Andrew E. McKechnie

Non-evaporative heat loss across the beak is an important thermoregulatory pathway among birds. Recent studies have revealed that blood flow to the beak is regulated so as to dissipate heat under h...

Energy is fundamental for life and accurately predicting animals' energy expenditure under changing environments is central to ecology and global change biology. Energy expenditure in free‐living animals can be directly quantified using the doubly labelled water (DLW) technique or estimated by extrapolating respirometric measurements of metabolic rates or modelled using biophysical principles. However, the accuracy of these latter approaches depends on data quality and model parameterisation. We evaluated respirometry extrapolations and biophysical models for estimating daily energy expenditure (DEE) in a free‐living bird, the Southern Pied Babbler ( Turdoides bicolor ) against DLW measurements. Respirometry extrapolations and biophysical models were parameterised using: (a) assumed animal behaviour and assumed operative temperature ( T _e ), (b) assumed animal behaviour and measured T _e , (c) fine scale (focal) observations of behaviour under natural conditions and measured T _e , or (d) coarse scale (scan) behaviour observations and measured T _e . We then quantified the minimum field‐collected data needed to accurately predict measured DEE across a range of daily maximum air temperatures ( T _air ). Models parameterised with assumed behaviours and T _e failed to predict DEE with 95% confidence across all T _air values. Biophysical models using focal observations and measured T _e closely matched DLW measurements (within 94.1%–95.3% of measured DEE), while scan observations reduced accuracy (within 81.7%–88.1% of measured DEE). Respirometry extrapolations using focal or scan observations and measured T _e predicted DEE within 82.1%–88.9% and 69.0%–87.1% of observed values, respectively. Varying behavioural data inputs showed that improving the accuracy of DEE predictions does not require exhaustive sampling. A targeted approach prioritising behavioural observations and T _e measurements across representative T _air can minimise effort in the field, while maintaining model accuracy. Biophysical models, increasingly used to predict species interactions, distributions, survival and fitness under environmental change, must be biologically relevant, well‐informed and accurate to reliably inform ecological and conservation research. Read the free Plain Language Summary for this article on the Journal blog.

The severity of fitness costs resulting from behavioural trade-offs between heat dissipation and activities such as foraging is increasing with advancing climate change. In terrestrial habitats, shade and water may buffer individuals from the negative effects of heat exposure, as may load-lightening in group-living species. We tested the hypothesis that thermal and hygric properties of home ranges (shade and water availability) and social factors (group size) influence the costs associated with hot weather, using a population of white-browed sparrow-weavers ( Plocepasser mahali ) in the southern Kalahari Desert. Across all sparrow-weaver groups, heat avoidance (shade-seeking) and dissipation (panting) behaviours increased with increasing air temperature ( T _air ), whereas foraging declined. Birds occupying shadier home ranges delayed the onset of panting to higher T _air and foraged less while maintaining overall peck rates. Birds with access to water foraged more, maintained higher peck rates and sought shade at higher T _air compared to birds without. However, they did not pant more, making the mechanism underpinning their increased foraging effort unclear. Birds in larger groups both panted more overall and sought shade at lower T _air than birds in smaller groups but maintained similar overall peck rates. Taken together, these results suggest birds in shadier home ranges can forage more efficiently, buffering foraging costs at high T _air . Our data therefore suggest that some impacts of increasing T _air under climate change can be buffered by shade availability, but the impacts of water availability and social factors are less clear.

Frequent and intense heatwaves are causing heat-related avian mass mortality events to become more common, but the role of elevated humidity as a contributing factor remains unclear. Here, we quantified the effect of humidity on risks of lethal hyperthermia for blue waxbills ( Uraeginthus angolensis ), the species most common among the victims of South Africa’s first documented heat-related mass mortality event involving wild bird populations. We quantified waxbills’ body temperature ( T _b ), metabolic heat production and evaporative heat loss at air temperatures ( T _air ) approaching and surpassing normothermic T _b in dry (1.1 ± 0.9 g m ⁻³ ) and humid (21.3 ± 0.4 g m ⁻³ ) air. The humid treatment was associated with significant declines in evaporative cooling capacity, and maximum T _air tolerated by waxbills was approximately 2°C lower (45.7°C) compared to the dry air treatment (47.9°C). A model of end-Century exposure for the waxbills reveals that elevated humidity could increase the risks of lethal hyperthermia by threefold to sevenfold in some parts of southern Africa.

