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Abstract 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. 

Birds that use their wings for ‘flight’ in both air and water are expected to fly poorly in each fluid relative to single-fluid specialists; that is, these jacks-of-all-trades should be the masters of none. Alcids exhibit exceptional dive performance while retaining aerial flight. We hypothesized that alcids maintain efficient Strouhal numbers and stroke velocities across air and water, allowing them to mitigate the costs of their ‘fluid generalism’. We show that alcids cruise at Strouhal numbers between 0.10 and 0.40 – on par with single-fluid specialists – in both air and water but flap their wings ~ 50% slower in water. Thus, these species either contract their muscles at inefficient velocities or maintain a two-geared muscle system, highlighting a clear cost to using the same morphology for locomotion in two fluids. Additionally, alcids varied stroke-plane angle between air and water and chord angle during aquatic flight, expanding their performance envelope. 

Abstract Vulnerability to warming is often assessed using short‐term metrics such as the critical thermal maximum (CT_MAX), which represents an organism's ability to survive extreme heat. However, the long‐term effects of sub‐lethal warming are an essential link to fitness in the wild, and these effects are not adequately captured by metrics like CT_MAX.The meltwater stonefly,Lednia tumana, is endemic to high‐elevation streams of Glacier National Park, MT, USA, and has long been considered acutely vulnerable to climate‐change‐associated stream warming. As a result, in 2019, it was listed as Threatened under the U.S. Endangered Species Act. This presumed vulnerability to warming was challenged by a recent study showing that nymphs can withstand short‐term exposure to temperatures as high as ~27°C. But whether they also tolerate exposure to chronic, long‐term warming remained unclear.By measuring fitness‐related traits at several ecologically relevant temperatures over several weeks, we show thatL. tumanacannot complete its life‐cycle at temperatures only a few degrees above what some populations currently experience.The temperature at which growth rate was maximized appears to have a detrimental impact on other key traits (survival, emergence success and wing development), thus extending our understanding ofL. tumana's vulnerability to climate change.Our results call into question the use of CT_MAXas a sole metric of thermal sensitivity for a species, while highlighting the power and complexity of multi‐trait approaches to assessing vulnerability. Read the freePlain Language Summaryfor this article on the Journal blog.