Flight zone  The flight zone of an animal is the area surrounding an animal that if encroached upon by a potential predator or threat, including humans, will cause alarm and escape behavior. The flight zone is determined by the animal's flight distance, sometimes called flight initiation distance (FID) which extends horizontally from the animal and sometimes vertically. It may also be termed escape distance, alert distance, flush distance, and escape flight distance. Swiss zoologist Heini Hediger distinguished between "flight distance" (run boundary), "critical distance" (attack boundary), "personal distance" (distance separating members of non-contact species, as a pair of swans), and "social distance" (intraspecies communication distance). Flight distance can be used as a measure of the willingness of an animal to take risks. Escape theory predicts that the probability of fleeing and flight distance increase as predation risk increases and decrease as escape cost increases. Flight initiation distance is one measure of animals' fear responses to humans. In a study comparing 56 bird species with long flight distances, it was found these had declining populations in Europe. This indicates that standardized measures of flight distance can provide reliable information about the population consequences of risk-taking behaviour by individuals and the susceptibility of different species to increased levels of disturbance by humans. A further study analyzing 75 flight initiation distance studies of 212 species found that larger species are more tolerant of humans. When the flight zone of a group of bulls was invaded by a mechanical trolley, the bulls moved away and maintained a constant distance between themselves and the trolley. This indicates animals sometimes maintain a flight zone around inanimate objects. The flight initiation distance is being used as a tool in wildlife management. By studying flight zones, wildlife managers are able to reduce the impact of humans by creating buffer zones between human populations and animal habitats. The alert distance (AD) is the distance, by definition greater, within which the animal changes its behaviour in a manner enabling it to better observe the stimulus, as by raising the head in an alert posture, but does not necessarily flee unless the stimulus is also within the escape distance. These measures are usually used to quantify the tolerance of wildlife to humans. Animals faced with approaching predators must decide at which distance to initiate their flight, and they are expected to do so in a way that maximizes their fitness. As flight has both costs (including lost opportunity) and benefits, there will be in general an optimal flight initiation distance, defined as the first point in a predator's approach at which the benefit of flight exceeds the cost.