If You Can’t Take the Heat: Evaluating Thermoregulatory Behaviors Used by Birds

If You Can’t Take the Heat: Evaluating Thermoregulatory Behaviors Used by Birds | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 1 February 2025 V1 Latest version Share on If You Can’t Take the Heat: Evaluating Thermoregulatory Behaviors Used by Birds Authors : Emmy James 0009-0004-1962-5657 [email protected] and Liz Derryberry Authors Info & Affiliations https://doi.org/10.22541/au.173844726.60187268/v1 564 views 306 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract As global temperatures rise, many animals encounter longer and more intense heat. Behavioral thermoregulation offers a potential means of staying cool, but there is no clear consensus on how to evaluate when thermoregulatory behaviors are actually working. Here, we examine the efficacy of thermoregulatory behaviors in birds. We begin by broadening the definition of avian thermoregulatory behaviors to include actions related to self-maintenance, parental care, and social interactions. We then examine the limitations of these behaviors, focusing on ecological tradeoffs and synergies that constrain their efficacy. Finally, we review the methodologies used to test these behavioral strategies, considering their effectiveness at both proximate and ultimate levels. By examining the role of thermoregulatory behaviors across ecological contexts, we aim to illuminate their potential and limitations as shields in a warming world and highlight avenues for future research. ABSTRACT As global temperatures rise, many animals encounter longer and more intense heat. Behavioral thermoregulation offers a potential means of staying cool, but there is no clear consensus on how to evaluate when thermoregulatory behaviors are actually working . Here, we examine the efficacy of thermoregulatory behaviors in birds. We begin by broadening the definition of avian thermoregulatory behaviors to include actions related to self-maintenance, parental care, and social interactions. We then examine the limitations of these behaviors, focusing on ecological tradeoffs and synergies that constrain their efficacy. Finally, we review the methodologies used to test these behavioral strategies, considering their effectiveness at both proximate and ultimate levels. By examining the role of thermoregulatory behaviors across ecological contexts, we aim to illuminate their potential and limitations as shields in a warming world and highlight avenues for future research. Keywords: temperature, buffer, global warming, behavioral inertia, Bogert Effect, competition INTRODUCTION Climate change is accelerating, with global temperatures becoming warmer and more variable every day (Vasseur et al. 2014, IPCC 2021). To cope with this rapid warming, many animals rely on behavior to mitigate heat. By spending more time in the shade or adopting cooling postures, individuals can reduce the risk of lethal heat exposure (Weiss and Laties 1961, Kearney et al. 2009). These behaviors play a key role in thermoregulation at a proximate level. However, they also affect the evolution of thermal tolerance: behavioral thermoregulation can reduce selection pressure on physiological responses to heat (Bogert 1949, Huey et al. 2003, Muñoz 2022). Thus, behaviors that enhance heat dissipation allow organisms to respond to warming on both immediate and evolutionary timescales. Despite their utility, behavioral “shields” or “buffers” are not impenetrable. Environmental constraints, such as habitat homogeneity, can limit opportunities for thermoregulatory behavior (Logan et al. 2019) – animals can’t seek shade in environments devoid of shady spots. Additionally, thermoregulatory behaviors often involve tradeoffs. For example, a nest must be large enough to incubate offspring but small enough to minimize predation risk (Perez et al. 2020). Such constraints highlight the challenges of relying on behavior alone to combat rapid environmental change (Beever et al. 2017). Yet, despite their importance, there is no consensus on how to evaluate the effectiveness of thermoregulatory behaviors in mitigating heat stress. In this mini-review, we examine methods used to test the strength of behavioral shields, focusing on birds—a group with well-documented thermoregulatory behaviors across diverse ecological contexts (Beever et al. 2017). First, we extend the definition of thermoregulatory behavior to include behaviors that control the thermal environment of offspring or conspecifics. Next, we explore the limitations of behavioral shields, particularly their dependence on spatial and ecological contexts. Lastly, we synthesize current methods for evaluating behavioral shields at both proximate and ultimate levels. By addressing these questions, we aim to clarify the potential and limitations of behavioral strategies in a rapidly warming world and highlight avenues for future research. BIRD BEHAVIOR AS AN ARCHITECT OF THE THERMAL ENVIRONMENT For this mini-review, we define a thermoregulatory behavior as a whole-organism response aimed at maintaining thermal homeostasis, encompassing both autonomic (involuntary) and somatic (voluntary) actions. Our focus is on behaviors that involve interactions