Many birds migrate long distances to different locations at different times of the year in search of habitats with abundant food and favourable breeding conditions. One-fifth of the world's known bird species migrate long distances. The longest single migration distance is 13,000 kilometres, which is a Guinness World Record, and one that truly exemplifies a magnificent bird marathon. Undoubtedly, the rigours of long-distance migration place a significant strain on birds' physical endurance. Hence, migratory birds prepare extensively to successfully complete this demanding and long journey.
Migratory birds adjust their diets before long migrations to conserve enough energy for their lengthy flights, much like professional marathon runners who eat high-calorie foods before a race. They go on weeks of "hyperphagia", where they eat tonnes of food to increase their body's fat reserves, often quickly doubling their weight during this time! They even engage in self-digestion by absorbing some of their own tissues, shrinking their internal organs to make room for fat storage. This remarkable phenomenon is known as "autophagy".
Most migratory birds make brief stops along their journey at certain locations, which serve as midway refuelling stations allowing them some respite. This is remarkably similar to how there are several refuelling stations along a marathon route where athletes can replenish their energy. Hong Kong, located on the East Asia - Australasia Flyway, is an important stopover for migratory birds in the East Asian region. As a result, this provides an opportunity to observe a variety of valuable passage migrants, including the endangered Yellow-breasted Bunting, also known here as the "rice bird". According to research, the length of stay of migratory birds at a stopover site depends on the availability of food sources nearby. If there is not enough food at these stops, they may be forced to linger longer, delaying their journey, and even risk death because they cannot refuel sufficiently to recover their strength. Therefore, to conserve bird species, proactive measures must be taken to protect migratory bird habitats at these vital stopover sites.
Help from Air Currents
Relying on wingbeat alone for sustained flight depletes energy quickly. Some migratory birds take advantage of air currents to significantly reduce their wingbeat frequency. When the sun heats the earth's surface, the warm air near the ground creates rising thermal updraughts. Migratory birds catch these updraughts, ascend with the flow, and then extend their wings and propel forward in a soaring-gliding flight, almost without flapping their wings. Even though migration by air currents is more passive and time-consuming, energy consumption is only a quarter as compared to flapping flight, making it one of the most energy-efficient modes of flight.
Every winter, tens of thousands of Common Cormorants migrate to Hong Kong to over-winter. Careful observation of these flocks of birds reveals that they often fly together in large formations, forming a characteristic "V-shape". This type of flight formation is often observed in larger migratory birds, so by doing so, they selflessly share the aerodynamic lift generated during flight with their companions. In fact, when birds flap their wings, the difference in pressure between the upper and lower surfaces of their wings creates an uplift that enables the birds to maintain their flight. This aerodynamic force flows outward along the wing tips, and the birds immediately following in formation can freely benefit from this additional propulsion without having to exert extra effort. Scientists suggest that flocks can save up to half their energy by flying in formation, proving that unity really does mean strength!