Conventionally, the size, shape, and biomechanics of cartilages are determined by their voluminous extracellular matrix. By contrast, we found that multiple murine cartilages consist of lipid-filled cells called lipochondrocytes. Despite resembling adipocytes, lipochondrocytes were molecularly distinct and produced lipids exclusively through de novo lipogenesis. Consequently, lipochondrocytes grew uniform lipid droplets that resisted systemic lipid surges and did not enlarge upon obesity. Lipochondrocytes also lacked lipid mobilization factors, which enabled exceptional vacuole stability and protected cartilage from shrinking upon starvation. Lipid droplets modulated lipocartilage biomechanics by decreasing the tissue’s stiffness, strength, and resilience. Lipochondrocytes were found in multiple mammals, including humans, but not in nonmammalian tetrapods. Thus, analogous to bubble wrap, superstable lipid vacuoles confer skeletal tissue with cartilage-like properties without “packing foam–like” extracellular matrix.

Increasingly frequent and intense heatwaves are expected to elevate the risks of heat-related mortality among birds. Most studies have focused on arid-zone avifaunas and the extent to which risks will increase in other habitats, particularly humid lowlands, remains unclear. We tested the prediction that increasing air temperature and corresponding increases in humidity, and hence wet-bulb temperature (TW; lowest temperature achievable via adiabatic evaporation), will increase exposure to conditions associated with lethal hyperthermia. We empirically determined maximum TW (TW-max) for an Afrotropical forest frugivore, the trumpeter hornbill (Bycanistes bucinator) as TW-max = 31.7 ± 1.0°C. We then modelled current and future exposure to conditions associated with TW > TW-max across this species’ range. Under a business-as-usual emissions scenario and assuming no vegetation buffering of air temperature (Tair), trumpeter hornbills will experience TW > TW-max for at least 1 day year−1 over 46% of their current range, compared to 30% at present. However, the frequency of exposure will increase substantially and reach ~100 days year−1 in parts of the southern Democratic Republic of Congo. When we incorporated the thermal buffering effect of vegetation, end-century exposure to TW > TW-max decreased by 0.3–66.7%, emphasizing the role of cool microsites provided by vegetation. Our analyses reveal the exposure of birds inhabiting humid environments at low latitudes to conditions associated with a risk of lethal hyperthermia under resting conditions will increase substantially in coming decades, putting a large fraction of global avian biodiversity at risk of population declines and local extinctions.

Accurately predicting exposure of animals to climate change requires evaluating the effects of warming on the microclimates they occupy. Birds, like many other taxa, make extensive use of cool microsites in vegetation during hot weather. Taking advantage of recent advances in modelling tree canopy microclimates, we combined LiDAR-based individual tree canopy mapping and biophysical modelling to evaluate the current and future availability of cool microsites in a subtropical African savanna landscape. We constructed biophysical models for two common bird species, an ~ 40-g bulbul and an ~ 200-g hornbill, and modelled exposure to conditions under which the body temperature (T _b) of individuals resting in canopies exceeds 42 °C, equivalent to ~ 2 °C above resting thermoneutral T _b. At present, 34.5% of trees taller than 2 m in our 139-ha study site provide microclimates in which resting T _b remained below 42 °C for both species during our study period. Under a Representative Concentration Pathway 8.5 climate change scenario and assuming no change in vegetation structure, by the end of the Century the availability of microsites characterized by T _b < 42 °C will decrease to just 0.4% and 3.8% for bulbuls and hornbills, respectively. The proportion of trees in whose canopies bulbuls’ and hornbills’ exposure to T _b > 42 °C is limited to < 10 d summer^− 1 will decrease from 98 to 99% currently to 3.0% and 24.3% by end-century, respectively. These findings reveal the magnitude of changes for birds in a savanna thermal landscape under a business-as-usual emissions scenario.