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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. 

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. 

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.