Bird Migration by Matthew Brennan
Book of Job (39:26) – Doth the hawk fly by Thy wisdom and stretch her wings toward the south?”
Early ornithologists noted changes in the birds based on the different seasons, references to migration date back to 3000 years ago when Aristotle postulated the phenomenon of birds disappearing and reappearing every year at the same time. He noted cranes travelled from the steppes of Scythia to the marshes at the headwaters of the Nile, and pelicans, geese, swans, rails, doves, and many other birds likewise passed to warmer regions to spend the winter. Aristotle can also lay claim to many of the superstitions that surround bird migration, as he concluded that birds hibernate during the cold months as well as what he termed transmutation, the theory of transmutation is the seasonal change of one species into another. Frequently one species would arrive from the north just as another species departed for more southerly latitudes. From this he reasoned the two different species were actually one and assumed different plumages to correspond to the summer and winter seasons.
We have come a long way from the days of Aristotle and research these days mainly focus on the way birds navigate their way around, they are able to fly in a particular constant direction, regardless of the position of the release point with respect to the bird’s home area. It has also been shown that birds are capable of relating the release point to their home area and of determining which direction to take, then maintaining that direction in flight. The navigational ability of birds has long been understood in terms of a presumed sensitivity to both the intensity and the direction of the Earth’s magnetic field. It has also been suggested that birds are sensitive to forces produced by the rotation of the Earth (Coriolis Effect); however, no sense organ or physiological process sensitive to such forces has yet been demonstrated to support this hypothesis.
Experiments have shown that the orientation of birds is based on celestial bearings. The Sun is the point of orientation during the day, and birds are able to compensate for the movement of the Sun throughout the day. A so-called internal clock mechanism in birds involves the ability to gauge the angle of the Sun above the horizon. Similar mechanisms are known in many animals and are closely related to the rhythm of daylight, or photoperiodism. When the internal rhythm of birds is disturbed by subjecting them first to several days of irregular light/dark sequences, then to an artificial rhythm that is delayed or advanced in relation to the normal rhythm, corresponding anomalies occur in the homing behaviour. Two theories have been formulated to explain how birds use the Sun for orientation. Neither, however, has so far been substantiated with proof. One theory holds that birds find the right direction by determining the horizontal angle measured on the horizon from the Sun’s projection. They correct for the Sun’s movement by compensating for the changing angle and thus are able to maintain the same direction. According to this theory, the Sun is a compass that enables the birds to find and maintain their direction. This theory does not explain, however, the manner in which a bird, transported and released in an experimental situation, determines the relationship between the point at which it is released and its goal.
The second theory, proposed by British ornithologist G.V.T. Matthews, is based on other aspects of the Sun’s position, the most important of which is the arc of the Sun, for example the angle made by the plane through which the Sun is moving in relation to the horizontal. Each day in the Northern Hemisphere, the highest point reached by the Sun lies in the south, thus indicating direction; the highest point is reached at noon, thus indicating time. In its native area a bird is familiar with the characteristics of the Sun’s movement. Placed in different surroundings, the bird can project the curve of the Sun’s movement after watching only a small segment of its course. By measuring maximum altitude (the Sun’s angle in relation to the horizontal) and comparing it with circumstances in the usual habitat, the bird obtains a sense of latitude. Details of longitude are provided by the Sun’s position in relation to both the highest point and position it will reach as revealed by a precise internal clock.
Migrant birds that travel at night are also capable of directional orientation. Studies have shown that these birds use the stars to determine their bearings. In clear weather, captive migrants head immediately in the right direction using only the stars. They are even able to orient themselves correctly to the arrangement of night skies projected on the dome of a planetarium; true celestial navigation is involved because the birds determine their latitude and longitude by the position of the stars. In a planetarium in Germany, blackcaps and garden warblers, under an artificial autumn sky, headed southwest toward their normal direction; lesser whitethroats headed southeast, their normal direction of migration in that season.
It is known, then, that birds are able to navigate by two types of orientation. One, simple and directional, is compass orientation; the second, complex and directed to a point, is true navigation, or goal orientation. Both types apparently are based on celestial bearings, which provide a navigational grid.
The types of migrants here in South Africa are known as;
Palearctic migrants (species that migrate between Europe/Asia and southern Africa)
Intra ‐ African migrants (species that migrate within Africa)
Altitudinal migrants– Species that tend to follow rainfall patterns up the varying altitudes.
An example of one of our migratory species is the European Roller (Coracias garrulus) is the only member of the roller family of birds to breed in Europe. Its overall range extends into the Middle East and Central Asia and Morocco.
There are two subspecies: the nominate garrulus, which breeds from north Africa from Morocco east to Tunisia, southwest and south-central Europe and Asia Minor east through northwest Iran to southwest Siberia; and semenowi, which breeds in Iraq and Iran (except northwest) east to Kashmir and north to Turkmenistan, south Kazakhstan and northwest China (west Xinjiang). The European Roller is a long-distance migrant, wintering in southern Africa in two distinct regions, from Senegal east to Cameroon and from Ethiopia west to Congo and south to South Africa.
It is a bird of warm, dry, open country with scattered trees, preferring lowland open countryside with patches of oak Quercus forest, mature pine Pinus woodland with heathery clearings, orchards, mixed farmland, river valleys, and plains with scattered thorny or leafy trees. It winters primarily in dry wooded savanna and bushy plains, where it typically nests in tree holes.
The European Roller is a stocky bird, the size of a Jackdaw at 29–32 cm in length with a 52–58 cm wingspan; it is mainly blue with an orange-brown back. Rollers often perch prominently on trees, posts or overhead wires, like giant shrikes, whilst watching for the large insects, small reptiles, rodents and frogs that they eat.
The display of this bird is a lapwing-like display, with the twists and turns that give this species its English name. It nests in an unlined tree or cliff hole, and lays up to six eggs.
The European Roller (Coracias garrulus) is the only member of the roller family of birds to breed in Europe. Its overall range extends into the Middle East and Central Asia and Morocco.
The call is a harsh crow-like sound. It gives a raucous series of calls when nervous.
To be continued………..
The word migration comes from the Latin migratus that means “to change” and refers to how birds change their geographic locations seasonally.