Control mechanisms

Yea, the stork in the heavens knoweth her appointed time; and the Turtle (Dove), and the Crane, and the Swallow observe the time of their coming. (Jeremiah (viii.7).)

This chapter is concerned with the control of migration - with the factors that stimulate migration at appropriate times of year and influence the preferred directions. It is concerned with the external factors to which birds respond, such as daylength and food supply, and also with the internal regulating mechanisms. It thus attempts to integrate the findings from both field and laboratory studies, and develops some aspects of this subject area already touched upon in Chapter 11.

The behavioural changes necessary for migration involve an urge to depart given suitable weather, and a tendency to fly in one particular direction rather than others. In addition, many normally diurnal birds also become active and migrate at night. The symptoms of this 'migratory state' are easily noticed in captive birds which at appropriate times of year develop 'migratory restlessness', when they hop and flutter round their cages and show long periods of wing-whirring (fluttering the wings rapidly while perched). Some species, such as White-crowned Sparrow Zonotrichia leucophrys, also spend long periods pointing the bill skywards, and fluttering upwards, which in a netting-topped cage provides a broad view of the night sky (Ramenofsky et al. 2003).

Because most birds cannot forage while flying, the chief physiological change necessary for migration involves the accumulation of fat and other body reserves to sustain the bird and fuel its flight (Chapter 5). The symptoms of this state include increases in the food intake and weight of the bird, and the appearance of a yellow colour (due to fat) beneath the skin. Fat also accumulates at other sites around the body, mainly in the tracheal pit (at the base of the neck) and among the viscera (Berthold 1996). Once adequate fat has accumulated, migration (or migratory restlessness) usually follows but, in the wild, adverse weather might delay the date of departure (Chapter 4).

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