though other explanations of their unusual life-cycle have been proposed. Alarm calls Animals that live in groups often give alarm calls that give warning of an attack. For example, vervet monkeys give different calls depending on the nature of the attack: for an eagle, a disyllabic cough; for a leopard or other cat, a loud bark; for a python or other snake, a "chutter". The monkeys hearing these calls respond defensively, but differently in each case: to the eagle call, they look up and run into cover; to the leopard call, they run up into the trees; to the snake call, they stand on two legs and look around for snakes, and on seeing the snake, they sometimes mob it. Similar calls are found in other species of monkey, while birds also give different calls that elicit different responses. Improved vigilance In the improved vigilance effect, groups are able to detect predators sooner than solitary individuals. For many predators, success depends on surprise. If the prey is alerted early in an attack, they have an improved chance of escape. For example, wood pigeon flocks are preyed upon by goshawks. Goshawks are less successful when attacking larger flocks of wood pigeons than they are when attacking smaller flocks. This is because the larger the flock size, the more likely it is that one bird will notice the hawk sooner and fly away. Once one pigeon flies off in alarm, the rest of the pigeons follow. Wild ostriches in Tsavo National Park in Kenya feed either alone or in groups of up to four birds. They are subject to predation by lions. As the ostrich group size increases, the frequency at which each individual raises its head to look for predators decreases. Because ostriches are able to run at speeds that exceed those of lions for great distances, lions try to attack an ostrich when its head is down. By grouping, the ostriches present the lions with greater difficulty in determining how long the ostriches' heads stay down. Thus, although individual vigilance decreases, the overall vigilance of the group increases. Predator confusion Individuals living in large groups may be safer from attack because the predator may be confused by the large group size. As the group moves, the predator has greater difficulty targeting an individual prey animal. The zebra has been suggested by the zoologist Martin Stevens and his colleagues as an