High temperatures are associated with substantial reductions in breeding success and offspring quality in an arid-zone bird

Mapping tree canopy thermal refugia for birds using biophysical models and LiDAR

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 (Tb) of individuals resting in canopies exceeds 42 °C, equivalent to ~ 2 °C above resting thermoneutral Tb. At present, 34.5% of trees taller than 2 m in our 139-ha study site provide microclimates in which resting Tb 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 Tb < 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 Tb > 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.

Plastic use as nesting material can alter incubation temperature and behaviour but does not affect yellow-legged gull chicks

Optimal incubation temperature is crucial for embryos' development and survival. With the increasing use of plastics in gulls' nests, it is essential to understand how their incorporation affects incubation temperature, parental behaviour, and hatching success. Considering this, we conducted an experiment where plastic was introduced into yellow-legged gulls (Larus michahellis) nests. The experiment comprised three groups: a control group, a group with low amount of plastic, and a third with a high amount of plastic. This design allowed us to investigate the effects of plastic on 1) the heart rate of incubating adults, 2) the number and duration of adults' absences from their nest, 3) how the presence or absence of the adult influenced egg temperature, and 4) chick hatching success, physiological parameters, and bill colour phenotype. We observed that incubation temperature was consistently higher in nests with plastic. The number of absences was higher in the low plastic group at increased temperatures, though the duration was significantly lower in both plastic groups than in the control, possibly to mitigate the effects of heat stress. During higher environmental temperatures, heart rate was higher for the high plastic group. The increase in heart rate in the low plastic group was less pronounced with increasing environmental temperatures. No significant effects were observed on hatching success or in the health condition of young chicks, except for high values of haemoglobin in both plastic groups, which might indicate dehydration.

Effects of lead on avian thermoregulation in the heat: An experimental test with pied crows (Corvus albus)

Many of the negative physiological effects of lead involve the hypothalamus, but the possibility that thermoregulation is affected has received little attention. We tested the hypothesis that lead exposure reduces avian thermoregulatory performance under hot conditions in pied crows (Corvus albus) experimentally exposed to lead in their diet. Crows in our high lead treatment (blood [Pb] = 87.3 ± 44.7 μg dL-1) showed significantly higher air temperature (Tair) inflections for evaporative water loss (EWL) and resting metabolic rate (RMR) compared to control (6.4 ± 1.8 μg dL-1) or intermediate (53.9 ± 23.7 μg dL-1) lead groups, which did not differ. EWL, RMR and body temperature (Tb) all increased more rapidly at Tair > Tb in the high lead treatment. In contrast, neither maximum Tair tolerated by the crows nor maximum Tb varied with treatment. Our data reveal that water and energy balance during hot weather is affected by lead exposure.

An arctic breeding songbird overheats during intense activity even at low air temperatures

Birds maintain some of the highest body temperatures among endothermic animals. Often deemed a selective advantage for heat tolerance, high body temperatures also limits birds' thermal safety margin before reaching lethal levels. Recent modelling suggests that sustained effort in Arctic birds might be restricted at mild air temperatures, which may require reductions in activity to avoid overheating, with expected negative impacts on reproductive performance. We measured within-individual changes in body temperature in calm birds and then in response to an experimental increase in activity in an outdoor captive population of Arctic, cold-specialised snow buntings (Plectrophenax nivalis), exposed to naturally varying air temperatures (- 15 to 36 °C). Calm buntings exhibited a modal body temperature range from 39.9 to 42.6 °C. However, we detected a significant increase in body temperature within minutes of shifting calm birds to active flight, with strong evidence for a positive effect of air temperature on body temperature (slope = 0.04 °C/ °C). Importantly, by an ambient temperature of 9 °C, flying buntings were already generating body temperatures ≥ 45 °C, approaching the upper thermal limits of organismal performance (45-47 °C). With known limited evaporative heat dissipation capacities in these birds, our results support the recent prediction that free-living buntings operating at maximal sustainable rates will increasingly need to rely on behavioural thermoregulatory strategies to regulate body temperature, to the detriment of nestling growth and survival.

Thermoregulatory consequences of growing up during a heatwave or a cold snap in Japanese quail

Changes in environmental temperature during development can affect growth, metabolism and temperature tolerance of the offspring. We know little about whether such changes remain to adulthood, which is important to understand the links between climate change, development and fitness. We investigated whether phenotypic consequences of the thermal environment in early life remained in adulthood in two studies on Japanese quail (Coturnix japonica). Birds were raised under simulated heatwave, cold snap or control conditions, from hatching until halfway through the growth period, and then in common garden conditions until reproductively mature. We measured biometric and thermoregulatory [metabolic heat production (MHP), evaporative water and heat loss (EWL, EHL) and body temperature] responses to variation in submaximal air temperature at the end of the thermal acclimation period and in adulthood. Warm birds had lower MHP than control birds at the end of the thermal acclimation period and, in the warmest temperature studied (40°C), also had higher evaporative cooling capacity compared with controls. No analogous responses were recorded in cold birds, although they had higher EWL than controls in all but the highest test temperature. None of the effects found at the end of the heatwave or cold snap period remained until adulthood. This implies that chicks exposed to higher temperatures could be more prepared to counter heat stress as juveniles but that they do not enjoy any advantages of such developmental conditions when facing high temperatures as adults. Conversely, cold temperature does not seem to confer any priming effects in adolescence.

