Vigilance, in the field of behavioural ecology, refers to an animal's examination of its surroundings in order to heighten awareness of predator presence. Vigilance is an important behaviour during foraging as animals must often venture away from the safety of shelter to find food. However being vigilant comes at the expense of time spent feeding so there is a trade-off between the two. The length of time animals devote to vigilance is dependent on many factors including predation risk and hunger. Vigilance is often observed in animals that forage in groups, such as yellow-eyed juncos (Junco phaeonutus) and meerkats (Suricata suricatta). Foraging in groups dilutes an individual's risk of predation, and allows them to reduce their own vigilance while the vigilance of the group is maintained. Alarm signals may be used to alert the group to the presence of predators. Groups of some species have at least one individual on sentry duty who looks out for predators on a perch while the rest of the group forages. Feeding and predation risk trade-off Vigilance and feeding (both searching for and handling food) are generally mutually exclusive activities, leading to foragers facing a trade-off between energy intake and safety from predation. As time allocated to scanning reduces the time spent feeding, vigilant individuals must devote more time on foraging to obtain the required food intake. This impedes on other activities in their time budget such as mating and prolongs their exposure to predation as foraging occurs away from shelter. When foraging time is limited, vigilant animals are left with a reduced energy intake. Optimality models can be used to predict foraging decisions of an animal based on costs (predation risk, starvation) and benefits (safety, food), which are also affected by physiology such as hunger levels. Grey squirrels (Sciurus carolinensis) alter their behaviour according to the relative costs and benefits when foraging in the open. Small food items are consumed immediately to maximise energy intake, as they require little handling time so the cost of predation risk is low. Large items that require a long handling time, and hence time exposed to predators, are carried back to the safety of a tree to minimise predation risk. Although there is an energetic cost to transporting food, large food items have a high contribution to nutrient intake so the cost outweighs the benefits. The overall predation risk is a function of the abundance, activity and