Hydrodynamic Drag of Diving Birds: Effects Of Body Size, Body Shape And Feathers At Steady Speeds

Abstract

For birds diving to depths where pressure has mostly reduced the buoyancy of air spaces, hydrodynamic drag is the main mechanical cost of steady swimming. Drag is strongly affected by body size and shape, so such differences among species should affect energy costs. Because flow around the body is complicated by the roughness and vibration of feathers, feathers must be considered in evaluating the effects of size and shape on drag. We investigated the effects of size, shape and feathers on the drag of avian divers ranging from wing-propelled auklets weighing 75 g to foot-propelled eiders weighing up to 2060 g. Laser scanning of body surfaces yielded digitized shapes that were averaged over several specimens per species and then used by a milling machine to cut foam models. These models were fitted with casts of the bill area, and their drag was compared with that of frozen specimens. Because of the roughness and vibration of the feathers, the drag of the frozen birds was 2–6 times that of the models. Plots of drag coefficient (C(D)) versus Reynolds number (Re) differed between the model and the frozen birds, with the pattern of difference varying with body shape. Thus, the drag of cast models or similar featherless shapes can differ both quantitatively and qualitatively from that of real birds. On the basis of a new towing method with no posts or stings that alter flow or angles of attack, the dimensionless C(D)/Re curves differed among a size gradient of five auklet species (75–100g) with similar shapes. Thus, extrapolation of C(D)/Re curves among related species must be performed with caution. At lower speeds, the C(D) at a given Re was generally higher for long-necked birds that swim with their neck extended (cormorants, grebes, some ducks) than for birds that swim with their head retracted (penguins, alcids), but this trend was reversed at high speeds. Because swimming birds actually travel at a range of instantaneous speeds during oscillatory strokes, species variations in drag at different speeds must be considered in the context of accelerational stroking.

The relationship between sternum variation and mode of locomotion in birds

“…We suggest this sternum shape allows for a dorsoventrally flatter body cross-section, which may result in a more hydrodynamic, streamlined profile [61][62][63]. Diving birds typically also have a number of other adaptations for streamlining, including smoother feathers, hindlimbs placed far back on the body, and a laterally compressed tarsus [61,64]. Therefore, we hypothesize that selective pressures on streamlining of diving bird body shape produce a convergence in sternum shape regardless of the use of the wings from underwater propulsion or not.…”

“…11A, B). We suggest this sternum shape allows for a dorsoventrally flatter body cross-section, which may result in a more hydrodynamic, streamlined profile [61][62][63]. Diving birds typically also have a number of other adaptations for streamlining, including smoother feathers, hindlimbs placed far back on the body, and a laterally compressed tarsus [61,64].…”

Fossil Evidence for Evolution of the Shape and Color of Penguin Feathers

“…Early modifications in the wing feathering seen in Inkayacu created a narrow streamlined flipper and changed the hydrodynamic properties of the body feathers. Broad flat contour feather rachises create less turbulent flow (12), an effect hypothesized to explain a lower observed drag coefficient than predicted from streamlined penguin body shape alone (14). They also facilitate reduction of intraplumage air (15).…”

Trends of avian locomotion in water – an overview of swimming styles

“…For example, in shallow water, both penguins and cormorants are positively buoyant, but in addition, this can help in ascending and supports the bird to reach the water surface (Ribak et al 2005, Kato et al 2006. Deep divers descend beyond the depth where buoyancy is minimal or neutral and can spend most of their energy searching and capturing prey (Lovvorn et al 2001), while shallow divers remain in the zone of maximal buoyancy, and must work against it during the entire dive sequence (Kato et al 2006).…”