Understanding how information spreads in animal groups is key to understand both the mechanisms underlying grouping patterns and the ultimate reasons of animal group living. Vultures offer a spectacular instance of group foraging - vultures may spend several days without feeding while searching alone or in small groups for carcasses along huge areas. Carcasses can lay untouched several hours to days without being detected. However, once discovered, carcasses are often quickly crowded of vultures feeding at the same time, clearly suggesting some sort of social information transfer. It has been hypothesized in a recent theoretical model that vultures follow other vultures who show directed flight, which is taken as a signal that the other individual has discovered food. Here, we suggest an alternative: vultures become attracted only by the presence of other flying vultures already above the carcass because of their characteristic cycling and descending flight with the legs down. There is also the hypothesis that vultures follow other individuals only within but not between groups. During three summers we studied a population of ca. 10,500 griffons vultures (Gyps fulvus) living in an area of ca. 10,000 Km2 in the Ebro Valley, northern Spain. We gathered data on vulture density and group size both during detailed monitoring of 25 carcasses from its availability in the field until their complete consumption by vultures, and by recording the presence and group size of vultures flying above 141 randomly distributed observation points. We also built an individual-based model where we simulated the behavior of vultures according to the three hypotheses outlined above.
Results/Conclusions
The model shows that these hypotheses predict different dynamics of vulture arrival and group sizes to the carcasses. Moreover, we parameterized the model with the real density of vultures in our study area, creating quantitative predictions for the hypotheses in terms of latent time from the occurrence of a carcass and the arrival of the first vulture, the total number of vultures arriving to the carcass and the shape of the dynamics of vulture arrivals to the carcasses. Comparing these model outputs with real data we show that vultures not only search in groups but also that there is transfer of information between vulture foraging groups, and we also hypothesize how vultures gather this information between groups.
