No abstract

Quantifying organismal sensitivity to heat stress provides one means for predicting vulnerability to climate change. Birds are ideal for investigating this approach, as they display quantifiable fitness consequences associated with behavioural and physiological responses to heat stress. We used a recently developed method that examines correlations between readily‐observable behaviours and air temperature (Tair) to investigate interspecific variation in avian responses to heat stress in seasonally hot, arid regions on three continents: the southwestern United States, the Kalahari Desert of southern Africa and the Gascoyne region of western Australia. We found substantial interspecific variation in heat dissipation behaviours (wing‐drooping, panting, activity‐reduction, shade‐seeking) across all three regions. However, pooling the data revealed that little of this interspecific variation was systematically explained by organismal traits (foraging guild, diet, drinking dependency, body mass or activity levels) at the scale we tested. After accounting for phylogeny, we found that larger birds engaged in wing‐drooping behaviour at lower Tair and had lower activity levels at high Tair compared to smaller birds, indicating an effect of body mass on heat dissipation behaviour (HDB). In the Kalahari, reliance on drinking was correlated with significantly lower Tair at which panting commenced, suggesting a key role of water acquisition in HDB in that region. Birds also tended to retreat to shade at relatively lower Tair when more active, suggesting a behavioural trade‐off between activity, heat load and microsite selection. Our results imply that the causes underlying interspecific variation in heat dissipation behaviours are complex. While the variation we observed was not systematically explained by the broad scale organismal traits we considered, we predict that the indices themselves will still reflect vulnerability to potential fitness costs of high air temperatures. Further research is needed on a species‐specific basis to establish the functional significance of these indices.

Arid environments provide ideal ground for investigating the mechanisms of adaptive evolution. High temperatures and low water availability are relentless stressors for many endotherms, including birds; yet birds persist in deserts. While physiological adaptation probably involves metabolic phenotypes, the underlying mechanisms (plasticity, genetics) are largely uncharacterized. To explore this, we took an intraspecific approach that focused on a species that is resident over a mesic to arid gradient, the Karoo scrub‐robin (Cercotrichas coryphaeus). Specifically, we integrated environmental (climatic and primary productivity), physiological (metabolic rates: a measure of energy expenditure), genotypic (genetic variation underlying the machinery of energy production) and microbiome (involved in processing food from where energy is retrieved) data, to infer the mechanism of physiological adaptation. We that found the variation in energetic physiology phenotypes and gut microbiome composition are associated with environmental features as well as with variation in genes underlying energy metabolic pathways. Specifically, we identified a small list of candidate adaptive genes, some of them with known ties to relevant physiology phenotypes. Together our results suggest that selective pressures on energetic physiology mediated by genes related to energy homeostasis and possibly microbiota composition may facilitate adaptation to local conditions and provide an explanation to the high avian intraspecific divergence observed in harsh environments.

Rapid anthropogenic climate change potentially severely reduces avian breeding success. While the consequences of high temperatures and drought are reasonably well-studied within single breeding seasons, their impacts over decadal time scales are less clear. We assessed the effects of air temperature (Tair) and drought on the breeding output of southern yellow-billed hornbills (Tockus leucomelas; hornbills) in the Kalahari Desert over a decade (2008–2019). We aimed to document trends in breeding performance in an arid-zone bird during a time of rapid global warming and identify potential drivers of variation in breeding performance. The breeding output of our study population collapsed during the monitoring period. Comparing the first three seasons (2008–2011) of monitoring to the last three seasons (2016–2019), the mean percentage of nest boxes that were occupied declined from 52% to 12%, nest success from 58% to 17%, and mean fledglings produced per breeding attempt from 1.1 to 0.4. Breeding output was negatively correlated with increasing days on which Tmax (mean maximum daily Tair) exceeded the threshold Tair at which male hornbills show a 50% likelihood of engaging in heat dissipation behavior [i.e., panting (Tthresh; Tair = 34.5°C)] and the occurrence of drought within the breeding season, as well as later dates for entry into the nest cavity (i.e., nest initiation) and fewer days post-hatch, spent incarcerated in the nest by the female parent. The apparent effects of high Tair were present even in non-drought years; of the 115 breeding attempts that were recorded, all 18 attempts that had ≥ 72% days during the attempt on which Tmax > Tthresh failed (equivalent to Tmax during the attempt ≥ 35.7°C). This suggests that global warming was likely the primary driver of the recent, rapid breeding success collapse. Based on current warming trends, the Tmax threshold of 35.7°C, above which no successful breeding attempts were recorded, will be exceeded during the entire hornbill breeding season by approximately 2027 at our study site. Therefore, our findings support the prediction that climate change may drive rapid declines and cause local extinctions despite the absence of direct lethal effects of extreme heat events.

18In north-temperate small passerines, overwinter survival is associated with a reversibly increased 19 maximum cold-induced metabolism (M sum ). This strategy may incur increased energy 20 consumption. Therefore, species inhabiting ecosystems characterized by cold winters and low 21 productivity (i.e., low available energy) may be precluded from displaying an increase in 22 Dawson 1989) (Swanson and Garland 2009). An alternative explanation is offered by the climate 52 variability hypothesis (Janzen 1967), (Bozinovic and Naya 2014), which posits that broader 53 climatic fluctuations results in wider flexibility in thermal tolerance as a means to cope with the 54 fluctuating environmental conditions. 55 Several mechanisms, from whole-organism level down to the biochemical level, have been 56 proposed to explain the high cold-induced M sum : i) increase in body condition assessed as body 57 tarsus-length (calliper, d = 0.01 mm). Thus, M b-scaled = M i x [L 0 /L i ] bSMA , where M i and L i are the 127 body mass and tarsus-length of individual i, respectively; L 0 is the tarsus-length arithmetic mean 128for the study populations to which index is standardized; b SMA is the scaling exponent estimated 129 by the standardized major axis regression of mass-length. 130We checked fat accumulation at the furcular depression and abdomen in all 85 adult birds, and 131 quantified it using a scale that ranges from zero (no fat) to eight (flight muscles not visible with 132