Birds are better at regulating heat loss through their legs than their bills: implications for body shape evolution in response to climate

Non-evaporative heat dissipation across the beaks and casques of large forest hornbills

Heat loss across the beak is an important thermoregulatory mechanism among birds, particularly in large-beaked taxa such as toucans (Ramphastidae) and hornbills (Bucerotidae). The number of species investigated remains limited, as does our understanding of how the functional significance of this pathway varies with environmental variables such as humidity, with little previous research on species inhabiting humid environments. We used infrared thermography to test the hypothesis that large (600-1300 g) Afrotropical forest hornbills use their beaks and casques as thermal radiators. We collected data over air temperatures (Tair) of 15-34 °C for wild-caught trumpeter hornbills (Bycanistes bucinator) and captive-bred silvery-cheeked hornbills (Bycanistes brevis) and black-casqued hornbills (Ceratogymna atrata). Surface temperatures of the beaks and casques (Tbeak) tracked Tair below 24-25 °C, but at higher Tair, the Tbeak - Tair gradient increased to maximum values of 10-12 °C. Maximum rates of beak heat loss were 2.5-3.8 W, equivalent to 31-83 % of estimated resting metabolic heat production. Facial skin showed also evidence for active regulation of heat loss. We also analysed the scaling of the inflection Tair above which the Tbeak - Tair gradient increases (Tinflection) by combining our data with published and three unpublished values. We found that Tinflection decreases with increasing body mass (Mb), with the relationship best described by the linear regression model Tinflection = -9.134log10Mb + 50.83, with Mb in g.

Latitudinal gradients and sex differences in morphology of the Black Oystercatcher (Haematopus bachmani)

Environment and behavior are widely understood to affect bird morphology, which can lead to differences among subspecies or populations within a wide-ranging species. Several patterns of latitudinal gradients in morphology have been described, though Allen's and Bergmann's rules are the most well-known and have been tested and confirmed across a diversity of taxa and species. These state that individuals at higher latitudes will have larger bodies (Bergmann's Rule) but smaller extremities (Allen's Rule) to conserve heat in colder climates. Migratory behavior also can influence avian morphology, particularly wing shape, where migratory birds tend to have longer, more pointed wings than residents. The Black Oystercatcher (Haematopus bachmani) is a large, partially migratory shorebird species restricted to intertidal habitats and distributed from Alaska to Baja California, spanning about 35° of latitude. A large proportion of Black Oystercatchers that breed in Alaska are migratory, where nearly all individuals breeding in British Columbia through the southern end of their range remain resident through the annual cycle. Their broad latitudinal range and diversity in migratory behavior may drive geographic variation in morphology. Here we evaluate three explanations for geographic variation in morphology of the Black Oystercatcher using data from seven sites across two regions: Alaska and British Columbia. We found evidence consistent with Allen's but not Bergmann's rule; birds in Alaska have shorter bills than those in British Columbia, and these findings held when controlling for body size using wing length. Despite regional differences in migratory behavior, we detected no difference in the wing shape of birds in Alaska and British Columbia. Differences between sexes and among sites suggest that multiple factors drive patterns of morphological variation in the Black Oystercatcher.

Footedness in merlins: Raptors perching in a cold climate

ABSTRACTPerching or standing on one foot is commonly reported in birds but the level of consistency in using one foot over the other has been less-well documented in most species, particularly birds of prey. For birds experiencing colder temperatures, unipedal perching has been attributed to limiting heat loss through unfeathered legs and feet; individuals should spend longer periods of time perched on one foot as temperatures decrease. Using radio tracking, I collected 486 hours of observations on nine overwintering, free-living merlins (Falco columbarius) in Saskatoon, Canada. Five merlins displayed clear preferences to perch on one foot, however the direction of preference was not consistent and four birds were ambidextrous. There was a curvilinear response in the proportion of time spent in unipedal posture versus temperature, with a peak of ∼22% of the time at moderate temperatures (-10 to -19°C), but lower values at warmer and colder temperatures; the main effect of the squared term for temperature was highly influential while individual foot preference had no impact on the use of unipedal perching. Although preferential use of one foot for perching was displayed by some individuals, thermoregulation may not be the primary driver of this behaviour at colder temperatures.

Selective effect of winter weather on a songbird's morphology depends on individual sex and winter condition

Knowledge of the effect of harsh weather on the phenotypic traits of organisms is essential for understanding the environmental influence on phenotype evolution and holds implications for predicting how species respond to current climate change. For many birds, harsh weather in winter often imposes a strong selective effect on their survival, and only the individuals with certain phenotypes may survive. However, whether the selective effect on phenotype varies with winter weather conditions has been poorly investigated. Here, we explored the selective effect of winter weather on black-throated tit's (Aegithalos concinnus) morphological traits under winters with and without severe snowstorms. We found that for males, the sizes of their bills, heads and wings significantly affected their overwinter survival, but the effects varied with winter conditions. In relatively benign winters, males with smaller bill depths, smaller bill surface areas, and greater head lengths survived better; whereas, in winters with severe snowstorms, a reverse pattern was found. This phenomenon was likely driven by selection pressures from heat retention and foraging requirements, with their relative importance depending on winter conditions. Additionally, wing length was positively correlated with male survival and the relationship was stronger in harsher winters, which was probably due to longer wings' higher flight efficiency in adverse weather. By contrast, we found no correlation between morphological traits and survival in females. These results suggest a sex-specific and condition-dependent selective effect of environment on bird phenotypes, implying complicated interactions between different selection pressures and phenotype evolution.

Thermal Imaging as a Method to Indirectly Assess Peripheral Vascular Integrity and Tissue Viability in Veterinary Medicine: Animal Models and Clinical Applications

Infrared thermography (IRT) is a technique that indirectly assesses peripheral blood circulation and its resulting amount of radiated heat. Due to these properties, thermal imaging is currently applied in human medicine to noninvasively evaluate peripheral vascular disorders such as thrombosis, thromboembolisms, and other ischemic processes. Moreover, tissular damage (e.g., burn injuries) also causes microvasculature compromise. Therefore, thermography can be applied to determine the degree of damage according to the viability of tissues and blood vessels, and it can also be used as a technique to monitor skin transplant procedures such as grafting and free flaps. The present review aims to summarize and analyze the application of IRT in veterinary medicine as a method to indirectly assess peripheral vascular integrity and its relation to the amount of radiated heat and as a diagnostic technique for tissue viability, degree of damage, and wound care.