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1. High Wing-Loading Correlates with Dive Performance in Birds, Suggesting a Strategy to Reduce Buoyancy

   Lapsansky, A ; Warrick, D ; Tobalske, B ( July 2022 , Integrative and comparative biology)

   Diving birds are regarded as a classic example of morphological convergence. Divers tend to have small wings extending from rotund bodies, requiring many volant species to fly with rapid wingbeats, and rendering others flightless. The high wing-loading of diving birds is frequently associated with the challenge of using forelimbs adapted for flight for locomotion in a “draggier” fluid, but this does not explain why species that rely exclusively on their feet to dive should have relatively small wings, as well. Therefore, others have hypothesized that ecological factors shared by wing-propelled and foot-propelled diving birds drive the evolution of high wing-loading. Following a reexamination of the aquatic habits of birds, we tested between hypotheses seeking to explain high wing-loading in divers using new comparative data and phylogenetically informed analyses. We found little evidence that wing-propelled diving selects for small wings, as wing-propelled and foot-propelled species share similar wing-loadings. Instead, our results suggest that selection to reduce buoyancy has driven high wing-loading in divers, offering insights for the development of bird-like aquatic robots. 

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2. High Wing-Loading Correlates with Dive Performance in Birds, Suggesting a Strategy to Reduce Buoyancy

   https://doi.org/10.1093/icb/icac117  

   Lapsansky, Anthony B. ; Warrick, Douglas R. ; Tobalske, Bret W. ( July 2022 , Integrative And Comparative Biology)

   Diving birds are regarded as a classic example of morphological convergence. Divers tend to have small wings extending from rotund bodies, requiring many volant species to fly with rapid wingbeats, and rendering others flightless. The high wing-loading of diving birds is frequently associated with the challenge of using forelimbs adapted for flight for locomotion in a “draggier” fluid, but this does not explain why species that rely exclusively on their feet to dive should have relatively small wings, as well. Therefore, others have hypothesized that ecological factors shared by wing-propelled and foot-propelled diving birds drive the evolution of high wing-loading. Following a reexamination of the aquatic habits of birds, we tested between hypotheses seeking to explain high wing-loading in divers using new comparative data and phylogenetically informed analyses. We found little evidence that wing-propelled diving selects for small wings, as wing-propelled and foot-propelled species share similar wing-loadings. Instead, our results suggest that selection to reduce buoyancy has driven high wing-loading in divers, offering insights for the development of bird-like aquatic robots.

    

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3. Common Mergansers Mergus merganser use wings to pursue a fish underwater

   Lapsansky, A ; Armstrong, R ( October 2022 , Marine ornithology)

   Common Mergansers Mergus merganser dive into lakes, rivers, and coastal waters to feed on fish and other aquatic prey. This species and others in the genus Mergus are traditionally classified as foot-propelled divers. When submerged, mergansers are expected to swim by kicking their feet, holding their wings close to their bodies. Here, we report, with video evidence, an event in which four mergansers used their wings underwater to chase down and capture a large fish. Documentation of wing use by this classically defined “foot-propelled diver” illustrates the gaps in our understanding of avian diving physiology, hydrodynamics, and behavior. 

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4. Unexpected Performance in Developing Birds

   https://doi.org/10.1093/icb/icad064  

   Heers, Ashley M. ( July 2023 , Integrative And Comparative Biology)

   Birds are well known for their ability to fly, and flight-capable adult birds have many anatomical specializations for meeting the demands of aerial locomotion. Juvenile birds in altricial species typically acquire these specializations close to fledging and leave the nest with some flight capability. In contrast, juveniles in most precocial species begin navigating their environment with rudimentary anatomies and may not develop full-sized wings or musculoskeletal apparatuses for several months. This manuscript explores how juvenile birds achieve high levels of locomotor performance in the absence of flight specializations, by synthesizing work on two groups of precocial birds with very different developmental strategies. Galliforms like the Chukar Partridge (Alectoris chukar) have early wing development and are capable of flight within weeks. Compared with adults, juvenile chukars have less aerodynamically effective feathers and smaller muscles but compensate through anatomical, kinematic, and behavioral mechanisms. In contrast, waterfowl have delayed wing development and initially rely on leg-based locomotion. In Mallards (Anas platyrhynchos) and their domesticated derivatives, leg investment and performance peak early in ontogeny, but then decline when wings develop. Chukar and mallard juveniles thus rely on different mechanisms for negotiating their surroundings in the absence of flight specializations. In conjunction with work in other animals, these patterns indicate that juveniles with developing locomotor apparatuses can achieve surprisingly high levels of locomotor performance through a variety of compensatory mechanisms.

    

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5. Evolved increases in hemoglobin-oxygen affinity and the Bohr effect coincided with the aquatic specialization of penguins

   https://doi.org/https://doi.org/10.1073/pnas.2023936118  

   Signore, Anthony V ; Tift, Michael S ; Hoffman, Federico G ; Schmitt, Todd L ; Moriyama, Hideaki ; Storz, Jay F. ( March 2021 , Proceedings of the National Academy of Sciences of the United States of America)

   Somero, George N. (Ed.)

   Dive capacities of air-breathing vertebrates are dictated by onboard O2 stores, suggesting that physiologic specialization of diving birds such as penguins may have involved adaptive changes in convective O2 transport. It has been hypothesized that increased hemoglobin (Hb)-O2 affinity improves pulmonary O2 extraction and enhances the capacity for breath-hold diving. To investigate evolved changes in Hb function associated with the aquatic specialization of penguins, we integrated comparative measurements of whole-blood and purified native Hb with protein engineering experiments based on site-directed mutagenesis. We reconstructed and resurrected ancestral Hb representing the common ancestor of penguins and the more ancient ancestor shared by penguins and their closest nondiving relatives (order Procellariiformes, which includes albatrosses, shearwaters, petrels, and storm petrels). These two ancestors bracket the phylogenetic interval in which penguin-specific changes in Hb function would have evolved. The experiments revealed that penguins evolved a derived increase in Hb-O2 affinity and a greatly augmented Bohr effect (i.e., reduced Hb-O2 affinity at low pH). Although an increased Hb-O2 affinity reduces the gradient for O2 diffusion from systemic capillaries to metabolizing cells, this can be compensated by a concomitant enhancement of the Bohr effect, thereby promoting O2 unloading in acidified tissues. We suggest that the evolved increase in Hb-O2 affinity in combination with the augmented Bohr effect maximizes both O2 extraction from the lungs and O2 unloading from the blood, allowing penguins to fully utilize their onboard O2 stores and maximize underwater foraging time. 

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