Blair Wolf is an associate professor of biology at the University of New Mexico. Research in the Wolf lab focuses on the natural history and ecophysiology of desert animals. He agreed to blog with Dr. Dolittle on the topic of birds. Here are excerpts from their exchange.
Dr. Wolf: Australia has a very challenging environment, with a great expanse of hot desert and seems to be THE poster child for the bad things that are associated with climate warming. The continent has recently suffered through severe droughts, associated wildfires and frequent heat waves that have had mostly unknown, but potentially catastrophic consequences for their wildlife populations. Both birds in the west, and fruit bats in the east have suffered multiple die-offs associated with heat waves in recent years. The high local abundances and communal nature of many of Australia's birds and bats makes these very conspicuous although poorly studied events. Most of the events are briefly and sensationally recorded in the News (e.g. Budgies Boiled to Death!), but the scientific community has almost no knowledge of the size, duration and extent of these events, and very limited information on which species are affected. Because these die-offs have occurred with regularity in Australia in recent years, the region provides a natural laboratory that can be used to discover how heat waves affect individual species as well as whole bird communities and ecosystems.
Dr. Dolittle: Response from your work showing that smaller birds are more susceptible to water loss during heat waves prompted one blog reader to ask: "I guess this selection pressure would drive smaller birds to evolve larger bodies - assuming the timeframe of the climate change was sufficiently protracted. Is there any good palaeontological and palaeoclimatological data correlating high global temperatures to avian body size?" Please respond.
Dr. Wolf: You would think this would be the case, birds would evolve greater body sizes as it gets warmer-given adequate time; I don't know of any paleo-literature that shows that body size in birds was greater when it was warmer. This assumes that body size is being driven by water loss rates, which are relatively greater in small birds, and tolerance limits for the loss of body water. However, recent heat waves that produced die-offs of Ostriches in Africa and Peacocks in Northern India, suggest that large birds may be more affected by increasing air temperatures than small birds. And so the selective force may be the innate ability to dissipate heat (birds do this by evaporating water) and prevent heat stroke, which is an uncontrolled increase in body temperature that exceeds lethal limits. The apparent presence of drinking water at the location of multiple die-off events suggests that the deaths may be due to heat stroke and not the excessive loss of body water. Our problem is, as for most animals, we have almost no knowledge of the ability of birds and other animals to withstand heat stress and prevent heat stroke when exposed to high temperatures. We know that birds in deserts can allow their body temperature to rise to 113 Â°F (45Â°C), but that body temperatures of 116-117Â°F (46Â°C) are lethal. The critical problem for birds at these temperatures is that, say a 2Â° increase in air temperature from 50 Â°C to 52 Â°C on a very hot day in the future may require a doubling of the bird's evaporative water loss rate; if the bird can't actually double this rate then body temperature rises towards lethal limits and heat stroke occurs. This is what appears to be happening with most die-off events, and the evolution of higher lethal limits by birds is unlikely to keep up with the rapidly warming environment.