Group-size and distance-to-neighbour effects on feeding and vigilance in brown-headed cowbirds

Sleeping gulls monitor the vigilance behaviour of their neighbours

Individuals in groups are often thought to scan their surroundings for threats independently of one another. Models, however, suggest that foragers should monitor the vigilance level of their neighbours to prevent cheating, and to gather information about incipient predation risk. Evidence for monitoring of vigilance is scant. Here, I examined changes in vigilance levels in sleeping gulls (Larus sp.) surrounded by neighbours in various states of alertness. Controlling for group size and neighbour density, gulls interrupted sleep more often to scan their surroundings, and were therefore more vigilant, when their neighbours were alert rather than sleeping or preening. The results provide evidence for copying of vigilance within groups of birds, suggesting a complex flow of information about predation risk in groups.

Vigilance in a solitary marsupial, the common wombat (Vombatus ursinus)

We studied vigilance activity in a wild population of the common wombat (Vombatus ursinus), a large, solitary, burrow-using, marsupial prey species in which individuals tolerate the presence of conspecifics within their home range. For the first time, we report postures and rates of vigilance in common wombats; our results show a limited repertoire of vigilant postures and low overall rates of vigilance. Because few studies of birds and mammals that have reported the effect of distance to conspecifics on the vigilance of focal animals have considered solitary prey species, we tested this effect in wombats. Our results show that a model including distance to cover and distance to the nearest conspecific, but not time of day, best explained the variation in the proportion of time that focal individuals spent in vigilance. Individual vigilance decreased when distance to cover increased. Vigilance of wombats increased when there was a conspecific within a radius of 70 m of the focal individual. In addition, we tested whether pairs of nearby wombats scanned independently of one another, coordinated their activity in non-overlapping bouts of vigilance or synchronised their bouts of vigilance. Wombats in close proximity exhibited independent bouts of individual vigilant and foraging activity. Thus, in this solitary species, our results support the assumption that individuals scan independently of each other.