A researcher on a routine survey route discovered hundreds of dead birds on the ground on a January morning in the Australian outback—not the picturesque red-rock version from vacation advertising, but the flat, shrubby, unrelentingly hot interior. Finches with zebras. Budgerigars. Some little honeyeaters. It was 48 degrees Celsius in the air the day before. Neither sickness nor predators have affected the birds. They died because they had simply lost the ability to chill themselves. After making a note of the places and gathering samples, the researcher submitted a report. It was not the first recorded incident of its kind in those area. It won’t be the final one.
Even under normal circumstances, small desert birds live on the brink of what their bodies can support. There is very little margin for error for a twenty-gram bird in an already hot and dry environment. Panting is the bird’s major cooling strategy when the temperature rises above a species-specific critical threshold, which for many tiny arid-zone birds is between 40 and 44 degrees Celsius. Evaporative water loss across the respiratory system is fueled by rapid breathing. Up to a certain point, it works. However, it burns thru the moisture in the bird’s body at a pace that can surpass normal by half or more in small species, and that moisture is restricted and irreplaceable in an environment where free water is scarce or nonexistent.
The shutdown of foraging is a compounded issue in and of itself. Birds that attempt to forage during the hottest part of a desert day run the risk of raising their core temperature above the threshold for irreversible thermal injury by traveling thru exposed bushes and grass, using energy, and collecting radiant heat from the ground. Thus, they cease. They take refuge in any area that provides shade, such as a thick bush, the underside of a rock ledge, or sometimes a tree hollow. However, sheltering prevents one from eating, which prevents one from obtaining the water that comes from the metabolism of fleshy fruit and insect prey. A heatwave that lasts long enough—three or four days without a significant decrease in nighttime temperatures—can put a little bird in a deadly feedback loop where it is too hot to forage, too thirsty to effectively control its body temperature, and too exhausted to wait it out.
The ensuing mass mortality episodes are not gradual. They happen quickly. After a long enough heat wave, researchers in southern Africa and the Sonoran Desert in North America have observed local bird colonies that were virtually unaltered one day and significantly reduced the next. Because these occurrences rely on the interplay of several variables, including ambient temperature, humidity, wind, cloud cover, time of year in relation to breeding season, and the state of individual birds at the start of the event, they are challenging to accurately forecast. However, the general pattern is becoming so well-known that ecologists are beginning to characterize it as an expected characteristic of desert ecosystems under ongoing warming, as opposed to aberrations that need to be explained after the fact.
Another level of concern that isn’t typically included in mortality figures is the reproductive dimension. Even if an adult bird survives a heatwave, the chicks and eggs are more heat-sensitive than the adults, and high temperatures make it difficult for the parents to provide constant shade and attend to the nest. Without killing a single adult, a heat event that occurs during the breeding season can completely eradicate a year’s worth of reproduction throughout the whole local population. Prior to and following such an incident, population numbers are identical. Only a year or two later, when recruiting has failed and the population starts to decline, does the impact become apparent.
