b. Managing the energy budget

Once the birds have set off, there are several things they can to save energy en route and ensure their fat reserves don’t fall critically low.

i. Stop-overs

Not all birds pack their bodies with enough fat to get them to their destination. Heavy fat loads are costly to carry. So whilst some birds (waders, for instance) migrate in a single non-stop flight of up to 7,500km, others divide their journey up into smaller distances with stop-overs for resting and feeding. Wetlands, with their rich vegetation and buzzing insect communities, are often favoured stop-over habitats. These places are key parts of the migration journey, and birds may return to the same sites year after year.

Stop-overs at suitable ‘service-station habitat’ may only last a few days, but the total stop-over time can dominate the journey: for instance the marsh warbler spends only 13% of it’s 4 month migration on the wing.

ii. Group savings

The v-formations (skeins) of geese, so characteristic of our temperate spring and autumn skies, serve an important purpose. These feathered formations are an energy-saving device used for flapping flight. Flying behind another bird in this way can save about 20% of the energy used when flying alone. There may be other benefits of flying with company, for instance raptors have a better chance of finding thermals if they migrate together.

iii. Night-riders

Short-distance migrants generally migrate by day. Most long-distance migrants however, migrate at night, even if they are normally active during the day. This includes almost all insect-eating passerines and waders, ducks and geese, cuckoo, quail and wryneck. There are several theories to explain this, which may be important for different species²:

1) Predator avoidance: most soaring birds, including raptors, migrate by day in order to utilise thermals. However, such predators are unlikely to feed on migrating birds, with the exception of Eleonora's falcon, which breeds around the Mediterranean specifically to take advantage of the flux of migrants.
2) Time gain: flying at night frees up more time for feeding during the day, which may minimise the overall journey time.
3) Energy saving: it may be easier to fly at night. At night the air becomes cooler and more dense, which causes a drop in wind-speed and turbulence, and the wind direction is less variable. It may be more energetically efficient to fly in these conditions.
4) Increased safety: birds are less likely to suffer from hypothermia or dehydration if they fly at night when the temperature is lower and humidity higher.