Individual and social learning of migration patterns in whooping cranes
Differences in migration strategies within a single population are well documented in a variety of bird species. In long-lived migratory species, migration patterns might change over time as individuals learn and optimize their route and site selection; furthermore, in social species that travel in groups, experienced individuals may transfer information to younger, less experienced birds. We analyzed data from a reintroduced population of whooping cranes to assess the relative importance of genetic, environmental, social, and individual factors in determining migration patterns. Whooping cranes are a long-lived social species of conservation concern; the Eastern Migratory Population (EMP) is a reintroduced population where young-of-year birds are trained to migrate by following an ultralight aircraft and each individual is monitored over its entire lifetime, yielding an extensive dataset of movement and demographic data. We examined the migratory performance, measured as the distance to a straight-line path between breeding and wintering ranges, and migration distance of the individuals in the EMP over time, as related to characteristics of individual birds, characteristics of flight groups, and environmental conditions.
We found that migratory strategies were influenced by both individual and group learning. Migratory performance was best predicted by the age of the oldest bird in a migratory group, where flight groups with older individuals flew closer to a straight-line path than those composed of younger individuals. First-year migrants travelling with older birds deviated 34% less from a straight-line patch than those travelling in single-age groups. We found a clear northward trend in the location of overwintering sites over time, with a reduction of the average migration distance from about 1800 km in 2002 to about 900 km in 2013. In contrast to other reports of changing migration strategies over time, we found no effect of temperature on migration distance; instead, the strongest predictor of migration distance was the age of birds using a site, where older birds migrated shorter distances than younger birds. Our results show that in social species, migration strategies are learned over time and are transmitted from older to younger individuals in a population. Training and assessment of reintroduced populations should take into account the potential changes in migration behavior over time that occur as a result of individual and group learning.