between an individual and the external environment – for instance, panting, which facilitates gas exchange with cooler surrounding air. Most reviews emphasize thermoregulatory behaviors associated with adult self-maintenance, where an individual uses behavior to directly manage the thermal environment it experiences (Stillman 2019, Cunningham et al. 2021, Blackburn et al. 2024). Such behaviors often involve immediate heat regulation strategies, including moving between warm and cool microclimates, adopting cooling postures, exposing thermal windows, or panting to dissipate heat (Blackburn et al. 2024). These actions enable individuals to lower their body temperature and buffer the immediate effects of intense heat. Over evolutionary timescales, behavioral mechanisms may also adapt to tolerate heat. For example, populations in warmer climates often shift activity patterns toward nocturnality, reducing exposure to high midday temperatures (O’Connor et al. 2018, Levy et al. 2019, Brivio et al. 2024). Thus, both immediate and long-term behavioral adjustments play vital roles in helping animals cope with sublethal heat. Not all thermoregulatory behaviors are related to self-maintenance (Fig. 1). Across the animal kingdom, behaviors are often used to regulate the thermal environment of offspring. Nests provide a clear example: parents invest time and energy into constructing nests, with variations in size, shape, and material influencing the thermal conditions experienced by offspring (reviewed by Perez et al. 2020). Additional behaviors, such as time spent incubating (DuRant et al. 2019), timing of egg-laying (Both and Visser 2005), or wing-shading (Brown and Downs 2003), further modify the temperatures experienced by offspring. Similarly, individuals may use behavior to buffer conspecifics from thermal extremes. Often considered as a shield against cold temperatures, huddling or group roosting can change the thermal conditions experienced by conspecifics (Gilbert et al. 2010). In these cases, thermoregulatory behaviors may confer ecological benefits that extend beyond the individual performing them to offspring or social groups, but these are often overlooked when discussing individual responses to climate warming. Though many studies focus on adults, thermoregulatory behaviors have also been described at early life stages. For instance, chicks pant and move throughout the nest to access cool microclimates (Woodruff et al. 2025). Other means of behavioral thermoregulation have been described by poultry scientists monitoring captive chicks (e.g. Rovee-Collier et al. 1997), illustrating the importance of thermoregulatory behaviors at early life stages. Given that birds may occupy diverse habitats throughout their lifetimes, selection on thermal tolerance may also vary across life stages (Schou and Cornwallis 2024). However, few studies examine thermoregulatory behavior in chicks, leaving the degree of variation unclear. Behavioral thermoregulation is a crucial mechanism for heat tolerance, both in birds and across the broader animal kingdom. However, emerging research reveals imperfections in these behavioral shields, highlighting constraints on their effectiveness as defenses against heat. CONSTRAINTS ON THERMOREGULATORY BEHAVIORS Landscape Heterogeneity In some populations, the physical landscape can significantly constrain opportunities for behavioral thermoregulation (Sears and Angilletta 2015, Logan et al. 2019, Muñoz 2022). Many animals rely on moving between warm and cool microclimates to dissipate heat. However, when the environment is predominantly sunny with little shade (i.e. homogeneous), the potential for behavioral thermoregulation diminishes. Indeed, this trend is particularly evident in ectotherms, where reduced thermal variation among accessible microclimates correlates with less frequent use of behavioral thermoregulation (Caillon et al. 2014, Pincebourde and Suppo 2016, Logan et al. 2019, Chan et al. 2024). Studies of endotherms have similarly begun to document the importance of thermal refugia, illustrating that environmental heterogeneity is critical for effective behavioral thermoregulation (Pattinson and Smit 2017, Raynor et al. 2018, Milling et al. 2018, Olsen et al. 2018, Verzuh et al. 2021, 2023, Sharpe et al. 2022). Across taxa, the availability of thermal heterogeneity is a key determinant of thermoregulatory behavior. Refuge Access and Competition Even when thermal refugia are present, they may not always be accessible. Limited availability of shady microclimates can intensify competition, as observed in birds (Bourne and Soravia 2023). This suggests that environmental heterogeneity may not consistently predict the use of behavioral thermoregulation, at least for all individuals. Competitive interactions introduce additional constraints: dominant individuals may be able to monopolize cooler microclimates, maintaining lower body temperatures and thermoregulating more effectively than subordinates (Cunningham et al. 2017). Dominance hierarchies, long studied in the context of communal roosting (Swingland 1977, Feare et al. 1995, Yackel Adams et al. 2000, Calf et al. 2002, McKechnie et al. 2006, McGowan et al. 2006, Lu and Zheng 2007, van