Thermoregulatory strategies of songbird nestlings reveal limited capacity for cooling and high risk of dehydration

How the accelerating pace of global warming will affect animal populations depends on the effects of increasing temperature across the life cycle. Developing young are sensitive to environmental challenges, often with life-long consequences, but the risks of climate warming during this period are insufficiently understood. This may be due to limited insight into physiological sensitivity and the temperatures that represent a thermal challenge for young. Here we examined the physiological and behavioural effects of increasing temperatures by measuring metabolic rate, water loss, and heat dissipation behaviours between 25-45 °C in nestlings of a small free-living songbird of temperate SE-Australia, the superb fairy-wren. We found a high and relatively narrow thermoneutral zone from 33.1 to 42.3 °C, with metabolic rate increasing and all nestlings panting above this range. Evaporative water loss sharply increased above 33.5 °C; at the same temperature, nestlings changed their posture (extended their wings) to facilitate passive heat loss. However, at all temperatures measured, water loss was insufficient to dissipate metabolically produced heat, indicating poor cooling capabilities, which persisted even when individuals were panting. While nestlings are relatively tolerant to higher temperatures, with no evidence for hyperthermia at temperatures below 42 °C, they are at a high risk of dehydration even at lower temperatures, with limited ability to mitigate this. Thus, climate warming is likely to elevate the risk dehydration, which is concerning, since it is accompanied by drier conditions.

Born without a Silver Spoon: A Review of the Causes and Consequences of Adversity during Early Life

A huge amount of research attention has focused on the evolution of life histories, but most research focuses on dominant individuals that acquire a disproportionate level of reproductive success, while the life histories and reproductive tactics of subordinate individuals have received less attention. Here, we review the links between early life adversity and performance during adulthood in birds, and highlight instances in which subordinate individuals outperform dominant conspecifics. Subordinate individuals are those from broods raised under high risk of predation, with low availability of food, and/or with many parasites. Meanwhile, the broods of many species hatch or are born asynchronously and mitigation of the asynchrony is generally lacking from variation in maternal effects such as egg size and hormone deposition or genetic effects such as offspring sex or parentage. Subordinate individuals employ patterns of differential growth to attempt to mitigate the adversity they experience during early life, yet they overwhelmingly fail to overcome their initial handicap. In terms of surviving through to adulthood, subordinate individuals employ other "suboptimal" tactics, such as adaptively timing foraging behaviors to avoid dominant individuals. During adulthood, meanwhile, subordinate individuals rely on "suboptimal" tactics, such as adaptive dispersal behaviors and competing for partners at optimal times, because they represent the best options available to them to acquire copulations whenever possible. We conclude that there is a gap in knowledge for direct links between early life adversity and subordination during adulthood, meaning that further research should test for links. There are instances, however, where subordinate individuals employ "suboptimal" tactics that allow them to outperform dominant conspecifics during adulthood.

Parental behavior and newborn attachment in birds: life history traits and endocrine responses

In birds, parental care and attachment period differ widely depending on the species (altricial or precocial), developmental strategies, and life history traits. In most bird species, parental care can be provided by both female and male individuals and includes specific stages such as nesting, laying, and hatching. During said periods, a series of neuroendocrine responses are triggered to motivate parental care and attachment. These behaviors are vital for offspring survival, development, social bonding, intergenerational learning, reproductive success, and ultimately, the overall fitness and evolution of bird populations in a variety of environments. Thus, this review aims to describe and analyze the behavioral and endocrine systems of parental care and newborn attachment in birds during each stage of the post-hatching period.

Effects of meteorological conditions on brood care in cooperatively breeding carrion crow and consequences on reproductive success

Meteorological stressors (e.g., temperature and rain shortage) constrain brood provisioning in some bird species, but the consequences on reproductive success have been rarely quantified. Here we show, in a cooperatively breeding population of carrion crow Corvus corone in Spain, that individual feeding rates decreased significantly with rising air temperatures both in breeders and helpers, while lack of rain was associated with a significant reduction in the effort of the male helpers as compared to the other social categories. Group coordination, measured as the degree of alternation of nest visits by carers, was also negatively affected by rising temperature. Furthermore, we found that the body condition of the nestlings worsened when temperatures were high during the rearing period. Interestingly, the analysis of a long-term data set on crow reproduction showed that nestling body condition steadily deteriorated over the last 26-years. Although many factors may concur in causing population changes, our data suggest a possible causal link between global warming, brood caring behaviour and the decline of carrion crow population in the Mediterranean climatic region of Spain.

Helpers don't help when it's hot in a cooperatively breeding bird, the Southern Pied Babbler

Cooperative breeding, where more than two individuals invest in rearing a single brood, occurs in many bird species globally and often contributes to improved breeding outcomes. However, high temperatures are associated with poor breeding outcomes in many species, including cooperative species. We used data collected over three austral summer breeding seasons to investigate the contribution that helpers make to daytime incubation in a cooperatively breeding species, the Southern Pied Babbler Turdoides bicolor, and the ways in which their contribution is influenced by temperature. Helpers spent a significantly higher percentage of their time foraging (41.8 ± 13.7%) and a significantly lower percentage of their time incubating (18.5 ± 18.8%) than members of the breeding pair (31.3 ± 11% foraging and 37.4 ± 15.7% incubating). In groups with only one helper, the helper's contribution to incubation was similar to that of breeders. However, helpers in larger groups contributed less to incubation, individually, with some individuals investing no time in incubation on a given observation day. Helpers significantly decrease their investment in incubation on hot days (>35.5°C), while breeders tend to maintain incubation effort as temperatures increase. Our results demonstrate that pied babblers share the workload of incubation unequally between breeders and helpers, and this inequity is more pronounced during hot weather. These results may help to explain why recent studies have found that larger group size does not buffer against the impacts of high temperatures in this and other cooperatively breeding species.

