Fat is primarily used to fuel the flight engine, the breast muscle. Migrants can store huge amounts of fat to help ensure they can keep going, keep flying across deserts or oceans safely, where food may not be available. Small, non-migrant songbirds generally have fat loads of 3-5% of their body mass. Before departure, long-distance migrants regularly increase their fat loads ten times, to 30-50% of their body mass! To compensate for this increase in weight, birds may lose other less-essential components, such as body water, protein and carbohydrate content.

In general, the longer the migration journey, the fatter a bird will become beforehand. However, the optimum amount of fat lies in a delicate balance, as carrying too much fat will hinder flying and may make birds more susceptible to predators. Large birds have another problem: if their body weight reaches over 20kg, they become too heavy to use flapping flight at all! So large birds, hindered with additional fat deposits are more or less forced to use gliding flight, which uses only 15-30% of the energy required for flapping flight. This applies to both raptors and storks2, and explains why vast numbers of such birds congregate at the shortest crossings of the Mediterranean, as there are no thermals to support their gliding flight over the sea (see ‘when and where’).

2) Taking the right fuel

Ordinarily a bird will use carbohydrate as its principle source of energy. During migration, fat becomes the main fuel type as it is the most efficient energy store, - with one gram of fat producing more energy than one gram of carbohydrate. A different chain of chemical reactions is required to break down fats and release energy. So a bird’s whole metabolism switches during migration.

Just to give an idea of how costly migration can be in terms of energy, Bar-tailed Godwits use 48% of the energy they need for a whole year on their winter migration to Africa. Those wintering in Europe use only 22% of their annual energy consumption for the flight.

3) Behavioural preparation

These changes in a bird’s internal workings also produce outward changes in behaviour. Some birds change their whole behavioural routine prior to departure: birds normally active in the daylight hours start to show nocturnal activity in preparation for night migration. Birds usually solitary become social: swallows start to flock more than before; finches, pipits, geese and cranes, even flock together. See section B. iii for possible advantages of such behaviour.