Dijk et al. 2013, Xu et al. 2013), likely mediate access to thermal refugia, though experimental evidence in the context of heat is limited. Taken together, variation in microsite availability and social dynamics may shape the effectiveness of behavioral thermoregulation, altering the selective pressures experienced by populations. For species that utilize cool microclimates for heat dissipation, an individual must be able to both acquire and maintain a cool microclimate to thermoregulate successfully. Thus, selection would favor traits associated with dominance and the ability to access cool microclimates (Muñoz and Losos 2018). For example, auditory or visual cues of territorial defense may become doubly important when individuals are defending a thermal resource. The evolution of other traits associated with dominance, such as a larger body size, would also accelerate if they provide a competitive edge in a warmer world. When thermoregulation is constrained by dominance, traits that confer competitive ability should evolve in parallel with behavior. To our knowledge, however, no studies have examined this pattern in birds. Fortunately, behavior is not the only avenue for thermal tolerance. Animals also rely on morphological and physiological traits to regulate body temperature (e.g. Tattersall et al. 2018, Logan et al. 2019, Vianna et al. 2020, Alattal 2024), often in synergy with behavioral strategies. For instance, morphological adaptations, such as a large bill, can enhance passive evaporative cooling, a physiological response to heat, which can be accelerated by panting, a behavioral response (for recent examples in birds, see Oswald and Arnold 2012, Ryeland et al. 2017, Playà-Montmany et al. 2021). However, when traits serve multiple ecological functions, selective forces may conflict, constraining the effectiveness of behavioral thermoregulation and generating tradeoffs. Ecological Tradeoffs Some tradeoffs are direct. For example, panting is associated with mass loss in both adult (Pessato et al. 2020) and altricial (Woodruff et al. 2025) songbirds. Other tradeoffs, such as those associated with heat-conserving postures, include slower response times to approaching predators (Carr and Lima 2012, Yorzinski et al. 2018, Timmis et al. 2022). A larger body size, which may help an individual defend cool microclimates, may come with its own ecological tradeoffs (Weathers 1981). Opportunity costs also arise when animals must forgo foraging or caring for young to thermoregulate (see du Plessis et al. 2012, Cunningham et al. 2015, 2021, Mason et al. 2017, van de Ven et al. 2019, 2020). Chicks in nests may also experience these opportunity costs. In cooler environments, spending time near the entrance of the cavity may allow better access to food while also exposing chicks to cold temperatures. Tradeoffs relevant for adults may also be more extreme for chicks. For example, mass at fledging is the strongest predictor of future survival and fecundity in tree swallows ( Tachycineta bicolor ) (McCarty 2001). Panting, which leads to mass loss in adults, may be even riskier at earlier life stages. If tradeoffs lead to reduced survival or reproductive success, natural selection may favor individuals who rely less on behavioral thermoregulation, altering the population’s response to heat. Parental and social contexts introduce additional complexities. Adults may face competing demands of thermoregulating themselves or caring for offspring, particularly when nest visits expose them to higher temperatures and require energy expenditure (Wiley and Ridley 2016, Cook et al. 2020, Tapper et al. 2020, Oswald et al. 2021, Heldbjerg et al. 2022). Nest construction also presents tradeoffs (Mainwaring et al. 2014, Perez et al. 2020). Though exposed nests may help keep offspring warm in cool climates, they may also increase the threat of predation, prompting shifts toward building nests in colder, more concealed microclimates (Eggers et al. 2005, Lima 2009, but see Prokop and Trnka 2011). Nest material may also complicate thermoregulation. In warm climates, piping plovers ( Charadrius melodus ) select light-colored nest materials to minimize heat absorption, but this can make offspring more conspicuous to predators (Mayer et al. 2009). These tradeoffs may also arise in group dynamics. In communal roosts, position is often governed by dominance, with individuals in the middle of the roost maintaining the warmest body temperatures (Calf et al. 2002, but see Gilbert et al. 2006). However, position may also be associated with predation risk (McGowan et al. 2006, Xu et al. 2013), lessening the thermoregulatory benefits of communal roosting. These competing selective forces may slow the evolution of thermoregulatory behaviors, even in populations vulnerable to heat. Ecological Synergies In some cases, behaviors fulfill multiple ecological functions, creating synergies. For instance, adding feathers to nests can increase nest temperature, attract mates, and improve nestling immunity, although these effects vary across taxa (Lombardo et al. 1995, Peralta-Sanchez et al. 2010, Deeming et al. 2020, but also see Mainwaring et al. 2016). Similarly, incubation behavior keeps eggs warm and reduces