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The therory is they instinctively know the profile of a raptor. My roo gives the growl at buzzards also.
Direction shape moves also important. Hawk shape moving backward not as scary. Read that in a paper.
My birds appear to produce distinctive calls reguarding hawks. Hawks flying past ellicit a different call from one on the attack. Perching hawk in tree causes yet another call. My birds even seem to distinguish between red-tailed and red-shouldered hawks which are almost the same size. Former is a threat, latter is not. During winter months, Coopers Hawks not a threat to my adult game chickens but during growing season they are major threat to chicks. The chickens seem to learn from experience as they see hawks multiple times a day. Reaction also a function of available cover and who has your back. Lone chickens much more scared of hawks and less likely to vocalize warnings. Game rooster will hold his ground when his harem and chicks are around but slip away otherwise.