Migration By Walking Or Swimming

While most birds migrate by flight, others migrate by walking or swimming. These include not only flightless birds, but also some birds which are able to fly, but in some circumstances opt to walk or swim, for at least part of their journey. For example, Prill (1931) described a pedestrian migration of American Coots Fulica americana in the Warner Valley of Oregon during May 1929. At least 10 000 individuals were seen walking northward over a period of four days. They did not swim or fly (unless alarmed), but followed the shore, 6-25 abreast. They may have been engaged in a moult migration, and some may not have flown because their flight feathers were loosened or already shed. In western North America, the Blue Grouse Dendragapus obscurus performs an altitudinal migration, moving several hundred metres up and down mountainsides between the breeding and non-breeding areas (Cade & Hoffman 1993). Although the bird can fly, the radio-tracking of individuals revealed that this journey is often undertaken mainly on foot, which is perhaps not surprising in a bird that spends most of its time walking. In flightless landbirds, such as ratites, all movement is inevitably by walking or running. Emus Dromaius novaehollandiae in central Australia have been found to cover hundreds of kilometres at times of drought, with a mean speed of 13.5 km per day (Marchant & Higgins 1990).

Most species of penguins perform regular migrations by swimming, some of more than 1000 km. Having lost the power of flight, penguins have developed flippers instead of wings, enabling them to 'fly under water'. They are well streamlined and can travel under water at high speeds, larger species faster than smaller ones. Large King Penguins Aptenodytes patagonicus were filmed in an aquarium swimming at 3.4 m per second (more than 12 km per hour). Although penguins can float and swim at the surface, they apparently undertake long sea journeys mainly under water, where the drag on the body is less than on the surface, but they have to come up frequently for air. As they approach the surface, the drag on the body increases, and it has been suggested that leaping clear of the water (porpoising) is energetically less costly than surfacing, although this may hold only at speeds over about 2.5 m per second (9 km per hour).

Some other seabirds that are normally able to fly, such as auks, may migrate entirely or partly by swimming, remaining in suitable habitat throughout. Young auks have been found to travel by paddling at 40 km per day (Gaston 1983). The extinct Great Auk Pinquinus impennis, which nested at high latitudes, was probably a long-distance swimming migrant, wintering in the western Atlantic as far south as Florida and in the east Atlantic south to Spain, probably involving journeys of more than 1500 km (Brown 1985).

0 0

Post a comment