Acute stress and restricted diet reduce bill-mediated heat dissipation in the song sparrow (Melospiza melodia): implications for optimal thermoregulation

We used thermal imaging to show that two environmental factors - acute stress and diet - influence thermoregulatory performance of a known thermal window, the avian bill. The bill plays important roles in thermoregulation and water balance. Given that heat loss through the bill is adjustable through vasoconstriction and vasodilation, and acute stress can cause vasoconstriction in peripheral body surfaces, we hypothesized that stress may influence the bill's role as a thermal window. We further hypothesized that diet influences heat dissipation from the bill, given that body condition influences the surface temperature of another body region (the eye region). We measured the surface temperature of the bills of song sparrows (Melospiza melodia) before, during and after handling by an observer at 37°C ambient temperature. We fed five birds a restricted diet intended to maintain body mass typical of wild birds, and we fed six birds an unrestricted diet for 5 months prior to experiments. Acute stress caused a decrease in the surface temperature of the bill, resulting in a 32.4% decrease in heat dissipation immediately following acute stress, before recovering over approximately 2.3 min. The initial reduction and subsequent recovery provide partial support for the hemoprotective and thermoprotective hypotheses, which predict a reduction or increase in peripheral blood flow, respectively. Birds with unrestricted diets had larger bills and dissipated more heat, indicating that diet and body condition influence bill-mediated heat dissipation and thermoregulation. These results indicate that stress-induced vascular changes and diet can influence mechanisms of heat loss and potentially inhibit optimal thermoregulation.

Heat tolerance limits of Mediterranean songbirds and their current and future vulnerabilities to temperature extremes

Songbirds are one of the groups most vulnerable to extreme heat events. Although several recent studies have assessed their physiological responses to heat, most of them have focused solely on arid-zone species. We investigated thermoregulatory responses to heat in eight small-sized songbirds occurring in the Mediterranean Basin, where heatwaves are becoming more frequent and intense. Specifically, we determined their heat tolerance limits (HTLs) and evaporative cooling efficiency, and evaluated their current and future vulnerabilities to heat in southwestern Iberia, a Mediterranean climate warming hotspot. To do this, we exposed birds to an increasing profile of air temperatures (Ta) and measured resting metabolic rate (RMR), evaporative water loss (EWL), evaporative cooling efficiency (the ratio between evaporative heat loss and metabolic heat production) and body temperature (Tb). HTL ranged between 40 and 46°C across species, and all species showed rapid increases in RMR, EWL and Tb in response to increasing Ta. However, only the crested lark (Galerida cristata) achieved an evaporative cooling efficiency greater than 1. The studied songbirds currently experience summer Ta maxima that surpass the upper critical temperatures of their thermoneutral zone and even their HTL. Our estimates indicate that five of the eight species will experience moderate risk of lethal dehydration by the end of the century. We argue that the limited heat tolerance and evaporative cooling efficiency of small-sized Mediterranean songbirds make them particularly vulnerable to heatwaves, which will be exacerbated under future climate change scenarios.

Diversity of Avian Species in Peri-Urban Landscapes Surrounding Fez in Morocco: Species Richness, Breeding Populations, and Evaluation of Menacing Factors

In this study, we investigated the avian diversity and threatening factors in five peri-urban sites around Fez city (Morocco) for 2 years (2018–2019). The study hosted 131 avian species, including 64.88% breeding species, 19.84% migrant winterers, and 11.45% migrant breeders. Five species of conservation concern such as the vulnerable European turtle dove and the European goldfinch, the near-threatened ferruginous duck and bar-tailed godwit, and the endangered white-headed duck were recorded. Most bird species were recorded at the Oued Fez River (26.89%) and the El Mehraz dam (25%), followed by the El Gaada dam (17.4%), the Ain Bida garbage dump (15.5%), and the Ain Chkef Forest (15.18%). About 44.44% of the breeding species were found at Oued Fez, along with 33.33% at the El Mehraz dam, while El Gâada, Ain Chkef, and Ain Bida hosted only 7.40% of species. An important breeding population of the endangered white-headed duck was recorded at El Mehraz and Oued Fez. The extension of farmlands, urbanization, touristic activities, and drought constitute the most menacing factors for the avian diversity and their habitats in Fez.

Forest microclimate and composition mediate long-term trends of breeding bird populations

Climate change is contributing to biodiversity redistributions and species declines. However, cooler microclimate conditions provided by old-growth forest structures compared with surrounding open or younger forests have been hypothesized to provide thermal refugia for species that are sensitive to climate warming and dampen the negative effects of warming on population trends of animals (i.e., the microclimate buffering hypothesis). In addition to thermal refugia, the compositional and structural diversity of old-growth forest vegetation itself may provide resources to species that are less available in forests with simpler structure (i.e., the insurance hypothesis). We used 8 years of breeding bird abundance data from a forested watershed, accompanied with sub-canopy temperature data, and ground- and LiDAR-based vegetation data to test these hypotheses and identify factors influencing bird population changes from 2011 to 2018. After accounting for imperfect detection, we found that for 5 of 20 bird species analyzed, abundance trends tended to be less negative or neutral at sites with cooler microclimates, which supports the microclimate buffering hypothesis. Negative effects of warming on two species were also reduced in locations with greater forest compositional diversity supporting the insurance hypothesis. We provide the first empirical evidence that complex forest structure and vegetation diversity confer microclimatic advantages to some animal populations in the face of climate change. Conservation of old-growth forests, or their characteristics in managed forests, could help slow the negative effects of climate warming on some breeding bird populations via microclimate buffering and possibly insurance effects.

