Gular Flutter Research Articles

Keeping cool with poop: Urohidrosis lowers leg surface temperature by up to 6°C in breeding White storks

AbstractStorks (Ciconiidae) are renowned for defecating on their legs when exposed to high temperatures, a phenomenon known as “urohidrosis.” Previous work suggested that this behavior can cool down the blood supply to the legs and thus prevent hyperthermia in captive storks when overheated. However, no study has quantified the magnitude or duration of its evaporative cooling effect in free‐ranging birds. Here, we combine urohidrosis data with thermal imaging and microclimate data to investigate the thermoregulatory significance of urohidrosis in White storks Ciconia ciconia during the breeding season. We show that urohidrosis can reduce leg surface temperature by up to 6.7°C (4.40 ± 1.04°C). Yet its cooling effect was of short duration (lasting no more than 2.5 min) and decreased with time since defecation. Thus, for urohidrosis to significantly contribute to heat dissipation, storks must perform it repeatedly when overheated. Indeed, an individual can perform up to 11 urohidrosis events per hour; our estimates indicate that repeated urohidrosis could represent a significant amount of heat loss during short time span, equivalent to 4% of daily field metabolic rate for an adult stork. Our results point to an absence of differences in the cooling efficiency of urohidrosis between adults and nestlings, probably because all nestlings were recorded during the last phase of the ontogeny of thermoregulation. Besides, during the hottest days, adult storks delivered water to their nestlings, which might allow them to replenish body water reserves to sustain urohidrosis and other heat dissipation behaviors such as panting or gular fluttering. Our results indicate that urohidrosis might buffer the impact of high temperatures in breeding storks, preventing overheating and lethal hyperthermia. Gaining knowledge about behavioral thermoregulation in the heat is therefore crucial to better predict the future persistence and vulnerability of species under different climate warming scenarios.

Efficient Evaporative Cooling and Pronounced Heat Tolerance in an Eagle-Owl, a Thick-Knee and a Sandgrouse

Avian evaporative cooling and the maintenance of body temperature (Tb) below lethal limits during heat exposure has received more attention in small species compared to larger-bodied taxa. Here, we examined thermoregulation at air temperatures (Tair) approaching and exceeding normothermic Tb in three larger birds that use gular flutter, thought to provide the basis for pronounced evaporative cooling capacity and heat tolerance. We quantified Tb, evaporative water loss (EWL) and resting metabolic rate (RMR) in the ∼170-g Namaqua sandgrouse (Pterocles namaqua), ∼430-g spotted thick-knee (Burhinus capensis) and ∼670-g spotted eagle-owl (Bubo africanus), using flow-through respirometry and a stepped Tair profile with very low chamber humidities. All three species tolerated Tair of 56–60°C before the onset of severe hyperthermia, with maximum Tb of 43.2°C, 44.3°C, and 44.2°C in sandgrouse, thick-knees and eagle-owls, respectively. Evaporative scope (i.e., maximum EWL/minimum thermoneutral EWL) was 7.4 in sandgrouse, 12.9 in thick-knees and 7.8 in eagle-owls. The relationship between RMR and Tair varied substantially among species: whereas thick-knees and eagle-owls showed clear upper critical limits of thermoneutrality above which RMR increased rapidly and linearly, sandgrouse did not. Maximum evaporative heat loss/metabolic heat production ranged from 2.8 (eagle-owls) to 5.5 (sandgrouse), the latter the highest avian value yet reported. Our data reveal some larger species with gular flutter possess pronounced evaporative cooling capacity and heat tolerance and, when taken together with published data, show thermoregulatory performance varies widely among species larger than 250 g. Our data for Namaqua sandgrouse reveal unexpectedly pronounced variation in the metabolic costs of evaporative cooling within the genus Pterocles.

