The waterbird tucked its wings.
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[一] + [v] depicts a 'single' movement (implies starting abruptly or carrying out decisively)
挥利剑砍下敌人头颅 = "swing a sharp sword and cut off the enemy's head"
一挥利剑，砍下敌人头颅 = "with one swing of a sharp sword, cut off the enemy's head" (you can imagine this movement must be starting abruptly and carried out decisively)
展翅飞上天空 - "spread the wings and fly up to the sky"
一展翅便即飞上天空 - "once spread the wings, it started flying up to the sky "
水乌儿一抿翅膀，轻轻降落在我的前草坪上 - The waterfowl with one gentle tuck of its wings, landed softly on my front lawn