Birds and other migrating species may be as dependent on social networks as we are, according to a study released by PNAS on Monday. By studying migration simulations of everything from bison to bacteria, a team of researchers found that very few individuals in a group migrate because of environmental cues. Most are just following the leader, which may be the reason many species eventually stop migrating as their habitable areas become more distant and fragmented.

Modeling the migration process of a few different species suggested to the authors that an impulse to migrate can be motivated by two things. One is termed a "gradient," and consists of whatever factors we normally think spurs a flock or herd of animals to migrate: a drop or rise in temperature, diminishing of food sources, and so on.

The second motivation was much larger, and unexpected by the authors. It seemed many organisms had little or no sense of the first type of motivator, the gradients. Instead, they relied on social interaction to tell them when wanderlust was the prevailing sentiment in the herd or flock. They deferred to the few members of the group that were sensitive to gradients, and followed the ad-hoc leaders to their new habitat.

While humans are intimately familiar with the follower-leader relationship, the phenomenon may be relatively new in migratory animal behavior. The authors think the mix of social and gradient factors may explain the issue of habitat fragmentation, where animals that have to travel increasing distances eventually stop migrating altogether.

The problem may be that the group leaders, who are a small minority in migratory groups, are eventually unable to rise to the occasion. They no longer detect gradients and are loath to risk leading their peers to increasingly distant areas. With no leaders to latch on to, the group becomes locked down to one area.

Loss of migratory ability is often long-lived or permanent, and can change the evolutionary path of many organisms. Considering both this social reliance on leaders with increased habitat destruction and habitat fragmentation, the authors expect declines in migratory species.

PNAS, 2010. DOI: 10.1073/pnas.1006874107 (About DOIs).