Early in the morning I was apprised by my servant that an extraor-dinary flock of birds was passing over, such as he had never seen before. Hurrying out and ascending the grassy ramparts, I was perfectly amazed to behold the air filled and the sun obscured by millions of pigeons, not hovering about, but darting onwards in a straight line with arrowy flight, in vast mass a mile or more in breadth, and stretching before and behind as far as the eye could reach. (Major King, Ontario, late nineteenth century, writing of the migration of the Passenger Pigeon.)
No one can fail to be impressed by the migrations undertaken by birds. Some journeys extend over distances of more than 10 000 km, and may involve the crossing of seas and other inhospitable areas. Such migrations require not only extraordinary navigational skills, but massive body reserves to fuel the flights, coupled with sustained non-stop effort for tens of hours at a time. Migration differs from ordinary day-to-day flight, not only in the much greater length of journey, but in the greater altitude at which it usually occurs, with most small birds flying at heights well beyond the range of human vision. Once underway, therefore, birds are usually exposed to a cooler and thinner atmosphere, with reduced buoyancy and oxygen levels. While en route, migrants must stop and feed to replenish depleted body reserves, often in unfamiliar places; they must respond appropriately to prevailing weather, and correct for any off-course drift. Moreover, areas of favourable habitat, where a migrant can feed rapidly and safely, may be limited and widely spaced. Little wonder that mortality rates are often high at migration times (Chapter 28).
Astonishingly, some species, such as grebes, rails and gallinules, may have hardly flown for months before they set off on migration, having moved around mainly by walking or swimming. Yet at the appropriate time, and having accumulated the necessary fuel reserves, they suddenly ascend into the night sky and fly for hundreds of kilometres non-stop. In Barnacle Geese Branta leucopsis, tele-metrically measured heartbeats revealed that in the weeks before autumn departure the birds flew for no more than a few minutes per day. Yet on migration, they flew non-stop for up to 13 hours at a time, with only occasional breaks in their 2500-3000 km journeys (Butler et al. 2000). Evidently, the amount of practice needed by such birds is minimal, compared with that needed by a human athlete to perform for much shorter periods.
One of the main advantages of flight is its speed. Whether by flapping or gliding, flight is fast compared with walking, running or swimming. It thereby facilitates long-distance travel, and allows migration to be accommodated as a twice-yearly event within the annual cycle. Nevertheless, the lengths and types of journeys that birds can undertake are greatly influenced by the body size, wing shape, flight powers and other features of the bird. These various features constrain the speed and mode of flight and the amount of fuel that can be carried as body reserves. This chapter is concerned with: (1) the relationships between body weight and fuel reserves, flight speed and duration; (2) the type of flight, whether mainly by flapping or soaring-gliding; and (3) migration by walking or swimming. These different aspects involve costs and benefits, and the interest is in finding where the balance is drawn in different species according to their sizes and shapes, and the particular conditions and constraints under which they operate.
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