Preparing for migration
Before setting off on their long journey south in the autumn, the many summer birds that visit the UK must prepare themselves.
Fattening for flight
The process is not a conscious one - it is controlled by hormones which subtly alter the metabolism of the birds, enabling them to store the energy they need for the long flight and build up the muscles they need to undertake the rigorous journey.
The amount of energy reserves needed differs between species depending on how far they fly in one go. Some birds will stop off on the way to feed while others will undertake the whole journey in a single non-stop flight. Birds like swifts and swallows store very little fuel as they can feed while they fly.
Most energy is stored in the form of fat, which is built up before the journey and used en route, with refuelling at stopping off places if needed.
Fat is an ideal way for birds to store fuel and one gram of it produces about nine kilocalories of energy. This is better than carbohydrate, which not only provides a fraction of the energy, but is also less easy to store as it weighs more due to its associated water molecules.
Weight poses a problem for migrating birds as the heavier a bird is, the more energy it uses in flight, and therefore the more fat must be stored for the journey.
Too fat to fly?
Many warblers will weigh twice their normal weight when they begin their migration and will need the best possible departure conditions when setting off. Fatter birds may also be more vulnerable to predators (and be a better meal for them!) as they cannot manoeuvre so easily.
Most small songbird migrants are insect-eaters and switch their diet to fruits in autumn. They gorge themselves deliberately by increasing their feeding rates and the hormone-induced changes in their metabolism mean they are able to utilise the food more efficiently, converting the sugars into fatty acids in the liver and then storing these as fats.
Those songbirds which feed on fruits will find plenty available at stop-over points as they progress south in the autumn, where they ripen earlier. There is also evidence that some Mediterranean fruits chosen by migrants have a high concentration of fatty compounds, meaning they can refuel quickly.
Some warblers will actually feed on nectar, rather than insects when preparing to migrate and others feed especially on aphids, which have a higher sugar content than many invertebrates. Research has shown that birds will select fruits that have a high concentration of unsaturated fatty acids as these are best for being stored as fat.
Those migrants which require stop-overs often use traditional sites which are essential to them. For waders it will be estuaries, and for warblers it may be areas with suitable habitat rich in insects, berries and fruit. The rate of fat storage (which can add more than 10 per cent of body weight per day) will be less than the daily loss during active flight, and so several days may be needed to refuel, depending on the type of food available.