选项They changed the direction of their flight at a rate of 15degrees per hour
是说小鸟以每小时15度角的速率改变方向.
而文中提到When the real Sun was visible,however,the birds
maintained a constant direction as it moved across the sky.
constant direction保持恒定的方向,说明小鸟的方向是不变的(在真的太阳可见的情况之下)