It’s a counterbalance.
When we bipedal humans walk, we swing the opposite arm to forward and the adjacent arm backward with each step so as to counterbalance the shifts in momentum created by taking that step and not lose our balance and fall over.
Birds are also bipedal, but they avoid swinging their wings around when they are on the ground, rather they keep their wings tucked up behind their backs to protect them. So instead they use their heads to counterbalance each step instead.
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