In the beginning of the 20th century,some school boys at a local private school devised a cruel prank. They somehow managed to capture a rook, a species from the crow or "Corvidae" family, or dress it in a homemade "coat" they had made from some scraps of red cloth before releasing it.
The rook,like all corvids or crows, is a social species that typically lives in groups known as "rookeries.” The species prove highly complex social behavior. When the rook flew back to its rookery wearing its "coat, and " it was attacked by the flock in a behavior known as "mobbing."Birds mob to coordinate an attack on a predatory animal they view as a threat. Emboldened by the safety of their superior numbers,the birds will attack raptors as large as golden eagles that they would never dare attack on their own.
However, as the case of the red coated rook sadly demonstrates, or this instinctual behavior can also be directed toward anything that deviates from the norm and invokes the impulse of fear. It’s likely that the oddly dressed avian alarmed his flock due to the startling colour of the "coat," which led to him being attacked.
The flock pursued him and attacked him in a nearby wood where pheasants were being raised for sport hunting. A game keeper responding to the noise saw the commotion and, fearing one of the pheasants was being attacked, and fired a shotgun into the flock,killing a number of birds, including the red coated rook.
Source: atlasobscura.com