The researchers went to great lengths to design the "threats" they used to test the crows. They set up feeding areas for urban wild crows and sent trained volunteers to visit, carrying different objects that were carefully selected to test the crows' alarm responses: taxidermy crows arranged in poses suggesting that they were dead, and taxidermy red-tailed hawks, which prey on crows, posed on a branch as though they were still alive.
Then, volunteers would visit the feeding areas. Sometimes they would carry the "dead" crow, sometimes they would carry the "live" hawk, and sometimes they would carry both at the same time. To make things easier for the researchers (and more surreal for passers-by) the volunteers were masked, eliminating the possibility that variations in their expressions would affect the crows' responses.
The crows reacted by vocally scolding and mobbing the volunteers carrying the "dead" crows, the posed hawks and the two taxidermy birds at the same time. After one of these encounters, the crows also appeared more watchful of the feeding area, taking longer to approach the food.
And the crows also appeared to remember the masks worn by the volunteers who held the dead birds. Even if a mask-wearer only carried a dead crow once, the crows continued to scold that person whenever they appeared, for up to six weeks.
The scientists conducted similar tests with another urban bird, the rock pigeon, and observed that they hardly even noticed when a dead pigeon was paraded in front of them, a dramatic contrast to the organized and negative reaction of the crows to the sight of a fallen comrade. And the crows were similarly unconcerned about the presence of a dead pigeon, reserving their scolding and mobbing for humans accompanying dead crows and suspected predators.
The study results suggest that not only do crows notice and react to the sight of a dead crow, but they also quickly learn to associate danger and threat with humans who appear in close proximity to dead crows, recognizing them as a distinct and different caws for alarm.
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