Warming in the land of the midnight sun: breeding birds may suffer greater heat stress at high- versus low-Arctic sites

Rising global temperatures are expected to increase reproductive costs for wildlife as greater thermoregulatory demands interfere with reproductive activities. However, predicting the temperatures at which reproductive performance is negatively impacted remains a significant hurdle. Using a thermoregulatory polygon approach, we derived a reproductive threshold temperature for an Arctic songbird-the snow bunting (Plectrophenax nivalis). We defined this threshold as the temperature at which individuals must reduce activity to suboptimal levels (i.e. less than four-time basal metabolic rate) to sustain nestling provisioning and avoid overheating. We then compared this threshold to operative temperatures recorded at high (82° N) and low (64° N) Arctic sites to estimate how heat constraints translate into site-specific impacts on sustained activity level. We predict buntings would become behaviourally constrained at operative temperatures above 11.7°C, whereupon they must reduce provisioning rates to avoid overheating. Low-Arctic sites had larger fluctuations in solar radiation, consistently producing daily periods when operative temperatures exceeded 11.7°C. However, high-latitude birds faced entire, consecutive days when parents would be unable to sustain required provisioning rates. These data indicate that Arctic warming is probably already disrupting the breeding performance of cold-specialist birds and suggests counterintuitive and severe negative impacts of warming at higher latitude breeding locations.

Complex associations of weather conditions with reproductive performance in urban population of a common waterbird

Weather conditions are recognized as one of key determinants of animal reproductive performance; however, the effect of weather on breeding success can be modulated by different features of breeding habitat. Constantly expanding urban areas cause significant changes in land cover and environmental conditions, but whether and how urban landscape mitigates weather impact on animal fitness remains little explored. The aim of this study was to investigate the association between weather parameters and reproductive performance in a reed-nesting waterbird species, the Eurasian coot Fulica atra. For this purpose, we performed a long-term monitoring of an urban coot population from central Poland, collecting data for over 400 breeding events. The results indicated that temperature may have contrasting effects on coot reproductive output at different stages of chick-rearing period (positive at early chick-rearing and negative at late chick-rearing). Also, contrary to our expectation, we found a positive relationship between mean daily precipitation in early chick-rearing period and reproductive output in our study population. Our study constitutes one of few examples showing how weather may affect fitness in urban wildlife and provides evidence for high complexity of associations between weather conditions and animal reproductive performance.

Collapse of Breeding Success in Desert-Dwelling Hornbills Evident Within a Single Decade

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 &gt; 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.

Experimental warming during incubation improves cold tolerance of Blue Tit (Cyanistes caeruleus) chicks

Climate change and increasing air temperature may alter environmental conditions for developing birds, with a range of phenotypic consequences for offspring. The thermal environment during incubation may affect the trade-off between growth and thermoregulation, but the effects of temperature on the ontogeny of endothermy are not fully understood. Therefore, we experimentally tested whether heating the nest cup of Eurasian blue tits (Cyanistes caeruleus) during incubation would influence cold tolerance of the chicks after hatching. Chicks from both heated and control nests showed a decrease in cooling rate with age as they became increasingly endothermic and homeothermic. However, chicks from previously heated nests cooled at a lower rate per unit surface area and from across the whole body. These chicks also had a greater body mass during the first 12 days of life compared with chicks from control nests. Lower cooling rates in heated chicks may reflect greater thermogenic capacity or a reduced surface area to volume ratio owing to a greater body mass. Future projections for climate change predict rising air temperature and increased likelihood of heatwaves, even in temperate regions. Our results indicate that nest microclimate can affect thermoregulation in offspring, and thus may be used to predict some of the future physiological responses of birds to climate change during breeding.

Summary: Chicks from experimentally heated nests demonstrate greater cold tolerance compared with control chicks when exposed to a series of post-hatch cooling challenges.

Climate alters the movement ecology of a non-migratory bird

Global climate change is causing increased climate extremes threatening biodiversity and altering ecosystems. Climate is comprised of many variables including air temperature, barometric pressure, solar radiation, wind, relative humidity, and precipitation that interact with each other. As movement connects various aspects of an animal's life, understanding how climate influences movement at a fine-temporal scale will be critical to the long-term conservation of species impacted by climate change. The sedentary nature of non-migratory species could increase some species risk of extirpation caused by climate change. We used Northern Bobwhite (Colinus virginianus; hereafter bobwhite) as a model to better understand the relationship between climate and the movement ecology of a non-migratory species at a fine-temporal scale. We collected movement data on bobwhite from across western Oklahoma during 2019-2020 and paired these data with meteorological data. We analyzed movement in three different ways (probability of movement, hourly distance moved, and sinuosity) using two calculated movement metrics: hourly movement (displacement between two consecutive fixes an hour apart) and sinuosity (a form of tortuosity that determines the amount of curvature of a random search path). We used generalized linear-mixed models to analyze probability of movement and hourly distance moved, and used linear-mixed models to analyze sinuosity. The interaction between air temperature and solar radiation affected probability of movement and hourly distance moved. Bobwhite movement increased as air temperature increased beyond 10°C during low solar radiation. During medium and high solar radiation, bobwhite moved farther as air temperature increased until 25-30°C when hourly distance moved plateaued. Bobwhite sinuosity increased as solar radiation increased. Our results show that specific climate variables alter the fine-scale movement of a non-migratory species. Understanding the link between climate and movement is important to determining how climate change may impact a species' space use and fitness now and in the future.

