Kin selection and reciprocity in flight formation?

The reasons for conspicuous V and other flight formations in birds are debated. Theory and recent empirical advances show that energy saving is one important function of flight formations, but some aspects remain poorly understood. Combining theories of animal flight and sociality, we suggest that some of the variation in flight formations has its base in kin selection and reciprocation. The bird leading an acute V formation saves less energy than does the trailing participants. The disadvantage of leading is reduced in more obtuse formations, and when the longitudinal distance between neighbors is small, the leading bird can save about as much energy as others. Therefore, acute V formations are predicted to occur mainly in circumstances conducive to kin selection or reciprocity. These mechanisms seem possible, for example, in small flocks of adults with offspring, such as in swans, geese, and cranes. Inclusive fitness advantages may then favor an energetically expensive leader role for adults. In small groups, reciprocity is also possible among unrelated adults that recognize each other and take turns leading the V formation. In contrast, obtuse formations are expected in large flocks of unrelated individuals, such as spring flocks of waders migrating long distances. Possibilities for testing these ideas are discussed.