Bird species with larger than average brains have lower levels of a key stress hormone, an analysis of nearly 200 avian studies has concluded. Such birds keep their stress down by anticipating or learning to avoid problems more effectively than smaller-brained counterparts, researchers suggest.
Birds in the wild lead a stressful life. Constantly spotting predators lurking in the trees or sensing dramatic changes in temperature is essential for survival, but can leave birds on the edge of a nervous breakdown. Reading these cues triggers changes in the birds’ metabolism, particularly increases in the stress hormone corticosterone. A sharp release of the hormone within 1 to 2 minutes after a cue triggers an emergency response and prepares birds to react quickly to the threat. However, regular exposure to the dangers of the wild and, hence, to high levels of this hormone, has serious health consequences and shortens life expectancy.
Not all birds respond to stress in the same way, however, notes Daniel Sol an ornithologist at the Centre for Ecological Research and Forestry Applications in Cerdanyola del Vallès, Spain. He and colleagues have for years looked at the differences between big-brained birds, such as crows and parrots, and those with smaller brains, such as chickens and quails. The former survive better in nature and are also more successful at establishing a community in a new environment.
In their new work, they connect brain size to handling stress. Sol; Ádám Lendvai, an evolutionary biologist at the College of Nyíregyháza in Hungary; and colleagues scoured the avian research literature to find studies that had measured corticosterone levels in birds in varying situations. They found 189 reports published before 2010 with comparable corticosterone and whole brain mass measurements for 119 bird species. The analysis, reported in the Proceedings of the Royal Society B, revealed that birds with large brains have lower circulating levels of the stress hormone, which rise only slightly in response to challenging situations, whereas these values can skyrocket in birds on the opposite end of the “brainy” scale.
An enlarged brain might be costly to develop and maintain, but could increase the bird's ability to face new challenges and cope with unpredictable situations. Higher cognitive skills “can be seen as an alternative mechanism to hormonal responses,” Sol explains. After all, he says, in multiple animal species “learning has long been associated with a reduction in stress.”
However, the literature review leaves lots of questions unanswered. The ultimate goal is to “try to understand what might be the mechanism that lowers the stress response” in larger brained birds, says Michaela Hau of the Max Planck Institute for Ornithology in Munich, Germany. It would be interesting to zoom in on a particular bird family, including species with different cognitive abilities, teach individuals a certain challenging task, and see if the bigger brained birds' stress response is smaller, she says. Still, Hau says, Sol’s hypothesis "is a cool idea."