predation risk (Smith et al. 2012, Basso and Richner 2015). Less is known about the synergies of heat dissipation behaviors. When experiencing heat, chicks in cavity nests move toward the entrance of the cavity to access cool air (Woodruff et al. 2025), which may also increase the likelihood of being fed by parents or detecting nearby predators. Likewise, sheltering in cool, shaded microclimates may assist with predator evasion. Synergies may also arise in interactions with conspecifics. Social species, who may derive ecological benefits from cooperative breeding or communal roosting, may also face fewer territorial disputes than less social taxa when sharing shady microclimates. Such ecological synergies can intensify selection on these behaviors, potentially driving their rapid evolution compared to behaviors that serve a purely thermoregulatory function. Thermoregulatory behaviors are constrained by countless ecological factors, generating tradeoffs (and occasional synergies). If behavioral shields are stronger in some contexts than others, we need clear, consistent methods for evaluating their effectiveness to determine how well populations can respond to rapid warming. METHODS OF EVALUATING SHIELDS VARY ACCORDING TO CONTEXT To measure thermoregulation directly, researchers often rely on physiological metrics, with body temperature being the most common. In environments with fluctuating temperatures, successful thermoregulators maintain stable body temperatures compared to individuals who conform more closely to ambient conditions. Internal or surface body temperature is frequently measured in birds, with increasing accuracy and precision as technology improves (Nord et al. 2016). These data can complement measures of microclimate temperatures, which are often obtained through visual or GPS tracking, attached data loggers, or thermal imaging (Pollock et al. 2015, Arnold and Oswald 2018, van de Ven et al. 2019, Ramos et al. 2023). Together, these approaches help identify the realized thermal conditions experienced by birds, offering insights into their thermoregulatory strategies. While body temperature is a standard metric in thermoregulatory studies of ectotherms, its application to birds requires caution. Endotherms maintain body temperatures that are decoupled from ambient environmental temperatures; even when not engaging in behavioral thermoregulation, birds maintain a relatively constant body temperature (McKechnie and Wolf 2019, Levesque and Marshall 2021). Birds also exhibit regional heterothermy, where body temperature varies across different anatomical regions. As a result, the placement of sensors can influence data accuracy (McCafferty et al. 2015); different places on an individual’s body may yield different temperature readings (Andreasson et al. 2023, but also see Oswald et al. 2018). Thermal imaging provides a promising solution, enabling simultaneous measurements of surface temperature across multiple body regions (McCafferty 2013, Rogalla et al. 2021, Tabh et al. 2021, Zuluaga and Danner 2023). However, thermal imaging has limitations, particularly in capturing internal body temperatures of wild birds. Each method is useful and should be deployed according to the constraints of each investigation (McCafferty et al. 2015). Given the limitations of measuring endothermic body temperature, it may be valuable to supplement body temperature data with other simultaneous responses to heat. For example, body temperature data could be coupled with heat-shock protein (HSP) expression, a physiological response to heat in birds (Woodruff et al. 2023). If behavior completely shields an individual from heat, HSP expression should be low. If, however, behavioral thermoregulation works in tandem with physiological mechanisms, HSP expression may be largely unchanged by behavioral thermoregulation. Integrating physiological and behavioral measurements is crucial for identifying responses to heat, determining if behavior is truly a shield, or simply acting in tandem with other thermoregulatory mechanisms deployed by endotherms. Proxies of fitness may also indicate the strength of behavioral thermoregulation. When thermoregulatory behaviors aren’t working, the consequences of heat – adult mass loss or offspring mortality, for example – should be more apparent. Adult body mass is a widely used fitness proxy in avian studies (du Plessis et al. 2012, van de Ven et al. 2019). For behaviors linked to parental care, offspring quality may also be an important metric of success. Parental behaviors, such as nest site selection to shield offspring from heat, are hypothesized to improve offspring survival, though empirical evidence remains limited. Even without directly measuring body temperature, there are still opportunities to gauge the efficacy of behavioral thermoregulation. Lastly, the evolutionary consequences of thermoregulation offer a broader perspective on its efficacy. Effective thermoregulation should reduce selective pressures on other traits associated with thermal tolerance, thereby slowing their evolutionary rates (Bogert 1949, Huey et al. 2003). In ectotherms, critical thermal limits (CTmin and CTmax) are commonly used to assess these evolutionary