Migration can be energetically expensive, requiring birds to lay down significant fat deposits. Fat is a highly efficient fuel, delivering up to nine times more energy than other alternatives.
Oxidation – the process that delivers energy rom fat – also produces an equal weight of water, which is essential during flight.
There are costs to using fat as fuel. First, there’s the extra weight; second, its metabolism requires small amounts of protein to be broken down – at least five per cent of the energy released during a migratory flight comes from protein.
- How high do birds fly?
- How do birds fly? We explain how they get - and stay - airborne
- How do flocks of birds fly so close together without colliding? The mystery explained
Birds must balance the amount of fat deposited to ensure they have sufficient fuel to make the journey but aren’t carrying unnecessary reserves. Individuals that migrate over areas of favourable habitat make a series of short flights and carry small amounts of fat to fuel them; those that cross unfavourable habitat lay down larger reserves, sometimes almost doubling their bodyweight.