Waking and roosting behaviour of the Indian Myna

Abstract

Waking and roosting of Acridotheres tristis at communal roosts were studied during two years in Auckland. Mynas sleep communally in trees all the year, the only exceptions being incubating and brooding females. Birds use some roosts during late spring and winter only and some roosts throughout the year. The temporary roosts are formed just before reproduction and, when they are formed, numbers of birds visiting year-round roosts drop sharply; they are gradually deserted during the summer long before the end of breeding. At year-round roosts, the times of assembly and of departure show trends relative to civil twilight and are latest and earliest respectively when breeding is at a peak. The intensities of light at which birds arrive and depart are the lowest at this time. These trends are probably because reproductive hormones affect the birds' sensitivity to physical cues or cause an increased attraction to the territory and nest. Throughout the breeding season changes in the status of arriving birds (singles, pairs and groups) correlate with the changing numbers of breeding pairs.

Light is probably the only physical factor that could provide consistent cues to times of waking and roosting. Other factors, such as temperature and humidity, may have an effect, especially on the periods of assembly and departure, An endogenous clock, set by light and possibly a social Zeitgeber, allows early departures, especially important to breeding individuals, it prevents a waste of time and energy during the day that might come from premature roosting.

The time available to breeding birds for feeding and for collecting food for nestlings is increased by leaving early from, and arriving late at, roosts. An early defence of the territory may also advance the time of departure. Vocalizations at roosts do not advertise them but are more proximate socially, as when these calls are given during the day. Gatherings before roosting provide protection from predation, which may be greater in the evening, but do not always prevent foraging. Communal roosting protects Mynas more from predators than if they slept solitarily. Other advantages, such as maintaining the unity of juvenile flocks and an increased efficiency in finding food, come from sleeping communally, but at any one time most birds in Auckland do not exploit these additional benefits.