Pilon, L. | bioRxiv Science

bioRxiv2026

Peak performance is repeatable and captures large individual differences in ruby-throated hummingbirds

Locomotor performance often determines the outcome of interactions with competitors, predators, and prey. In flying animals, the asymptotic load-lifting assay measures maximal muscle power output in vertical flight. Previous studies of small birds have shown that load-lifting performance is linked to flight maneuverability and the outcome of competitive species interactions. Here, we quantify sources of performance variation within a species, namely repeatability, and determine the number of trials that accurately capture individual differences. We conducted 124 load-lifting trials on 13 wild-caught male ruby-throated hummingbirds (Archilochus colubris), testing each individual repeatedly over a 3-day period. We report large individual differences in peak performance, with 70% of the total variation in lifted mass attributed to differences among males. Notably, these differences in muscle performance are independent of body mass and size. An additional 23% of the variation in lifted mass was due to short-term fluctuations, wherein a given males performance varied across trials and days. We find no systematic effects of experience or time on load-lifting performance. Using simulation, we test the effect of different sampling protocols for measuring individual performance, and show that single-trial protocols yield the highest repeatabilities, but are less suitable for capturing the true underlying differences among individuals. We discuss recommendations for future studies that aim to measure maximum performance. Overall, our results show that the asymptotic load-lifting assay reveals large individual differences and can closely reflect individuals true maximum capacity. Summary statementMale ruby-throated hummingbirds exhibit large, consistent differences in load-lifting performance, highlighting peak flight performance as a potential driver of differences in competitive abilities between conspecifics.

zoology↗

bioRxiv2024

How does Gloger's rule predict near- and mid-infrared radiation in birds?

Animal coloration has diverse functions such as camouflage, communication, thermoregulation, protection from UV damage and more, and can be shaped by environmental selective pressures. Some climactic selective pressures are strong enough to produce consistent patterns in many species across large-scale geographic gradients. One pattern in endothermic animals is Glogers rule, which predicts that populations in hot, humid areas will be darker than those in cool, dry areas. This pattern has been demonstrated in several species across latitudinal gradients and is expected to relate to the selective effects of both local thermoregulatory pressures and humidity. However, shortwave radiation from sunlight extends beyond the visible spectrum [400-700 nm] into the near-infrared; thus, thermal pressures often result in changes in surface reflectance characteristics beyond the visible [e.g., 700-2500 nm]. Further, heat exchange with the environment extends into the mid-infrared, including MIR heat loss through the atmospheric transmission window [7.5 - 14 um]. Here, we examine both UV-NIR absorption and MIR emittance in five species of birds that have been shown to follow, or not follow, Glogers rule. We show that NIR absorption varies by species and population in ways that correspond to their habitat and thermoregulatory strategies. MIR emittance, by contrast, was very stable across both species and populations but differed across populations of Northern Bobwhites. We conclude by highlighting the importance of extending coloration research into the NIR and MIR. Further consideration of infrared radiation is necessary for a complete view of animals phenotypic diversity and possible responses to thermal challenge.

ecology↗