Urban rooftop-nesting Common Nighthawk chicks tolerate high temperatures by hyperthermia with relatively low rates of evaporative water loss

Abstract Heat tolerance for many birds under climate and land use change scenarios could be compromised in the future. Common Nighthawks (Chordeiles minor) belong to the Caprimulgiformes, a generally heat-tolerant order, but few studies have assessed heat tolerance in Caprimulgiform chicks, which might be particularly susceptible to heat stress. In the midwestern United States, nighthawks primarily nest on flat graveled rooftops in urban areas, as natural nesting habitats are limited. Urban rooftop-nesting nighthawks are likely exposed to higher environmental temperatures than birds nesting at more thermally buffered natural sites, and evaporative cooling might be impeded by the typically high summer humidity in their Midwest breeding range. This combination of heat and humidity might negatively impact heat tolerance of nighthawk chicks. We exposed 7- to 14-day-old nighthawk chicks (n = 15) from rooftop nests to ambient temperatures (Tas) up to 51°C and at typical summer dew points. Chicks initiated gular flutter at a mean ambient temperature of 42.4 ± 3.4°C (mean ± SE). Evaporative water loss (EWL) rates increased significantly with increasing temperature above 44.0 ± 1.5°C. Chicks showed little evidence of lower and upper bounds of the thermal neutral zone over the range of temperatures (30–44°C) for which we measured oxygen consumption. Body mass loss was significantly positively correlated with temperature during heat exposure trials. Chicks tolerated Tas up to 51°C and body temperatures (Tbs) up to 48°C, which, along with the high temperatures at which gular flutter and high rates of EWL were initiated, suggest that nighthawk chicks are tolerant of high ambient temperatures, even with relatively high humidity. Given the high rates of mass loss and high Tbs at hot ambient temperatures, chick heat tolerance mechanisms could be detrimental for rooftop-nesting nighthawks given projected increasing trends for both heat and humidity in the midwestern United States.

Parenting in a warming world: thermoregulatory responses to heat stress in an endangered seabird.

The frequency of extreme weather events, including heat waves, is increasing with climate change. The thermoregulatory demands resulting from hotter weather can have catastrophic impacts on animals, leading to mass mortalities. Although less dramatic, animals also experience physiological costs below, but approaching, critical temperature thresholds. These costs may be particularly constraining during reproduction, when parents must balance thermoregulation against breeding activities. Such challenges should be acute among seabirds, which often nest in locations exposed to high solar radiation and predation risk. The globally endangered bank cormorant Phalacrocorax neglectus breeds in southern Africa in the winter, giving little scope for poleward or phenological shifts in the face of increasing temperatures. Physiological studies of endangered species sensitive to human disturbance, like the bank cormorant, are challenging, because individuals cannot be captured for experimental research. Using a novel, non-invasive, videographic approach, we investigated the thermoregulatory responses of this seabird across a range of environmental temperatures at three nesting colonies. The time birds spent gular fluttering, a behaviour enhancing evaporative heat loss, increased with temperature. Crouching or standing birds spent considerably less time gular fluttering than birds sitting on nests (ca 30% less at 22°C), showing that postural adjustments mediate exposure to heat stress and enhance water conservation. Crouching or standing, however, increases the vulnerability of eggs and chicks to suboptimal temperatures and/or expose nest contents to predation, suggesting that parents may trade-off thermoregulatory demands against offspring survival. We modelled thermoregulatory responses under future climate scenarios and found that nest-bound bank cormorants will gular flutter almost continuously for several hours a day by 2100. The associated increase in water loss may lead to dehydration, forcing birds to prioritize survival over breeding, a trade-off that would ultimately deteriorate the conservation status of this species.

Avian thermoregulation in the heat: metabolism, evaporative cooling and gular flutter in two small owls.