patterns. In birds, recent studies have begun exploring taxon-level variations in CTmax as a proxy for thermoregulation (see McKechnie and Wolf 2019). Populations that rely heavily on behavioral thermoregulation are expected to experience reduced selection for physiological tolerance, resulting in lower CTmax values compared to populations without effective thermoregulatory behaviors—known as “The Bogert Effect” (Huey et al. 2003). Similarly, physiological traits like metabolic rate plasticity may reflect the strength of thermoregulation; populations with limited behavioral options often exhibit greater metabolic plasticity (McKechnie et al. 2007, Norin and Metcalfe 2019). Physiological trait evolution can reveal the strength of behavioral shields, showcasing the degree to which behavior mitigates a full-body response to heat. By integrating physiological, fitness-related, and evolutionary metrics, researchers can better assess the effectiveness of behavioral shields. These methods collectively demonstrate how thermoregulatory behaviors mitigate heat stress, offering insights into the role of behavior in maintaining thermal homeostasis and driving adaptation. FUTURE DIRECTIONS Though behavioral thermoregulation in birds is well-described, its efficacy as a shield against rapid warming remains uncertain. To address this gap, three key areas require further exploration. First, the definition of thermoregulatory behavior in birds must be expanded. Self-maintenance behaviors, such as panting and seeking shade, are crucial for managing heat. However, birds also modify the thermal environment experienced by young and by conspecifics. Examples of parental and social thermoregulation are often considered separately from more canonical examples of thermoregulation for self-maintenance. Isolating self-maintenance behaviors from parental care or group dynamics risks overlooking critical tradeoffs or synergies that may influence the overall strength of these behavioral shields. Additionally, thermoregulatory behaviors should be closely examined across life stages. Chicks use thermoregulatory behavior to cope with heat, which may impact adulthood fitness. Broadening the scope of research to integrate these components will provide a more comprehensive understanding of thermoregulatory strategies, as well as the selective forces that amplify or diminish them. Second, more empirical evidence is needed to evaluate how avian thermoregulatory behaviors affect proxies of fitness. While many observational studies document avian behavior in heat, experimental manipulations are essential for delineating the specific causes and consequences of these behaviors. Controlled experiments can uncover how behavioral thermoregulation impacts fitness and identify the environmental conditions under which these behaviors are most effective. Lastly, understanding the evolution of thermoregulatory behaviors requires examining individual variation in responses to heat (Buchholz et al. 2019). Heritable individual variation is the foundation of natural selection; without it, populations cannot adapt to changing environments. Therefore, identifying how siblings or neighbors vary in response to the same thermal conditions is a crucial next step for predicting evolutionary responses to global warming. Comparing populations across different thermal regimes may also provide valuable insight into the evolution of thermal tolerance. Populations under different thermal regimes have likely experienced different selective pressures for thermoregulatory behaviors, but we are aware of few such comparative studies. 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FIGURES Figure 1: Some of the ecological contexts in which birds deploy thermoregulatory behaviors, even within the same ecosystem. These may include solitary behaviors such as wing-spreading and moving to shady spots (bottom right), parental behaviors such as wing-shading and use of light-colored nesting material (left), or social behaviors (top right). Though birds may huddle to preserve warmth, here we depict the huddling that occurs as a result of competition for shady microclimates. We also depict panting across adult (top right) and juvenile (left) life stages. Images compiled in BioRender. Supplementary Material File (untitled.pdf) Download 13.91 MB Information & Authors Information Version history V1 Version 1 01 February 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords behavioral inertia bogert effect buffer competition global warming temperature Authors Affiliations Emmy James 0009-0004-1962-5657 [email protected] The University of Tennessee Knoxville Collaborative for Animal Behavior View all articles by this author Liz Derryberry The University of Tennessee Knoxville Collaborative for Animal Behavior View all articles by this author Metrics & Citations Metrics Article Usage 564 views 306 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Emmy James, Liz Derryberry. If You Can’t Take the Heat: Evaluating Thermoregulatory Behaviors Used by Birds. Authorea . 01 February 2025. DOI: https://doi.org/10.22541/au.173844726.60187268/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. 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