Top M, Nulit R, Puan CL. Influence of Climatic Factors and Nest Tree Characteristics on the Nest Structures of the Baya Weaver (Ploceus philippinus) in Peninsular Malaysia

Simple Summary

The baya weavers of South and Southeast Asia, living in colonies, build overhanging nests in partially and fully completed forms on the same trees. The female birds will only select certain nests in which to complete breeding, with nest structure linked to the nest tree characteristics and the surrounding environment. We examined nest structure, nest tree characteristics, and climatic variables for colonies found in two climatically dissimilar sites in Peninsular Malaysia. Our results indicate that nest structure is linked to nest microclimate, possibly being influenced by the surrounding temperature.

Abstract

The baya weaver (Ploceus philippinus: Ploceidae), found across South and Southeast Asia, is known for its oblique-shaped overhanging nests. During the breeding season, females select from partially built (‘helmet’-stage) nests constructed by the males, after which nests are completed and used by the birds. Reproductive success is linked to an optimal microclimate within these nest structures. We recorded nest tree and nest structure characteristics of 66 fully completed nests for 22 colonies located in two climatically dissimilar sites in Peninsular Malaysia and examined how these factors affected the microclimate within six nests that were randomly selected at each location. Total vertical length of the nests, the height of nests from the ground, and the diameter at breast height of the nest trees recorded for the Selangor and Perlis colonies (in the southwest and north, respectively), were significantly different. The climatic variables inside and outside the nests correlated significantly in both sites. Our findings indicate that nest structures differed in Selangor and Perlis and were linked to nest microclimate, possibly being influenced by the surrounding temperature.

How to Stay Cool: Early Acoustic and Thermal Experience Alters Individual Behavioural Thermoregulation in the Heat

Climate change is pushing organisms closer to their physiological limits. Animals can reduce heat exposure – and the associated risks of lethal hyperthermia and dehydration – by retreating into thermal refuges. Refuge use nonetheless reduces foraging and reproductive activities, and thereby potentially fitness. Behavioural responses to heat thus define the selection pressures to which individuals are exposed. However, whether and why such behavioural responses vary between individuals remains largely unknown. Here, we tested whether early-life experience generates inter-individual differences in behavioural responses to heat at adulthood. In the arid-adapted zebra finch, parents incubating at high temperatures emit “heat-calls,” which adaptively alter offspring growth. We experimentally manipulated individual early life acoustic and thermal experience. At adulthood, across two summers, we then repeatedly recorded individual panting behaviour, microsite use, activity (N = 2,402 observations for 184 birds), and (for a small subset, N = 23 birds) body temperature, over a gradient of air temperatures (26–38°C), in outdoor aviaries. We found consistent inter-individual variation in behavioural thermoregulation, and show for the first time in endotherms that early-life experience contributes to such variation. Birds exposed prenatally to heat-calls started panting at lower temperatures than controls but panted less at high temperatures. It is possible that this corresponds to a heat-regulation strategy to improve water saving at high temperature extremes, and/or, allow maintaining high activity levels, since heat-call birds were also more active across the temperature gradient. In addition, microsite use varied with the interaction between early acoustic and thermal experiences, control-call birds from cooler nests using the cooler microsite more than their hot-nest counterparts, whereas the opposite pattern was observed in heat-call birds. Overall, our study demonstrates that a prenatal acoustic signal of heat alters how individuals adjust behaviourally to thermal challenges at adulthood. This suggests that there is scope for selection pressures to act differently across individuals, and potentially strengthen the long-term fitness impact of early-life effects.

Disentangling climatic and nest predator impact on reproductive output reveals adverse high-temperature effects regardless of helper number in an arid-region cooperative bird

Climate exerts a major influence on reproductive processes, and an understanding of the mechanisms involved and which factors might mitigate adverse weather is fundamental under the ongoing climate change. Here, we study how weather and nest predation influence reproductive output in a social species, and examine whether larger group sizes can mitigate the adverse effects of these factors. We used a 7‐year nest predator‐exclusion experiment on an arid‐region cooperatively breeding bird, the sociable weaver. We found that dry and, especially, hot weather were major drivers of nestling mortality through their influence on nest predation. However, when we experimentally excluded nest predators, these conditions were still strongly associated with nestling mortality. Group size was unimportant against nest predation and, although positively associated with reproductive success, it did not mitigate the effects of adverse weather. Hence, cooperative breeding might have a limited capacity to mitigate extreme weather effects.

Experimental Manipulation of Air Temperature in Captivity Appears Unsuitable for Evaluating Fecal Glucocorticoid Metabolite Responses of Wild-Caught Birds to Heat Exposure