The thermoregulatory responses of owls to heat stress have been the subject of few studies. Although nocturnality buffers desert-dwelling owls from significant heat stress during activity, roost sites in tree and cactus cavities or in deep shade provide only limited refuge from high environmental temperatures during the day. We measured thermoregulatory responses to acute heat stress in two species of small owls, the elf owl (Micrathene whitneyi) and the western screech-owl (Megascops kennicottii), which occupy the Sonoran Desert of southwestern North America, an area of extreme heat and aridity. We exposed wild-caught birds to progressively increasing air temperatures (Ta) and measured resting metabolic rate (RMR), evaporative water loss (EWL), body temperature (Tb) and heat tolerance limits (HTL; the maximum Ta reached). Comparatively low RMR values were observed in both species, Tb approximated Ta at 40°C and mild hyperthermia occurred as Ta was increased toward the HTL. Elf owls and screech-owls reached HTLs of 48 and 52°C, respectively, and RMR increased to 1.5 and 1.9times thermoneutral values. Rates of EWL at the HTL allowed for the dissipation of 167-198% of metabolic heat production (MHP). Gular flutter was used as the primary means of evaporative heat dissipation and produced large increases in evaporative heat loss (44-100%), accompanied by only small increases (<5%) in RMR. These small, cavity-nesting owls have thermoregulatory capacities that are intermediate between those of the open-ground nesting nightjars and the passerines that occupy the same ecosystem.

Avian thermoregulation in the heat: phylogenetic variation among avian orders in evaporative cooling capacity and heat tolerance.

Little is known about the phylogenetic variation of avian evaporative cooling efficiency and heat tolerance in hot environments. We quantified thermoregulatory responses to high air temperature (Ta) in ∼100-g representatives of three orders, namely, the African cuckoo (Cuculus gularis, Cuculiformes), lilac-breasted roller (Coracias caudatus, Coraciiformes) and Burchell's starling (Lamprotornis australis, Passeriformes). All three species initiated respiratory mechanisms to increase evaporative heat dissipation when body temperature (Tb) approached 41.5°C in response to increasing Ta, with gular flutter observed in cuckoos and panting in rollers and starlings. Resting metabolic rate and evaporative water loss increased by quantitatively similar magnitudes in all three species, although maximum rates of evaporative water loss were proportionately lower in starlings. Evaporative cooling efficiency [defined as the ratio of evaporative heat loss (EHL) to metabolic heat production (MHP)] generally remained below 2.0 in cuckoos and starlings, but reached a maximum of ∼3.5 in rollers. The high value for rollers reveals a very efficient evaporative cooling mechanism, and is similar to EHL/MHP maxima for similarly sized columbids which very effectively dissipate heat via cutaneous evaporation. This unexpected phylogenetic variation among the orders tested in the physiological mechanisms of heat dissipation is an important step toward determining the evolution of heat tolerance traits in desert birds.

Avian thermoregulation in the heat: evaporative cooling capacity of arid-zone Caprimulgiformes from two continents.

Birds in the order Caprimulgiformes (nightjars and allies) have a remarkable capacity for thermoregulation over a wide range of environmental temperatures, exhibiting pronounced heterothermy in cool conditions and extreme heat tolerance at high environmental temperatures. We measured thermoregulatory responses to acute heat stress in three species of Caprimulgiformes that nest in areas of extreme heat and aridity, the common poorwill (Phalaenoptilus nuttallii: Caprimulgidae) and lesser nighthawk (Chordeiles acutipennis: Caprimulgidae) in the Sonoran Desert of Arizona, and the Australian owlet-nightjar (Aegotheles cristatus: Aegothelidae) in the mallee woodlands of South Australia. We exposed wild-caught birds to progressively increasing air temperatures (Ta) and measured resting metabolic rate (RMR), evaporative water loss (EWL), body temperature (Tb) and heat tolerance limit (HTL; the maximum Ta reached). Comparatively low RMR values were observed in all species (0.35, 0.36 and 0.40 W for the poorwill, nighthawk and owlet-nightjar, respectively), with Tb approximating Ta at 40°C and mild hyperthermia occurring as Ta reached the HTL. Nighthawks and poorwills reached HTLs of 60 and 62°C, respectively, whereas the owlet-nightjar had a HTL of 52°C. RMR increased gradually above minima at Ta of 42, 42 and 35°C, and reached 1.7, 1.9 and 2.0 times minimum resting values at HTLs in the poorwill, nighthawk and owlet-nightjar, respectively. EWL increased rapidly and linearly as Ta exceeded Tb and resulted in maximum rates of evaporative heat dissipation equivalent to 237-424% of metabolic heat production. Bouts of gular flutter resulted in large transient increases in evaporative heat loss (50-123%) accompanied by only small increments in RMR (<5%). The cavity-nesting/roosting owlet-nightjar had a lower HTL and less efficient evaporative cooling compared with the species that nest and/or roost on open desert surfaces. The high efficiency of gular flutter for evaporative cooling, combined with mild hyperthermia, provides the physiological basis for defending Tb well below Ta in extreme heat and is comparable to the efficient cooling observed in arid-zone columbids in which cutaneous EWL is the predominant cooling pathway.

