If birds are to survive their flight and arrive at their destinations, they need to know when to go, and to be able to manage their energy needs. Preparation is crucial as journeys are long. For instance, the bar-tailed godwit flies 11,000 kilometres from Alaska to New Zealand, while the Arctic tern flies more than 17,000 kilometres.
Birds need a trigger to prepare for migration. Changing day length between the seasons provides this by setting off hormonal activity that influences birds’ physiology and behaviour. Experiments with birds held in outdoor cages showed that at the appropriate time to migrate they became agitated, slept less and started facing the direction of their migratory route. This behaviour is known as zugunruhe (from German), or migratory restlessness.
Energy for the flight
Migrating to or from New Zealand means long flights over the ocean. While seabirds may be able to feed en route, other birds depend on their ability to stockpile and conserve energy. Before departure and at stopovers, they go into a feeding frenzy, putting on weight at a great rate.
The bar-tailed godwit puts on 60%–70% of its weight in preparation for migration. By the time to leave, 55% of their body mass is fat. This enables them to fly non-stop over the Pacific Ocean from Alaska to New Zealand, whereas most other Arctic migrants follow the East Asian–Australasian Flyway along the western Pacific edge, where they can make stopovers to feed.
Fat is light and yields about eight times as much energy per gram as protein (muscle). As weight is a big cost in flight, carrying fat rather than protein or carbohydrate for energy needs is an advantage. Many birds also grow bigger flight muscles with the help of natural steroids, so they have extra power for the demanding flight. Some weight is also saved by shrinkage of organs not needed on the flight, such as the gut, liver and kidneys.
When Arctic migrants return from New Zealand to their breeding grounds, they need to have enough reserves for the start of breeding, as food supplies in the high Arctic spring are unreliable. That is why even the bar-tailed godwits take the long route north, stopping to feed at estuaries on the coast of the Yellow Sea.
Survival of the fittest
Not all birds have equal access to food during stopovers. There is great competition when there is not enough to go around, with larger or more dominant birds making it difficult for inexperienced or weaker birds to get their fill. The stakes are high, as birds that run out of energy during a crossing will drop out of the sky.
Birds have developed a variety of ways to reduce the amount of energy they need for flight. Some species fly in formation, which reduces expended energy by around 20%. Non-gliders can increase their speed by flying at altitudes of around 1,500 metres, in air that is less dense than at sea level. Many seabirds glide and use wind and wave pressure for ‘dynamic soaring’. This needs much less energy than flapping their wings. Soaring on rising thermals is possible for birds crossing hot continents, and many birds also make good use of tail winds to reduce the amount of energy and time.