AbstractNoninvasive measurement of stress-related alterations in fecal glucocorticoid metabolite (fGCM) concentrations has considerable potential for quantifying physiological responses to very hot weather in free-ranging birds, but practical considerations related to sampling will often make this method feasible only for habituated study populations. Here we evaluate an alternate approach, the use of experimentally manipulated thermal environments for evaluating stress responses to high environmental temperatures in wild-caught birds housed in captivity. Using an enzyme immunoassay utilizing antibodies against 5ß-pregnane-3α,11ß,21-triol-20-one-CMO∶BSA (tetrahydrocorticosterone), we quantified fGCMs in captive individuals of three southern African arid-zone species (southern pied babblers [Turdoides bicolor], white-browed sparrow-weavers [Plocepasser mahali], and southern yellow-billed hornbills [Tockus leucomelas]) experiencing daily air temperature maxima (T_max) ranging from 30°-32°C to 42°-44°C. For none of the three species did T_max emerge as a significant predictor of elevated fGCM concentrations, and no stress response to simulated hot weather was evident. The apparent lack of a stress response to T_max = 42°C in captive southern pied babblers contrasts with linear increases in fGCMs at T_max > 38°C in free-ranging conspecifics. The lack of an effect of T_max on fGCM levels may potentially be explained by several factors, including differences in operative temperatures and the availability of water and food between free-ranging and captive settings or the stress effect of captivity itself. Our results suggest that experimental manipulations of thermal environments experienced by wild-caught captive birds have limited usefulness for testing hypotheses concerning the effects of hot weather events on fGCM (and, by extension, glucocorticoid) concentrations.

Direct and indirect effects of high temperatures on fledging in a cooperatively breeding bird

High temperatures and low rainfall consistently constrain reproduction in arid-zone bird species. Understanding the mechanisms underlying this pattern is critical for predicting how climate change will influence population persistence and to inform conservation and management. In this study, we analyzed Southern Pied Babbler Turdoides bicolor nestling survival, daily growth rate and adult investment behavior during the nestling period over three austral summer breeding seasons. High temperatures were associated with lower body mass, shorter tarsi, and reduced daily growth rates of nestlings. Our piecewise structural equation models suggested that direct impacts of temperature had the strongest influence on nestling size and daily growth rates for both 5-day-old and 11-day-old nestlings, followed by temperature-related adjustments to provisioning rates by adults. Rainfall and group size influenced the behavior of provisioning adults but did not influence nestling growth or survival. Adjustments to adult provisioning strategies did not compensate for direct negative effects of high air temperatures on nestling size or daily growth rates. Detailed mechanistic data like these allow us to model the pathways by which high temperature causes nest failure. In turn, this could allow us to design targeted conservation action to effectively mitigate climate effects.

Hot days are associated with short-term adrenocortical responses in a southern African arid-zone passerine bird

Relatively little effort has been directed towards elucidating the role of physiological stress pathways in mediating avian responses to global heating. For free-ranging southern pied babblers, Turdoides bicolor, daily maximum air temperatures (Tmax) between ∼35 and ∼40°C result in reduced foraging efficiency, loss of body mass and compromised breeding success. We tested the hypothesis that very hot days are experienced as stressors by quantifying relationships between Tmax and faecal glucocorticoid metabolite (fGCM) levels in naturally excreted droppings. On days when Tmax<38°C, fGCM levels were independent of Tmax (mean±s.d. 140.25±56.92 ng g-1 dry mass). At Tmax>38°C, however, fGCM levels increased linearly with Tmax and averaged 190.79±70.13 ng g-1 dry mass. The effects of Tmax on fGCM levels did not carry over to the following morning, suggesting that very hot days are experienced as acute stressors.

Flexible breeding performance under unstable climatic conditions in a tropical passerine in Southwest China

Parents may adjust their breeding time to optimize reproductive output and reduce reproductive costs associated with unpredictable climatic conditions, especially in the context of global warming. The breeding performance of tropical bird species in response to local climate change is relatively understudied compared with that of temperate bird species. Here, based on data from 361 white-rumped munia (Lonchura striata) nests, we determined that breeding season onset, which varied from 15 February to 22 June, was delayed by drought and high temperatures. Clutch size (4.52±0.75) and daily survival rate but not egg mass (0.95±0.10 g) were negatively affected by frequent rainfall. Daily nest survival during the rainy breeding season in 2018 (0.95±0.04) was lower than that in 2017 (0.98±0.01) and 2019 (0.97±0.00). The overall nesting cycle was 40.37±2.69 days, including an incubation period of 13.10±1.18 days and nestling period of 23.22±2.40 days. The nestling period in 2018 (25.11±1.97 days) was longer than that in 2017 (22.90±2.22 days) and 2019 (22.00±2.48 days), possibly due to the cooler temperatures. Climate also affected the total number of successful fledglings, which was highest under moderate rainfall in 2017 (115 fledglings) and lowest during prolonged drought in 2019 (51 fledglings). Together, our results suggest that drought and frequent rainfall during the breeding season can decrease reproductive success. Thus, this study provides important insights into bird ecology and conservation in the context of global climate change.

Thermoregulation in desert birds: scaling and phylogenetic variation in heat tolerance and evaporative cooling

Evaporative heat dissipation is a key aspect of avian thermoregulation in hot environments. We quantified variation in avian thermoregulatory performance at high air temperatures (T _a) using published data on body temperature (T _b), evaporative water loss (EWL) and resting metabolic rate (RMR) measured under standardized conditions of very low humidity in 56 arid-zone species. Maximum T _b during acute heat exposure varied from 42.5±1.3°C in caprimulgids to 44.5±0.5°C in passerines. Among passerines, both maximum T _b and the difference between maximum and normothermic T _b decreased significantly with body mass (M _b). Scaling exponents for minimum thermoneutral EWL and maximum EWL were 0.825 and 0.801, respectively, even though evaporative scope (ratio of maximum to minimum EWL) varied widely among species. Upper critical limits of thermoneutrality (T _uc) varied by >20°C and maximum RMR during acute heat exposure scaled to M _b ^0.75 in both the overall data set and among passerines. The slope of RMR at T _a>T _uc increased significantly with M _b but was substantially higher among passerines, which rely on panting, compared with columbids, in which cutaneous evaporation predominates. Our analysis supports recent arguments that interspecific within-taxon variation in heat tolerance is functionally linked to evaporative scope and maximum ratios of evaporative heat loss (EHL) to metabolic heat production (MHP). We provide predictive equations for most variables related to avian heat tolerance. Metabolic costs of heat dissipation pathways, rather than capacity to increase EWL above baseline levels, appear to represent the major constraint on the upper limits of avian heat tolerance.

