PREDICTABILITY AND CHAOS IN BIRD VIGILANT BEHAVIOR

While foraging, many animals alternate between feeding and scanning. S pectral analyses of sequences of successive scan and inter-scan durati ons suggest the existence of a significant non-random, periodic compon ent in the scanning dynamics of some birds. Evidence for cyclic vigila nce remains controversial, however. Here data obtained from a purple s andpiper, Calidris maritima, two Barbary doves, Streptopelia risoria, and two choughs, Pyrrhocorax pyrrhocorax were re-analysed by making us e of statistical methods from non-linear dynamics. Predictability port raits obtained by subjecting the data to non-linear forecasting suppor t the view that the vigilant behaviour of the two choughs is periodic with superimposed noise. By contrast, the hypothesis of periodic scann ing is rejected for the sandpiper and doves, as well as that of pure r andomness. Instead, the vigilant behaviour of the sandpiper and doves bears the signature of deterministic chaos: high, short-term, decaying predictability of scan and inter-scan durations. The sequential struc ture of the data sets makes them well suited for reliable computation of the rate at which predictability declines; results support the conc lusion of chaotic patterns. Finally, a mathematical model is developed to investigate some possible functional benefits of periodicity and c haos in vigilant behaviour, compared with random scanning, in terms of the optimization of corporate vigilance of birds foraging in flocks. If individual vigilance is chaotic, then even loose coordination, base d on predictions only one foraging bout ahead, can dramatically reduce individual predictability and enhance the level of group surveillance . (C) 1996 The Association for the Study of Animal