with large fat reserves can fly further in a single flight and will cover distances quicker than those with smaller fat reserves that need to stop frequently to acquire energy. Indirect routes may also be chosen to make use of energy-saving prevailing winds. Single flights of up to 6500 kilometres can be accomplished where birds are flying for up to 40 hours non-stop.
Excerpts from ‘Beat about the Bush – Mammals and Birds’ by Trevor Carnaby, Published by Jacana Media, Second Edition reprint 2018.
