Population- and Species-Level Variation in Near- and Mid-infrared Radiation in Birds: A Preliminary Analysis

Animal coloration has diverse functions, such as camouflage, communication, thermoregulation, and protection from UV damage and more, and can be shaped by environmental selective pressures. Some climatic selective pressures are strong enough to produce consistent patterns in many species across large-scale geographic gradients, leading to the creation of macrophysiological rules such as Gloger's rule, which predicts that endothermic populations in hot, humid areas will be visibly darker than those in cool, dry areas, and the thermal melanism hypothesis, which predicts that ectothermic animals will be visibly darker in cooler areas. While these rules often capture trends in animal absorptance in the visible spectrum, wavelengths of visible light are not the only relevant wavelengths to an animal's energy budget: solar radiation extends beyond the visible spectrum [0.4-0.7 μm] into the near-infrared; thus, thermal pressures may result in changes in surface reflectance characteristics beyond the visible [e.g., 0.7-2.5 μm] in birds. Further, heat exchange with the environment extends into the mid-infrared (MIR), including heat loss through the atmospheric transparency window [8-14 μm]. It is unknown whether animal absorptance in the NIR or emittance in MIR might also follow macrophysiological rules, as seen in the visible spectrum, such as more absorptance of NIR and less emittance of MIR in cooler areas for ectotherms under the thermal melanism hypothesis. Here, we examine both UV-NIR absorptance and MIR emittance in five species of birds: the Great Horned Owl, Northern Bobwhite, Steller's Jay, Song Sparrow, and Common Raven. We show that NIR absorptance varies by species and population, corresponding to their habitat and thermoregulatory strategies. MIR emittance, in contrast, was stable across species and populations but differed slightly across populations of Northern Bobwhites. We conclude by highlighting the importance of considering the full spectrum from UV to MIR in research on animal adaptation. Further consideration of infrared radiation is necessary for a complete view of animals' phenotypic diversity and possible responses to thermal challenge.