Span-Ratio Analysis Used to Estimate Effective Lift: Drag Ratio in the Double-Crested Cormorant Phalacrocorax Auritus From Field Observations

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Abstract
A method is described for measuring the effective lift:drag ratio of a bird in flapping flight from the bird's span ratio; that is, the ratio of the wing span in the upstroke to that in the downstroke. The method depends on the assumptions that the circulation on the wing is constant, and that the lift varies in proportion to the span. With some simplifications, it can be applied in the field to birds that cruise in steady, continuous flapping flight. Double-crested cormorants, observed on foraging nights, flew at a mean airspeed of 14.7 ms−1, with a span ratio of 0.70. The effective lift:drag ratio, taking account of mechanical components of power only, was estimated to be 14.8. A much lower estimate (8.96) was obtained from a power curve program (Pennycuick, 1989). The discrepancy could be due (at least partly) to bias in the observations, but it is also suggested that one or more of the default values, used in the program for calculating induced, profile and parasite power, may be in need of downward revision.