A desert-dwelling hornbill has been shown for the first time to use its bill as a cooling device when temperatures rise above 87°F (30.7°C).
By Rex Graham
A study published May 18 in the open-access journal PLOS ONE reported the stunning details of the cooling adaptation in the beautiful Southern Yellow-billed Hornbill of the Kalahari Desert.
Since the time of Charles Darwin, the size and shape of birds’ bills was thought to evolve to most efficiently find and retrieve food. However, several years ago the Toco Toucan, the bird with the largest bill in relation to its body size, changed that simple view.
On hot days in the Tropics of South America scientists discovered the toucan radiates 60 percent of its body heat non-evaporatively by pumping blood through its oversized bill. The signature feature of the Amazon bird is one of nature’s most efficient radiators. The toucans can cool themselves even more by panting, releasing water on the inner bill surface as an evaporative-cooling feature.
The study in PLOS ONE by a team of South African scientists found that the thicker, harder hornbill bills are about one-third as effective as cooling devices as toucan bills, but hornbills use the toucan’s cooling techniques.
“We have found that, like toucans, hornbills can use their beak as a controllable thermal radiator,” said Tanja van de Ven from the Percy FitzPatrick Institute of African Ornithology and the first author of the study. “We think this might provide an advantage in the arid Kalahari by reducing the amount of water the birds need to spend on evaporative cooling.”
Birds burn energy at 5 to 10 times the rate of reptiles, which allows birds to live in colder climates. They maintain their body temperatures in the range of 100°F (37.8°C), which would be a fever in humans. However, when air temperatures approach or exceed a bird’s body temperature, it must cool itself to avoid succumbing to heat stroke.
Infrared thermal imaging is permitting scientists like van de Ven to understand for the first time how birds and other animals conserve body heat in cold weather and dissipate it on hot days. Thermal imaging cameras allowed the scientist to vividly see the hornbills’ bills appear much brighter against the darker colors of the cooler background. The imaging also showed that the birds radiate heat from the bare skin around their eyes and on their throats.
As the hornbill researchers gradually heated temperature-controlled cages, the imaging cameras revealed that the hornbills beaks began to fill with blood through a network of vessels.
While toucans use their light-weight bills to eat primarily soft fruit, hornbills use their stronger beaks to break away pieces of tree bark in search of invertebrates. The additional hardness of hornbills’ bills reduced their cooling capacity, but it is still highly efficient, the researchers said.
Last resort – panting
When temperatures rose higher than the hornbills’ ability to radiate body heat, the birds opened their mouths and began to pant. Birds don’t have sweat glands, but imaging cameras revealed that hornbills’ panting involves the evaporation of water on the interior structures of their bills.
The researchers theorize that hornbills, toucans and other birds may be able to shut off blood flow to their bills when temperatures soar above the birds’ body temperatures. Such vasoconstriction ability would, in theory, benefit the birds by not absorbing excess heat, “but we were unable to confirm this using thermal imaging data,” the researchers said.
The energy metabolism of hornbills and most other birds keeps running even when they become overheated. A smarter strategy might be to simply adjust their internal body temperatures to fit the circumstance, but hornbills and most other birds can’t do that. The continually revved-up metabolism primes them to flee a predator in an instant at full speed, which is something reptiles and other cold-blooded animals can do.
Avian air conditioning
On the other hand, hummingbirds and some other birds routinely slow their metabolism to conserve energy on chilly nights.
Almost all bird species shed feathers temporarily on their breast or abdomen during the nesting season. The bare patches transfer body heat to the incubating eggs. Parents replace the lost feathers after the breeding season.
When temperatures climb too high, parents that build nest on exposed ledges simply stand up to offer cooling shade to their eggs.
Cactus Wrens in the U.S. Southwest build dome-shaped nests with entrance holes facing away from the often-cold prevailing winds in the early spring. However, the wrens build a second nest in the hottest half of the season that faces into the wind to help cool their chicks.
The wrens have another cooling trick to cool their chicks. Instead of immediately removing the chicks’ “fecal sacs” on hot days, the parents leave the poop in the nest. The waste material provides a significant evaporative cooling effect.
Revealing animal survival strategies
In a 2010 paper in The Imaging Science Journal, Canadian scientists said infrared thermal imaging in animal research is still in its infancy. “This technology has the potential to contribute a greater understanding of animal thermal adaptations, not only since it provides live information on surface temperatures, but more importantly because its non-invasive nature allows measurements to be obtained with minimal disturbance,” said the paper’s authors, Brock University biologists G.J. Tattersall and V. Cadena.