How hornbills handle heat: sex-specific thermoregulation in the southern yellow-billed hornbill

At a global scale, thermal physiology is correlated with climatic variables such as temperature and aridity. There is also evidence that thermoregulatory traits vary with fine-scale microclimate, but this has received less attention in endotherms. Here, we test the hypothesis that avian thermoregulation varies with microclimate and behavioural constraints in a non-passerine bird. Male and female southern yellow-billed hornbills (Tockus leucomelas) experience markedly different microclimates while breeding, with the female sealing herself into a tree cavity and moulting all her flight feathers during the breeding attempt, becoming entirely reliant on the male for provisioning. We examined interactions between resting metabolic rate (RMR), evaporative water loss (EWL) and core body temperature (T _b) at air temperatures (T _a) between 30°C and 52°C in male and female hornbills, and quantified evaporative cooling efficiencies and heat tolerance limits. At thermoneutral T _a, neither RMR, EWL nor T _b differed between sexes. At T _a >40°C, however, RMR and EWL of females were significantly lower than those of males, by ∼13% and ∼17%, respectively, despite similar relationships between T _b and T _a, maximum ratio of evaporative heat loss to metabolic heat production and heat tolerance limits (∼50°C). These sex-specific differences in hornbill thermoregulation support the hypothesis that avian thermal physiology can vary within species in response to fine-scale microclimatic factors. In addition, Q ₁₀ for RMR varied substantially, with Q ₁₀ ≤2 in some individuals, supporting recent arguments that active metabolic suppression may be an underappreciated aspect of endotherm thermoregulation in the heat.

Dehydration risk is associated with reduced nest attendance and hatching success in a cooperatively breeding bird, the southern pied babbler Turdoides bicolor

High air temperatures have measurable negative impacts on reproduction in wild animal populations, including during incubation in birds. Understanding the mechanisms driving these impacts requires comprehensive knowledge of animal physiology and behaviour under natural conditions. We used a novel combination of a non-invasive doubly labelled water (DLW) technique, nest temperature data and field-based behaviour observations to test effects of temperature, rainfall and group size on physiology and behaviour during incubation in southern pied babblers Turdoides bicolor, a cooperatively breeding passerine endemic to the arid savanna regions of southern Africa. The proportion of time that clutches were incubated declined as air temperatures increased, a behavioural pattern traditionally interpreted as a benefit of ambient incubation. However, we show that (i) clutches had a <50% chance of hatching when exposed to daily maximum air temperatures of >35.3°C; (ii) pied babbler groups incubated their nests almost constantly (99% of daylight hours) except on hot days; (iii) operative temperatures in unattended nests frequently exceeded 40.5°C, above which bird embryos are at risk of death; (iv) pied babblers incubating for long periods of time failed to maintain water balance on hot days; and (v) pied babblers from incubating groups lost mass on hot days. These results suggest that pied babblers might leave their nests during hot periods to lower the risk of dehydration associated with prolonged incubation at high operative temperatures. As mean air temperatures increase and extreme heat events become more frequent under climate change, birds will likely incur ever greater thermoregulatory costs of incubation, leading to compromised nest attendance and increased potential for eggs to overheat, with implications for nest success and, ultimately, population persistence.

Not Singing in the Rain: Linking Migratory Songbird Declines With Increasing Precipitation and Brood Parasitism Vulnerability

Few empirical studies have quantified relationships between changing weather and migratory songbirds, but such studies are vital in a time of rapid climate change. Climate change has critical consequences for avian breeding ecology, geographic ranges, and migration phenology. Changing precipitation and temperature patterns affect habitat, food resources, and other aspects of birds’ life history strategies. Such changes may disproportionately affect species confined to rare or declining ecosystems, such as temperate grasslands, which are among the most altered and endangered ecosystems globally. We examined the influence of changing weather on the dickcissel (Spiza americana), a migratory songbird of conservation concern that is an obligate grassland specialist. Our study area in the North American Great Plains features high historic weather variability, where climate change is now driving higher precipitation and temperatures as well as higher frequencies of extreme weather events including flooding and droughts. Dickcissels share their breeding grounds with brown-headed cowbirds (Molothrus ater), brood parasites that lay their eggs in the nests of other songbirds, reducing dickcissel productivity. We used 9 years of capture-recapture data collected over an 18-year period to test the hypothesis that increasing precipitation on dickcissels’ riparian breeding grounds is associated with abundance declines and increasing vulnerability to cowbird parasitism. Dickcissels declined with increasing June precipitation, whereas cowbirds, by contrast, increased. Dickcissel productivity appeared to be extremely low, with a 3:1 ratio of breeding male to female dickcissels likely undermining reproductive success. Our findings suggest that increasing precipitation predicted by climate change models in this region may drive future declines of dickcissels and other songbirds. Drivers of these declines may include habitat and food resource loss related to flooding and higher frequency precipitation events as well as increased parasitism pressure by cowbirds. Positive correlations of June-July precipitation, temperature, and time since grazing with dickcissel productivity did not mitigate dickcissels’ declining trend in this ecosystem. These findings highlight the importance of empirical research on the effects of increasing precipitation and brood parasitism vulnerability on migratory songbird conservation to inform adaptive management under climate change.