Avian thermoregulation in the heat: efficient evaporative cooling in two southern African nightjars.

Nightjars represent a model taxon for investigating physiological limits of heat tolerance because of their habit of roosting and nesting in sunlit sites during the heat of the day. We investigated the physiological responses of Rufous-cheeked nightjars (Caprimulgus rufigena) and Freckled nightjars (Caprimulgus tristigma) to high air temperatures (T a) by measuring body temperature (T b), resting metabolic rate (RMR) and total evaporative water loss (TEWL) at T a ranging from 10 to 56°C. Both species became hyperthermic at T a>T b. Lower critical limits of thermoneutrality occurred at T a between 35 and 37°C, whereas we detected no clear upper critical limits of thermoneutrality. Between T a≈37.0 and 39.9°C, rates of TEWL increased rapidly with T a. At T a≥40°C, fractional increases in mass-specific TEWL rates were 78-106% of allometric predictions. Increasing evaporative heat dissipation incurred only small metabolic costs, with the RMR of neither species ever increasing by more than 20% above thermoneutral values. Consequently, both species displayed extremely efficient evaporative cooling; maximum evaporative heat dissipation was equivalent to 515% of metabolic heat production (MHP) at T a≈56°C in C. rufigena and 452% of MHP at T a≈52°C in C. tristigma. Our data reiterate that caprimulgids have evolved an efficient mechanism of evaporative cooling via gular fluttering, which minimizes metabolic heat production at high T a and reduces total heat loads. This likely aids in reducing TEWL rates and helps nightjars cope with some of the most thermally challenging conditions experienced by any bird.

Behavioural responses to heat in desert birds: implications for predicting vulnerability to climate warming

Temperature increases associated with climate change pose a substantial threat to arid-zone bird species. However, predicting vulnerability to high temperatures using species-specific, mechanistic data, and assessing how this varies within and among species, is complex, time consuming and expensive. Using the bird community of the southern Kalahari Desert, we investigated whether interspecific variation in heat dissipation behaviour (HDB) thresholds could provide an index of vulnerability to high temperatures. We fitted logistic regression models to presence/absence data for behaviours as a function of air temperature (T air) in order to determine, for each species, the T air at which panting and gular flutter (pant 50), wing-drooping (wing 50), resting (rest 50) and shade-seeking (shade 50) responses occurred in 50 % of instances. We show that pant 50 (n = 30 species) is higher in species that 1) are smaller, 2) maintain low activity levels at high T airs, and 3) rely mostly on food as their water source (i.e. non-drinking species) — the only predictor variable to remain significant in phylogenetically-independent analyses. Like pant 50, wing 50 (n = 30 species) was negatively correlated with body mass, but did not vary systematically with other organismal traits. There was no systematic variation in shade 50 (n = 33 species) or rest 50 (n = 14 species) values. Our findings suggest that evaporative cooling demands are relatively higher in larger birds and could limit activities (such as foraging) at T airs exceeding 30 °C, while the trade-off between thermoregulation and sustaining activity levels appears less pronounced in smaller species. Kalahari species that do not drink regularly show a relatively greater dependence on wing-drooping compared to panting/gular flutter, probably resulting from selective pressures to reduce evaporative cooling demands in an arid environment with scarce surface water. Two key questions remain to be answered to confirm whether variation in HDB thresholds provides an opportunity for rapid assessment of vulnerability to high temperatures in avian species from hot, arid environments. First, what are the relationships between HDB patterns and body temperature regulation? Second, is heat dissipation effort indicative of dehydration risk?

