On clusters and formations: birds flying in groups

Flocks of birds perform some aerial phenomena, which have amazed many of its viewers. This thesis is a literature review, which concerns the three major questions of ‘When’, ‘How’ and ‘Why’ birds fly in flocks. Flying flocks are roughly dividable in two types: ball shaped clusters and line formations. Whereas cluster flying is commonly used throughout the year by groups of birds, for example while transiting between their roosting and their foraging sites, formation flight is almost specifically used for migration. The departure time of migration is a critical moment in the annual cycle of the bird. The exact moment of departure is an optimization of several external and internal factors, including migrating distance, energy reserves and weather conditions. Clusters have a three-dimensional structure and tend to be rather disorganized. When a predator is nearby, clusters often start to perform rapid manoeuvres. Previously believed to be coordinated by telepathy or electromagnetic communication, due to high-speed filming scientists have determined that these manoeuvres are initiated by one individual bird and followed by the rest of the flock. The individual fulfilling the role of initiator changes from moment to moment. The most likely explanation for movement in a cluster is a self-organization mechanism where each bird applies a few behavioural rules in response to local information from neighbouring birds. There are different kinds of formations, ranging from an acute V-formation to a U-shaped bow. Although the total energy saving is the same for different formations, it differs for individual birds in the group. In an acute formation, the leader gains little or no energy savings, while in a U-formation the energy savings are more egalitarian. A reason for obtaining a specific formation may be related to kin-selection or reciprocity, although no empirical evidence for this hypothesis has been found yet. Several specific reasons have been put forward for birds to fly in clusters, mainly regarding predation prevention. These advantages include increased predator detection, a dilution effect, physical protection of neighbours and a confusion effect. Two hypotheses exist for a structured formation, like the well known V-shape. One is the aerodynamic advantage hypothesis, in which birds conserve energy by taking advantage of the upwash vortices created by the wings of the birds in front. The other is the communication and orientation hypothesis, in which birds make use of their position for their combined orientation experience and visual communication. These explanations are not mutually exclusive, and various studies have given support to both of them. However, no conclusive evidence has been found for any of the both hypotheses. There is still a debate going on between supporters and criticasters of both hypotheses which remains unsolved upon till now. A review of the literature concerning the subject of flock flying shows that the use (the ‘When’) and the organization (the ‘How’) of flock flying have been thoroughly researched, which has led to a consistent view on these subjects. However, thorough research on the main reasons for flock flying (the ‘Why’) has not given a clear view yet. Also behavioural aspects of the organization and departure are still shrouded. These subjects still raise enough questions to provide research options for years to come.