Vigilance and neighbour distance in foraging flocks of red-billed choughs, Pyrrhocorax pyrrhocorax

Foraging flocks of red-billed choughs (Pyrrhocorax pyrrhocorax) were studied in the Gran Sasso National Park (Apennines, Central Italy) to assess the relevance of neighbour distance and group size upon vigilance, after controlling for other confounding variables (feeding frequency, air temperature and time of the day). Both partial correlation and multiple regression analyses clearly suggested that neighbour distance was the major determinant of the vigilant behaviour in the red-billed chough, even though group size would also exert a certain influence. As no significant correlation between the two variables was found, it can be assumed that the relationship between vigilance and neighbour distance is not spurious. Feeding frequencies were not significantly correlated with the other variables, except for temperature, for which a positive correlation was observed. Since the red-billed chough usually feeds on insects, which become more active and available as temperature increases, we suggest that the feeding frequency of the red-billed chough mostly depends on food availability. Our results are discussed in the framework of the hypotheses put forward to explain vigilance behaviour. The conclusion is that the 'individual risk hypothesis' explains the behaviour of the red-billed chough better than the 'many eyes hypothesis'.