Avian thermoregulation in the heat: evaporative cooling capacity in an archetypal desert specialist, Burchell's sandgrouse (Pterocles burchelli).

Sandgrouse (Pterocliformes) are quintessential examples of avian adaptation to desert environments, but relatively little is known about the limits to their heat tolerance and evaporative cooling capacity. We predicted that evaporative cooling in Burchell's sandgrouse (Pterocles burchelli) is highly efficient and provides the basis for tolerance of very high air temperature (Ta). We measured body temperature (Tb), resting metabolic rate (RMR) and evaporative water loss (EWL) at Ta between 25°C and ∼58°C in birds exposed to successive increments in Ta Normothermic Tb averaged 39.0°C, lower than typical avian values. At Ta>34.5°C, Tb increased linearly to a maximum of 43.6°C at Ta=56°C. The upper critical limit of thermoneutrality (Tuc) was Ta=43.8°C, closely coinciding with the onset of panting and gular flutter. Above the Tuc, RMR increased 2.5-fold to 2.89 W at Ta=56°C, a fractional increase far exceeding that of many other species under comparable conditions. Rates of EWL increased rapidly at Ta>42.9°C to 7.84±0.90 g h(-1) at Ta=56°C, an 11-fold increase above minimal levels. Maximum evaporative cooling efficiency (ratio of evaporative heat loss to metabolic heat production) was 2.03, but could be as high as 2.70 if our assumption that the birds were metabolising lipids is incorrect. Thermoregulation at very high Ta in P. burchelli was characterised by large increases in RMR and EWL, and is much less efficient than in taxa such as columbids and caprimulgids.

Commentary: Yawning, acute stressors, and arousal reduction in Nazca booby adults and nestlings.

Commentary: Yawning, acute stressors, and arousal reduction in Nazca booby adults and nestlings.

DEFENSE STRATEGIES AGAINST ENVIRONMENTAL HEAT STRESS IN BIRDS

ABSTRACT When heat-acclimated pigeons are exposed to ambient temperatures of 50–60°C, extremely high cutaneous evaporative cooling, together with a high insulative capacity of the feather coat, creates for the bird a microclimate within which physiological processes can be regulated normally. By skillfully using this cooling garment, pigeons maintain their resting metabolic rate and regulate low skin and body temperatures, employing neither panting nor gular fluttering. These physiological achievements parallel those of arctic animals. Hence, during animal evolution towards inhabiting extreme thermal environments, selection seems to have favored adaptations at the lowest energy cost in both the extreme cold arctic and the hottest deserts of the world.

Functional morphology and evolution of aspiration breathing in tetrapods

In the evolution of aspiration breathing, the responsibility for lung ventilation gradually shifted from the hyobranchial to the axial musculoskeletal system, with axial muscles taking over exhalation first, at the base of Tetrapoda, and then inhalation as well at the base of Amniota. This shift from hyobranchial to axial breathing freed the tongue and head to adapt to more diverse feeding styles, but generated a mechanical conflict between costal ventilation and high-speed locomotion. Some "lizards" (non-serpentine squamates) have been shown to circumvent this speed-dependent axial constraint with accessory gular pumping during locomotion, and here we present a new survey of gular pumping behavior in the tuatara and 40 lizard species. We observed gular pumping behavior in 32 of the 40 lizards and in the tuatara, indicating that the ability to inflate the lungs by gular pumping is a shared-derived character for Lepidosauria. Gular pump breathing in lepidosaurs may be homologous with buccal pumping in amphibians, but non-ventilatory buccal oscillation and gular flutter have persisted throughout amniote evolution and gular pumping may have evolved independently by modification of buccal oscillation. In addition to gular pumping in some lizards, three other innovations have evolved repeatedly in the major amniote clades to circumvent the speed-dependent axial constraint: accessory inspiratory muscles (mammals, crocodylians and turtles), changing locomotor posture (mammals and birds) and respiratory-locomotor phase coupling to reduce the mechanical conflict between aspiration breathing and locomotion (mammals and birds).