Experimental evidence that hyperthermia limits offspring provisioning in a temperate-breeding bird

In many vertebrates, parental care can require long bouts of daily exercise that can span several weeks. Exercise, especially in the heat, raises body temperature, and can lead to hyperthermia. Typical strategies for regulating body temperature during endurance exercise include modifying performance to avoid hyperthermia (anticipatory regulation) and allowing body temperature to rise above normothermic levels for brief periods of time (facultative hyperthermia). Facultative hyperthermia is commonly employed by desert birds to economize on water, but this strategy may also be important for chick-rearing birds to avoid reducing offspring provisioning when thermoregulatory demands are high. In this study, we tested how chick-rearing birds balance their own body temperature against the need to provision dependent offspring. We experimentally increased the heat dissipation capacity of breeding female tree swallows (Tachycineta bicolor) by trimming their ventral feathers and remotely monitored provisioning rates, body temperature and the probability of hyperthermia. Birds with an experimentally increased capacity to dissipate heat (i.e. trimmed treatment) maintained higher feeding rates than controls at high ambient temperatures (greater than or equal to 25°C), while maintaining lower body temperatures. However, at the highest temperatures (greater than or equal to 25°C), trimmed individuals became hyperthermic. These results provide evidence that chick-rearing tree swallows use both anticipatory regulation and facultative hyperthermia during endurance performance. With rising global temperatures, individuals may need to increase their frequency of facultative hyperthermia to maintain nestling provisioning, and thereby maximize reproductive success.

High temperatures drive offspring mortality in a cooperatively breeding bird

An improved understanding of life-history responses to current environmental variability is required to predict species-specific responses to anthopogenic climate change. Previous research has suggested that cooperation in social groups may buffer individuals against some of the negative effects of unpredictable climates. We use a 15-year dataset on a cooperative breeding arid zone bird, the southern pied babbler Turdoides bicolor, to test (i) whether environmental conditions and group size correlate with survival of young during three development stages (egg, nestling, fledgling) and (ii) whether group size mitigates the impacts of adverse environmental conditions on survival of young. Exposure to high mean daily maximum temperatures (mean T_max) during early development was associated with reduced survival probabilities of young in all three development stages. No young survived when mean T_max > 38°C, across all group sizes. Low survival of young at high temperatures has broad implications for recruitment and population persistence in avian communities given the rapid pace of advancing climate change. Impacts of high temperatures on survival of young were not moderated by group size, suggesting that the availability of more helpers in a group is unlikely to buffer against compromised offspring survival as average and maximum temperatures increase with rapid anthropogenic climate change.

High temperatures drive offspring mortality in a cooperatively breeding bird

An improved understanding of life-history responses to current environmental variability is required to predict species-specific responses to anthopogenic climate change. Previous research has suggested that cooperation in social groups may buffer individuals against some of the negative effects of unpredictable climates. We use a 15-year dataset on a cooperative breeding arid zone bird, the southern pied babbler Turdoides bicolor, to test (i) whether environmental conditions and group size correlate with survival of young during three development stages (egg, nestling, fledgling) and (ii) whether group size mitigates the impacts of adverse environmental conditions on survival of young. Exposure to high mean daily maximum temperatures (mean T_max) during early development was associated with reduced survival probabilities of young in all three development stages. No young survived when mean T_max > 38°C, across all group sizes. Low survival of young at high temperatures has broad implications for recruitment and population persistence in avian communities given the rapid pace of advancing climate change. Impacts of high temperatures on survival of young were not moderated by group size, suggesting that the availability of more helpers in a group is unlikely to buffer against compromised offspring survival as average and maximum temperatures increase with rapid anthropogenic climate change.

Avian mortality risk during heat waves will increase greatly in arid Australia during the 21st century

Intense heat waves are occurring more frequently, with concomitant increases in the risk of catastrophic avian mortality events via lethal dehydration or hyperthermia. We quantified the risks of lethal hyperthermia and dehydration for 10 Australian arid-zone avifauna species during the 21st century, by synthesizing thermal physiology data on evaporative water losses and heat tolerance limits. We evaluated risks of lethal hyperthermia or exceedance of dehydration tolerance limits in the absence of drinking during the hottest part of the day under recent climatic conditions, compared to those predicted for the end of this century across Australia. Increases in mortality risk via lethal dehydration and hyperthermia vary among the species modelled here but will generally increase greatly, particularly in smaller species (~10-42 g) and those inhabiting the far western parts of the continent. By 2100 CE, zebra finches' potential exposure to acute lethal dehydration risk will reach ~ 100 d y^-1 in the far northwest of Australia and will exceed 20 d y^-1 over > 50% of this species' current range. Risks of dehydration and hyperthermia will remain much lower for large non-passerines such as crested pigeons. Risks of lethal hyperthermia will also increase substantially for smaller species, particularly if they are forced to visit exposed water sources at very high air temperatures to avoid dehydration. An analysis of atlas data for zebra finches suggests that population declines associated with very hot conditions are already occurring in the hottest areas. Our findings suggest that the likelihood of persistence within current species ranges, and the potential for range shifts, will become increasingly constrained by temperature and access to drinking water. Our model adds to an increasing body of literature suggesting that arid environments globally will experience considerable losses of avifauna and biodiversity under unmitigated climate change scenarios.