Costs of diving by wing and foot propulsion in a sea duck, the white-winged scoter.

Most birds swim underwater by either feet alone or wings alone, but some sea ducks often use both. For white-winged scoters (Melanitta fusca), we measured costs (V(O2)) of dives to 2 m with descent by feet only versus wings + feet (only feet are used at the bottom). Dive costs repaid during the recovery period after a dive bout were an important fraction (27-44%) of total dive costs, and removing costs of extraneous surface behaviors increased resolution of differences between dive types. Scoters using wings + feet had 13% shorter descent duration, 18% faster descent speed, 31% fewer strokes/m, and 59% longer bottom duration than with feet only. The cost of time underwater for dives using wings + feet was 32-37% lower than with feet only (P = 0.09 to 0.15). When indirect methods were used to partition descent costs from costs of ascent and bottom phases, using wings + feet lowered descent cost by an estimated 34%. Thus, using wings + feet increases descent speed and lowers descent cost, leaving more time and energy for bottom foraging. For birds in cold water, the large savings may result from both biomechanical and thermoregulatory factors.

Thermal substitution and aerobic efficiency: measuring and predicting effects of heat balance on endotherm diving energetics.

For diving endotherms, modelling costs of locomotion as a function of prey dispersion requires estimates of the costs of diving to different depths. One approach is to estimate the physical costs of locomotion (Pmech) with biomechanical models and to convert those estimates to chemical energy needs by an aerobic efficiency (eta=Pmech/Vo2) based on oxygen consumption (Vo2) in captive animals. Variations in eta with temperature depend partly on thermal substitution, whereby heat from the inefficiency of exercising muscles or the heat increment of feeding (HIF) can substitute for thermogenesis. However, measurements of substitution have ranged from lack of detection to nearly complete use of exercise heat or HIF. This inconsistency may reflect (i) problems in methods of calculating substitution, (ii) confounding mechanisms of thermoregulatory control, or (iii) varying conditions that affect heat balance and allow substitution to be expressed. At present, understanding of how heat generation is regulated, and how heat is transported among tissues during exercise, digestion, thermal challenge and breath holding, is inadequate for predicting substitution and aerobic efficiencies without direct measurements for conditions of interest. Confirming that work rates during exercise are generally conserved, and identifying temperatures at those work rates below which shivering begins, may allow better prediction of aerobic efficiencies for ecological models.

Substitution of heat from exercise and digestion by ducks diving for mussels at varying depths and temperatures.

Diving birds can lose significant body heat to cold water, but costs can be reduced if heat from exercising muscles or the heat increment of feeding (HIF) can substitute for thermogenesis. Potential for substitution depends jointly on the rate of heat loss, the rate of heat produced by exercise, and the level of HIF. To explore these interactions, we measured oxygen consumption by lesser scaup ducks (Aythya affinis) diving to depths of 1.2 and 2 m at thermoneutral (23 degrees C) and sub-thermoneutral (18 and 8 degrees C) temperatures. Birds dove while fasted and when feeding on blue mussels (Mytilus edulis). Substitution occurred if HIF or costs of diving above resting metabolic rate (RMR) were lower at 18 or 8 degrees C than at 23 degrees C, indicating reduction in the thermoregulatory part of RMR. For fasted scaup diving to 1.2 m, substitution from exercise heat was not apparent at either 18 or 8 degrees C. At 2 m depth, dive costs above RMR were reduced by 5% at 18 degrees C and by 40% at 8 degrees C, indicating substitution. At 1.2 m depth (with voluntary intake of only 14-17% of maintenance requirements), HIF did not differ between temperatures, indicating no substitution. However, at 2 m (intake 13-25% of maintenance), substitution from HIF was 23% of metabolizable energy intake at 18 degrees C and 22% at 8 degrees C. These results show that even with low HIF due to low intake rates, substitution from HIF can add to substitution from the heat of exercise.