Magpies, Pica pica, at the southern limit of their range actively select their thermal environment at high ambient temperatures

The effect of high ambient temperatures on the behaviour of Magpies (Pica pica) was assessed in northern Cyprus, which marks the southern limit of their European range. Magpies actively selected their thermal environment when temperatures rose above 31°C, by retreating to shaded areas. On days when the temperature was higher, Magpies appeared to spend more time perched in the shade and less time foraging. Flying and foraging activities were generally restricted to cooler parts of the day. Magpies were rarely seen panting or drinking and gular fluttering was not observed. We suggest that utilisation of shade is important for Magpies to regulate their body temperature, allowing them to remain common at the southern limit of their range despite experiencing high temperatures.

Does Shading Behavior of Incubating Shorebirds in Hot Environments Cool the Eggs or the Adults?

-Birds that nest on the ground in hot environments employ behavioral and physiological mechanisms to prevent the overheating of their eggs and themselves (e.g. egg covering, belly soaking, orientation on the nest, ptiloerection, panting, and gular fluttering). One of the least energetically expensive behaviors employed during incubation at high temperatures is nest shading. During shading, the birds raise themselves onto their tibiotarsi so that the incubation patch is about 2 to 3 cm above the eggs. It has frequently been assumed that birds adopt shading behavior to allow for direct convective cooling of the eggs. However, an alternative hypothesis is that birds rise up onto their tibiotarsi in order to place themselves out of the boundary layer. In doing so, adult birds cool themselves directly by convection. The lowering of the adult's body temperature during nest shading may, of course, cool the eggs indirectly when the bird resumes incubation. We tested these hypotheses using a taxidermic model of a Crowned Plover (Vanellus coronatus), which nests in hot environments in South Africa. Field observations have shown that this species spends 20 to 50% of its time shading when incubating at high environmental temperatures. When we alternated the bird between incubating and shading positions, we found that bird temperature, but not egg temperature, was lowered by shading behavior. Furthermore, wind speed had a significant negative effect on bird temperature but not egg temperature. Therefore, our results support the hypothesis that shading behavior functions to lower the temperature of incubating adults rather than of the eggs. Received 24 September 1996, accepted 14 May 1997. INCUBATION IN BIRDS usually is considered to be a warming process (e.g. Vleck 1981, Whittow 1986). However, bird species that nest on the ground in environments characterized by high daytime temperatures have to employ behavioral and physiological mechanisms to prevent the overheating of their eggs and themselves (Schmidt-Nielsen 1965, Grant 1982, Ward 1990). These mechanisms include egg covering (Maclean 1972), belly soaking (Jayakar and Spurway 1965, Howell 1979, Maclean 1975, Grant 1982, Ward 1988), orientation on the nest, ptiloerection, panting, and gular fluttering (Maclean 1967, 1975, 1976; Grant 1982; Ward 1989a,b). Eggshells of birds nesting in semiarid to arid regions also have reduced water-vapor conductance, increased thickness, and reduced effective pore area (Ar et al. 1974, Vleck et al. 1979, Ward 1987, Arad et al. 1988). Attentive behavior at the nest is thought to be important for maintenance of egg tempera1 E-mail: downs@zoology.unp.ac.za 2 Present address: Mitrani Center for Desert Ecology and Ramon Science Center, Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer 84990, Israel. tures below lethal levels in harsh, hot environments (Grant 1982, Ward 1989a,b). During the heat of the day, incubating shorebirds nesting near the Salton Sea in California spent most of their time shading or loosely sitting over their eggs (Grant 1982). Nest relief occurred about every 10 to 15 min during the heat of the day (Grant 1982). Indeed, many shorebirds that nest in areas of high ambient temperatures shade their eggs at moderate to high temperatures (Maclean 1976, Ward 1989a,b). The Crowned Plover (Vanellus coronatus) is a medium-sized shorebird (average body mass = 167 g; Maclean 1993) that commonly occurs in open grasslands and lightly wooded savannas in sub-Saharan Africa (Ward and Maclean 1989). The nest is a shallow scrape (1 to 2 cm deep) in the ground (Ward 1987). These ground nests may get cold at night, requiring the parent to incubate the eggs to keep them warm. A more stressful period for the incubating birds occurs during the main breeding season, when daytime surface temperatures usually exceed 50?C. During these periods, the eggs require cooling (Ward 1990). Typical egg temperatures in birds vary between 30 and 35?C, and hyperthermia evidently is more injurious to the de-

