Biomechanical conflicts between adaptations for diving and aerial flight in estuarine birds

Diving and aerial flight place conflicting physiological constraints on diving birds depending on their typical dive depths. The amount by which air volumes in the respiratory system and plumage are reduced by hydrostatic pressure decreases rapidly with depth. Thus, birds diving shallowly, and ascending passively by means of positive buoyancy, content with greater work against buoyancy as well as more unstable buoyancy as they move vertically in the water column. The buoyancy of air far exceeds that of tissues or blood, whose buoyancy does not change appreciably with depth. Accordingly, experiments on ducks suggest that birds adapt to shallow diving by increasing blood volume and thus blood oxygen stores while decreasing respiratory volume. During dives, increased inertia from greater mass of blood and associated muscle lowers the costs of foraging at the bottom by resisting the upward buoyant force, but raises the costs of descent because of higher inertial work in accelerating the body with each stroke. Thus, average dive depth (compression of buoyant air spaces), stroke kinematics (inertial effects), and the relative time spent descending versus bottom foraging will determine the appropriate balance between buoyancy and inertia for diving. Greater blood volume also increases wing loading, so elements of dive costs must be balanced against flight costs in optimizing allocation of oxygen stores to blood versus the respiratory system. For example, biomechanical models for ducks suggest that increasing blood volume while decreasing respiratory volume lowers dive costs only for dives to depths <5 m or for dives with extended time at constant depth. If flight costs are also considered, these anti-buoyancy mechanisms reduce daily energy expenditure only if average dive depth is <2 m. High wing-loading in many foot-propelled divers is probably not an adaptation to diving but rather a result of modifications in wing size and shape for high flight-speed. These wing modifications appear possible because competing demands on wing morphology (maneuverability, takeoff ability) are relaxed in open aquatic environments.