一般情况下，鸟类在空中飞行时是____和____等方式交替使用．

欣赏下面一首现代诗，回答小题。

瀑布

洪 源

                                                                                                   要下山！要出谷！

                                                                                                   一声呐喊，

                                                                                                   从云崖上扑下，

                                                                                                   哪怕它粉身碎骨！

                                                                                                   向往的是——浩浩江河、茫茫大海，

                                                                                                   想念的是——展翼的风帆、击浪的桡橹，

                                                                                                   啊，全凭一腔燃烧的信念，

                                                                                                   脚下，才开辟出一条奔腾的生命之路。

(节选自《中国当代名诗一百首》)