Thermoregulation in the Brown Noddy ( Anous stolidus)

1. 1. Oxygen consumption ( V ̇ O 2 ), body temperature ( T b), respiratory frequency ( f) and total evaporative water loss were measured in Brown Noddies exposed to air temperatures ( T a) between −0.2 and 41.9°C, in a climatic chamber. 2. 2. The thermoneutral zone of T a was approx. 22–37°C and the V ̇ O 2 in the thermoneutral zone (1.031 ml/g h) was 77–80% of predicted values. 3. 3. The body temperature was characterized by its lability: even within the thermoneutral zone, there was a significant correlation with T a. 4. 4. Exposure to low T a resulted in shivering and a maximal increase in V ̇ O 2 of 129%. The minimal thermal conductance of the tissues and plumage below the lower critical T a was 86% of the predicted value. 5. 5. At high T a, thermal polypnea and open-beak panting were observed but the birds did not gular flutter. In some birds evaporative heat loss exceeded concurrent metabolic heat production. 6. 6. The features of thermoregulation in adult Brown Noddies are compared with those of the hatchling and also with thermoregulation in the related Sooty Tern.

THE COOLING POWER OF THE PIGEON HEAD

Resting pigeons preheated to a stable core temperature of 43.2 &deg;C, which is within the range of body temperatures recorded during flight, were able to cool their body at high rates if their head and upper neck were exposed to an air stream at 23.5 &deg;C. The heat dissipation capacity of the head and neck, estimated from measurements made at a wind speed (100 km h-1) corresponding to fast flight, was 9.8 W, or 4.5 times the resting heat production. Since the greater part of this capacity, about 8 W, was attributable to the inner surfaces of the mouth, ram ventilation of the buccal cavity appears to be an important mechanism for increasing evaporative heat loss during flight. Accordingly, wind-assisted mouth cooling should be utilized by resting pigeons, since exposure to a slight breeze (approximately 10 km h-1) could augment their dissipating power by an amount equivalent to their resting metabolic rate. It is concluded that beak opening, together with a source of convection other than panting and gular flutter, is required to exploit fully the heat dissipation capacity of the buccopharyngeal mucosa of birds.

