Flock size measures of migrating Lesser White-fronted Geese Anser erythropus

摘要

Migrating in flocks has various benefits but also incurs costs, which depend on the conditions individuals experience in flocks of various sizes. Here we quantified such conditions by using recently developed methods to measure crowdedness in animal groups using data on flock sizes of the endangered Fennoscandian population of the globally threatened Lesser White-fronted Goose Anser erythropus . Data were gathered on migration in Hortobágy National Park (E Hungary) between 1994 and 2006. Data on 548 sightings of the species suggested that the distribution of flock sizes was skewed and that flock sizes were significantly larger in the autumn than in the spring. Despite this difference, mean crowding values, i.e., the group size in which an average individual occurs, were remarkably similar in the spring and in the autumn. These results show that even though flock size varied seasonally, the social environment experienced by the average individuals was surprisingly stable across seasons. Such stability can be important in the consistency of the benefits and costs of migrating in flocks. Our results also draw attention to the importance of quantifying crowdedness in animal groups as these measures can provide information different from those provided by looking at mean group size alone.