Predator versus prey: on aerial hunting and escape strategies in birds

Predator and prey attack-escape performance is likely to be the outcome of an evolutionary arms race. Predatory birds are typically larger than their prey, suggesting different flight performances. We analyze three idealized attack-escape situations between predatory and prey birds: climbing flight escape, horizontal speeding, and turning and escape by diving. Generally a smaller bird will outclimb a larger predator and hence outclimbing should be a common escape strategy. However, some predators such as the Eleonora's falcon (Falco elenorae) has a very high rate of climb for its size. Prey species with an equal or higher capacity to climb fast, such as the swift Apus apus, usually adopt climbing escape when attacked by Eleonora's falcons. To analyze the outcome of the turning gambit between predator and prey we use a Howland diagram, where the relative linear top speeds and minimum turning radii of prey and predator define the escape and danger zones. Applied to the Eleonora's falcon and some potential prey species, this analysis indicates that the falcon usually wins against the example prey species; that is, the prey will be captured. Level maneuvering hunting is the most common strategy seen in Eleonora's falcons. To avoid capture via use of this strategy by a predator, the prey should be able to initiate tight turns at high linear speed, which is facilitated by a low wing loading (weight per unit of wing area). High diving speed is favored by large size. If close enough to safe cover, a prey might still opt for a vertical dive to escape in spite of lower terminal diving speed than that of the predator. On the basis of aerodynamic considerations we discuss escape flight strategies in birds in relation to morphological adaptations.