____when we passed by its nest

B， Up into the blue sky是表示方位的介词短语开头，后面句子用完全倒装结构，即:主谓语交换位置，本来是 bird flew 变为flew the bird 而A答案是部分倒装。

类似的句子有:

Under the tree is sitting an old man..

.At the top of the hill stands a weather station.