A flock is a gathering of individual birds to forage or travel collectively. Avian flocks are typically associated with migration. Flocking also offers foraging benefits and protection from predators, although flocking can have costs for individual members. Flocks are often defined as groups consisting of individuals from the same species. However, mixed flocks consisting of two or more species are also common. Avian species that tend to flock together are typically similar in taxonomy and share morphological characteristics such as size and shape. Mixed flocks offer increased protection against predators, which is particularly important in closed habitats such as forests where early warning calls play a vital importance in the early recognition of danger. The result is the formation of many mixed-species feeding flocks. Mixed flocks While mixed flocks are typically thought to be composed of two different species, it is specifically the two different behaviours of the species that compose a mixed flock. Within a mixed flock there can be two different behavioural characteristics: sally and gleaner. Sallies are individuals that act as guards of the flock and consume prey in the air during flight. On the other hand, gleaners are those that consume prey living within vegetation. Studies have shown that as resources in the aerial environment increase, the flock will possess more sallies than gleaners. This has been shown to occur during forest fires in which insects have been flushed from vegetation, however this can also be done by the gleaners. When gleaners obtain meals from vegetation it causes the other prey within the vegetation to be flushed out into the aerial environment. It is through this specific behaviour of feeding among vegetation that the gleaners indirectly increase the foraging rate of the sallies. Those birds that are more rare and therefore less abundant in an environment are more likely to perform in this mixed flock behaviour. Despite the fact that this bird is more likely to be a subordinate, its ability to obtain food increases substantially. As well this bird is now less likely to be attacked by a predator because predators have a lower success rate when attacking large flocks. Safety from predation The ability to avoid predation is one of the most important skills necessary in order to increase one's fitness. It can be seen that by ground squirrels living in colonies, the ability to recognize a predator is rapid. The squirrel is then