Flight energetics, Marbled Murrelet

Abstract:

We measured flight speeds (n = 3000) of Marbled Murrelet, Brachyramphus marmoratus (J.F. Gmelin, 1789), to determine whether flight speeds of an exceptionably fast bird coincide with the maximum-range speeds (V-mr) predicted by aerodynamic theory. The mean (+/-SE) speed of 22.6 +/- 0.21 m.s(-1) was significantly higher than the V-mr predicted by four models, using conventional values for the parasite drag coefficient (C-Dpar). In order for the Penny cuick model to predict a V-mr of 22 m.s(-1), a C-Dpar of 0.05, which is lower than any previously reported, is necessary; the other models would need to assume even lower values for C-Dpar. We concluded that the cruising speed of Marbled Murrelets exceeds V-mr. Marbled Murrelets may exceed V-mr as a result of behavioural decisions, and we examined two behavioural hypotheses: that flight speeds exceed V-mr to (1) minimize predation rate and (2) maximize chick growth rate. However, there was no significant difference between flight speeds during high (daylight) and low (darkness) predation periods or between chick-rearing and non-breeding periods. Marbled Murrelets may also appear to fly at a speed that exceeds V-mr because the underlying aerodynamic theory is inaccurate for this species. To examine the reliability of aerodynamic theory for Marble Murrelets, we compared measured wingbeat frequencies (f) to those predicted by Pennycuick's model. The mean f was significantly lower than the f(ref) predicted by Pennycuick's model, and generally, f = 7.9m(-0.22) is a better model for auks than Pennycuick's model. In addition, the Strouhal number was particularly low (0.12 +/- 0.02). We conclude that the current aerodynamic models are insufficient for an exceptionally fast-flying bird.