Repeated evolution of drag reduction at the air-water interface in diving kingfishers

Piscivorous birds have a unique suite of adaptations to forage under the water. One method aerial birds use to catch fish is the plunge dive, wherein birds dive from a height to overcome drag and buoyancy in the water. The kingfishers are a well-known clade that contains both terrestrially foraging and plunge-diving species, allowing us to test for morphological and performance differences between foraging guilds in an evolutionary context. Diving species have narrower bills in the dorsoventral and sagittal plane and longer bills (size-corrected data, n = 71 species, p < 0.01 for all). Although these differences are confounded by phylogeny (phylogenetically corrected ANOVA for dorsoventral p = 0.26 and length p = 0.14), beak width in the sagittal plane remains statistically different (p < 0.001). We examined the effects of beak morphology on plunge performance by physically simulating dives with three-dimensional printed models of beaks coupled with an accelerometer, and through computational fluid dynamics (CFD). From physically simulated dives of bill models, diving species have lower peak decelerations, and thus enter the water more quickly, than terrestrial and mixed-foraging species (ANOVA p = 0.002), and this result remains unaffected by phylogeny (phylogenetically corrected ANOVA p = 0.05). CFD analyses confirm these trends in three representative species and indicate that the morphology between the beak and head is a key site for reducing drag in aquatic species

Effective Rheology of Bubbles Moving in a Capillary Tube

We calculate the average volumetric flux versus pressure drop of bubbles moving in a single capillary tube with varying diameter, finding a square-root relation from mapping the flow equations onto that of a driven overdamped pendulum. The calculation is based on a derivation of the equation of motion of a bubble train from considering the capillary forces and the entropy production associated with the viscous flow. We also calculate the configurational probability of the positions of the bubbles.Comment: 4 pages, 1 figur

Effects of El Niño-driven changes in wind patterns on North Pacific albatrosses

Changes to patterns of wind and ocean currents are tightly linked to climate change and have important implications for cost of travel and energy budgets in marine vertebrates. We evaluated how El Niño-Southern Oscillation (ENSO)-driven wind patterns affected breeding Laysan and black-footed albatross across a decade of study. Owing to latitudinal variation in wind patterns, wind speed differed between habitat used during incubation and brooding; during La Niña conditions, wind speeds were lower in incubating Laysan (though not black-footed) albatross habitat, but higher in habitats used by brooding albatrosses. Incubating Laysan albatrosses benefited from increased wind speeds during El Niño conditions, showing increased travel speeds and mass gained during foraging trips. However, brooding albatrosses did not benefit from stronger winds during La Niña conditions, instead experiencing stronger cumulative headwinds and a smaller proportion of trips in tailwinds. Increased travel costs during brooding may contribute to the lower reproductive success observed in La Niña conditions. Furthermore, benefits of stronger winds in incubating habitat may explain the higher reproductive success of Laysan albatross during El Niño conditions. Our findings highlight the importance of considering habitat accessibility and cost of travel when evaluating the impacts of climate-driven habitat change on marine predators

Foraging strategies of a generalist marine predator inhabiting a dynamic environment

Intraspecific variability is increasingly recognized as an important component of foraging behavior that can have implications for both population and community dynamics. We used an individual-level approach to describe the foraging behavior of an abundant, generalist predator that inhabits a dynamic marine ecosystem, focusing specifically on the different foraging strategies used by individuals in the same demographic group. We collected data on movements and diving behavior of adult female California sea lions (Zalophus californianus) across multiple foraging trips to sea. Sea lions (n&nbsp;=&nbsp;35) used one of three foraging strategies that primarily differed in their oceanic zone and dive depth: a shallow, epipelagic strategy, a mixed epipelagic/benthic strategy, and a deep-diving strategy. Individuals varied in their degree of fidelity to a given strategy, with 66&nbsp;% of sea lions using only one strategy on all or most of their foraging trips across the two-month tracking period. All foraging strategies were present in each of the sampling years, but there were inter-annual differences in the population-level importance of each strategy that may reflect changes in prey availability. Deep-diving sea lions traveled shorter distances and spent a greater proportion of time at the rookery than sea lions using the other two strategies, which may have energetic and reproductive implications. These results highlight the importance of an individual-based approach in describing the foraging behavior of female California sea lions and understanding how they respond to the seasonal and annual changes in prey availability that characterize the California Current System