The Adaptive Significance of Avian Mobbing V. An Experimental Test of the ‘Move On’ Hypothesis

AbstractThis paper presents the first experimental study of the effects of mobbing on predators. Two captive little owls Athene noctua and a tawny owl Strix aluco, having stabilized activity patterns and roosting sites, were singly confronted with one or two mobbing blackbirds Turdus merula. The little owls showed marked stress (gular fluttering, panting and panic flights), abandoned chosen roosts and remained unsettled for several days. A permanent alternative roost was chosen after one mobbing encounter, other encounters resulted in several days of ‘floating’ before returning to the original sites. One little owl used ‘camouflage’ by ‘freezing’ during repeated harassments, the other tended to retaliate if mobbing was close and weak. The tawny owl seemed to ignore the rather weak blackbird mobbing, but abandoned its roost for up to 36 h when mobbed by larger numbers of woodland birds at close range. It is concluded that mobbing causes profound distress, especially in small, vulnerable predators like the little owl and forces it to change its daytime roost. If abandoning the area is not possible, time and energy are expended in increased vigilance as a protection against further mobbing attacks. Thus there is strong support for the ‘Move on’ hypothesis.Results also support the ‘Perception advertisement’ hypothesis, implying that the detected hunter deliberately forfeits the surprise element. Which effect prevails depends on predator evaluation of the signals involved.

Energetics of mound-tending behaviour in the malleefowl, Leipoa ocellata (Megapodiidae)

Abstract. Malleefowl artificially incubate their eggs in mounds containing decaying vegetation. The effort involved in behaviourally regulating mound temperature seems prodigious. The energy cost of regulating mound temperature was estimated indirectly, using time to onset of an evaporative heat loss behaviour (gular flutter) to establish equivalent work rates for malleefowl running on a treadmill in the laboratory, and for malleefowl tending mounds in the field. Equivalency between laboratory and field thermal environments was established based on operative temperature and water vapour pressure. Mound work was equivalent to running at 0·57 m/s. The power input of birds running this speed was 12·9 W/kg, which is 3·1 times the resting metabolic rate, or 3·8 times the basal metabolic rate. In March, malleefowl tend their mounds for an average of 4·8-5·3 h/day and incur a daily energy cost of incubation about twice that of other birds. At other times of the year malleefowl devote little effort to incubation. Overall, the total work to construct a mound and tend it during the 5- to 6-month incubation period exceeds the total energy cost of incubation in typical birds by perhaps two-fold. The benefits that compensate the malleefowl's higher total incubation cost include higher fecundity and emancipation from post-hatching parental care.

Life in extreme environments: Investigations on the ecophysiology of a desert bird, the Australian Diamond Dove (Geopelia cuneata Latham)

The Diamond Dove, Geopelia cuneata, is the world's second smallest (ca. 35 g) species of the columbid order. The Diamond Dove is endemic in the arid and semiarid Mulga and Spinifex regions of Central and Western Australia. It regularly encounters ambient temperatures (T a ) in its habitat above +40° C, especially when foraging for seeds on bare ground cover, and may be found at up to 40 km from water. This entails extreme thermal stress, with evaporative cooling constrained by limited water supply. Energy metabolism (M), respiration, body temperature (T a ) and water budget were examined with regard to physiological adaptations to these extreme environmental conditions. The zone of thermal neutrality (TNZ) extended from +34° C to at least +45° C. Basal metabolic rate (BMR) was 34.10±4.19 J g-1h-1, corresponding to the values predicted for a typical columbid bird. Thermal conductance (C) was higher than predicted. Geopelia cuneata showed the typical breathing pattern of doves, a combination of normal breathing at a stable frequency (ca. 60 min-1) at low T a and panting followed by gular flutter (up to 960 min-1) at high T a . At T a > +36° C, T a increased to considerably higher levels without increasing metabolic rate, i.e. Q10=1. This enabled the doves not only to store heat but also to save the amout of water that would have been required for evaporative cooling if T a had remained constant. The birds were able to dissipate more than 100% of the metabolic heat by evaporation at T a ≥ +44° C. This was achieved by gular flutter (an extremely effective mechanism for evaporation), and also by a low metabolic rate due to the low Q10 value for metabolism during increased T b . At lower T a , Geopelia cuneata predominantly relied on non-evaporative mechanisms during heat stress, to save water. Total evaporative water loss over the whole T a range was 19-33% lower than expected. In this respect, their small body size proved to be an important advantage for successful survival in hot and arid environments.