Picture a bird that drops onto a shore for two or three days. It feeds, it rests, and one morning the branch is empty. It was never staying; the place was only a stop on a longer route. Hold on to the second half of the name — passage is a going-through, not a home — and both uses of the phrase follow from it.
「可以想成一隻鳥落在海邊，待了兩三天。牠吃東西，休息，然後某個早上，那根樹枝就空了。牠本來就沒打算留下。那個地方只是一段長路上的一個停靠點。記住名字的後半段：passage 是「經過」，不是「家」。這個說法的兩種用法，都是從這個畫面長出來的。」
From the word’s history
Probably a word-for-word rendering of Middle French oiseau de passage, from oiseau (bird) and passage (part of a journey or route). The literal sense is a bird that is on its way through.
「這個說法可能是把中古法文的 oiseau de passage 一個字一個字翻過來的：oiseau 是鳥，passage 是行程、經過的那一段。字面就是「正在路過的鳥」，所以「只是經過」這件事，一開始就寫在名字裡。來源記的是「可能」，不是確定。」
Wiktionary, "bird of passage"
登入就可以看每一支怎麼連回核心畫面。