Heat increment of feeding and thermal substitution in mallard ducks feeding voluntarily on grain.

The heat increment of feeding (HIF), including heat from digestion, assimilation, and nutrient interconversion, may substitute for thermogenesis and reduce thermoregulation costs. HIF and its substitution have been measured mainly in animals fed single large meals with high protein content, but many species such as some dabbling ducks (Anatini) feed more continuously in intermittent small meals with low protein content. We measured HIF in seven mallard ducks (Anas platyrhynchos) eating mixed grain (corn, wheat, milo) ad libitum while floating on water at 23 degrees C (thermoneutral) and 8 degrees C. HIF was calculated as the difference in oxygen consumption between fed and fasted birds, correcting for costs of behavior, heat storage (change in body temperature), and heating food. Substitution occurred if HIF was lower at 8 degrees C than at 23 degrees C. Food intake of mallards averaged 83% of that required for maintenance (zero energy balance) at 23 degrees C, and 68% of maintenance at 8 degrees C. Mean HIF (+/-1 SE) was 1.59+/-0.61 l O(2) at 23 degrees C and 1.48+/-0.68 l O(2) at 8 degrees C. These values were 4.9% and 3.9% of metabolizable energy intake, consistent with values expected for grain. HIF did not differ between temperatures (ANCOVA, birds as blocks, intake as covariate, P=0.51), indicating no measurable substitution at these intake levels in intermittent meals. For these large birds that feed on low-protein foods in intermittent small meals, the ecological importance of HIF substitution appears negligible during periods when food intake is below that required for energy balance.

Mechanical versus physiological determinants of swimming speeds in diving Brünnich's guillemots.

For fast flapping flight of birds in air, the maximum power and efficiency of the muscles occur over a limited range of contraction speeds and loads. Thus, contraction frequency and work per stroke tend to stay constant for a given species. In birds such as auks (Alcidae) that fly both in air and under water, wingbeat frequencies in water are far lower than in air, and it is unclear to what extent contraction frequency and work per stroke are conserved. During descent, compression of air spaces dramatically lowers buoyant resistance, so that maintaining a constant contraction frequency and work per stroke should result in an increased swimming speed. However, increasing speed causes exponential increases in drag, thereby reducing mechanical versus muscle efficiency. To investigate these competing factors, we have developed a biomechanical model of diving by guillemots (Uria spp.). The model predicted swimming speeds if stroke rate and work per stroke stay constant despite changing buoyancy. We compared predicted speeds with those of a free-ranging Brünnich's guillemot (U. lomvia) fitted with a time/depth recorder. For descent, the model predicted that speed should gradually increase to an asymptote of 1.5-1.6 m s-1 at approximately 40 m depth. In contrast, the instrumented guillemot typically reached 1.5 m s-1 within 10 m of the water surface and maintained that speed throughout descent to 80 m. During ascent, the model predicted that guillemots should stroke steadily at 1.8 m s-1 below their depth of neutral buoyancy (62 m), should alternate stroking and gliding at low buoyancies from 62 to 15 m, and should ascend passively by buoyancy alone above 15 m depth. However, the instrumented guillemot typically ascended at 1.25 m s-1 when negatively buoyant, at approximately 1.5 m s-1 from 62 m to 25 m, and supplemented buoyancy with stroking above 25 m. Throughout direct descent, and during ascent at negative and low positive buoyancies (82-25 m), the guillemot maintained its speed within a narrow range that minimized the drag coefficient. In films, guillemots descending against high buoyancy at shallow depths increased their stroke frequency over that of horizontal swimming, which had a substantial glide phase. Model simulations also indicated that stroke duration, relative thrust on the downstroke versus the upstroke, and the duration of gliding can be varied to regulate swimming speed with little change in contraction speed or work per stroke. These results, and the potential use of heat from inefficient muscles for thermoregulation, suggest that diving guillemots can optimize their mechanical efficiency (drag) with little change in net physiological efficiency.