A model of foraging efficiency and daily energy budget in the Black Skimmer (Rynchops nigra)

Abstract

A model of the energetics of foraging in the Black Skimmer (Rynchops nigra) is developed. The energy costs of flight and other activities are equated with the energy income from food. The model focuses on the influence of ground proximity during flight (ground effect) on the form of the velocity–power curves of the bird. Minimum power and maximum range speeds are reduced in ground effect but their relative values are unchanged. With increasing ground proximity, foraging efficiency (power input from food divided by flight power) for a given flight velocity increases, but maximal values of foraging efficiency occur at lower speeds. Peak values of foraging efficiency coincide with the position of the maximum range speed on the velocity–power curve. However, maximum values of the daily energy budget parameter (energy income from food divided by the energy cost of flight and of other activiites) occur at greater speeds. If the net daily energy surplus is to be maximized, the bird should fly at speeds greater than that for maximum range. Ground effect reduces the minimum food intake to ensure a nonnegative daily energy budget for any given foraging time. The foraging radius attainable on a given energy store is significantly increased in ground effect. Calculated foraging radii for a modest energy surplus exceed observed values. The energy budget of a breeding bird and the possible constraints on breeding are considered. It is concluded that the energy required to produce the eggs is not a major limitation. Model results are discussed in relation to observations of flight behaviour.