Animal groups vary in size from two magpies sitting on a branch to plagues of millions of locusts crossing the desert. Not only do the sizes of groups vary between species, but they can change dramatically within species. In some cases, a change in group size depends on changes in the environment. For example, locust outbreaks are thought to originate where resources are patchily distributed, causing locusts to move towards these limited resources (Collett et al. 1998; Despland et al. 2004). In other cases, individuals in similar environments are found in very different-sized groups. Fishermen are used to such intrinsic variation in fish school size. Some days a net contains three fish, while the next day it contains tens of thousands (Bonabeau & Dagorn 1995). Human settlements also show similar variety in size, from tiny villages to massive cities, with differences in size arising without large differences in the environments in which they were originally founded (Reed 2001).
Can we then make general predictions about animal group sizes? In this chapter I approach the group size question from the two directions of functional and mechanistic explanation. The functional approach looks at how the costs and benefits of group membership can be used to calculate the optimal group size, at which individuals maximise their fitness, and the stable group size, at which no individual can improve its fitness by moving to another group. The mechanistic approach attempts to explain the large variation in group sizes observed empirically. By describing the mechanisms by which individuals join and leave groups a distribution of group sizes is predicted.
There are many ways an individual can benefit from being a member of a group. The movement of a water skater as a predator approaches both confuses the predator and alerts other skaters of its presence (Treherne & Foster 1981); the starling in a flock can invest less time scanning for