Flocking in wintering Common Cranes Grus grus: influence of population size, food abundance and habitat patchiness

Abstract

We examined the relationships between flock structure and food availability, patch quality, spacing, and population size in wintering Common Cranes Grus grus. Using principal component analysis we simplify 10 flock variables to 4 major variation components which define respectively (1) dispersion pattern of the birds, (2) flock dispersal from the roost, (3) flock spacing pattern, and (4) feeding effort. We investigate the effects of number of birds, food abundance and spacing and quality of feeding patches on the principal components defined. The results indicate that an increase in patch quality correlates positively with flock size and density. A decrease in total food availability forces the birds to fly on average longer distances from the roost to forage. Independently, the same effect is observed in response to an increase in the number of birds. Flocks that fly to feed at longer distances from the roost select feeding patches of high quality, reduce the variance of their structural components, and increase their feeding intensity, probably to compensate for the long distance dispersal. We suggest that the higher unpredictability of the food location due to increased farming activity is the cause of the daily flocking enhancement in autumn and spring