Foraging behaviour of the Common Myna (Acridotheres tristis) in relation to vigilance and group size

Abstract

Foraging individuals allocate time to foraging behaviour or anti-predator vigilance, but rarely both at the same time. If we assume that foraging rate (that is, the number of foraging attempts per minute) decreases as a function of time spent vigilant, an individual can directly increase food intake by reducing time spent vigilant. Alternatively, it may increase food intake by increasing foraging effort, that is, making more foraging attempts in the time remaining after time devoted to anti-predator vigilance is excluded. I studied the foraging behaviour of the Common Myna (Acridotheres tristis) to determine foraging behaviour under different conditions of human disturbance (indirect predation risk), time of day and group size. The results showed that Mynas generally increased their food intake by increasing foraging effort, rather than decreasing vigilance behaviour. Although joining a group resulted in significantly lower levels of vigilance behaviour, it did not increase foraging rate or food intake. Despite being well adapted to the human environment, and having few natural predators, Mynas showed high levels of anti-predator vigilance, at least within the resource rich urban environment. These results are discussed in the light of possible management options to control this highly invasive bird.