One way to hold preen: a bird takes a single feather into its and pulls it all the way through, from the base to the tip, then starts on the next one. The work is slow, patient and repeated, it is aimed at the bird's own body, and it is done out in the open — on a wire, on a rock, where anyone can watch. When the bird is finished it sits up, shakes once, and looks smooth and very satisfied. Most of what preen does in English seems to grow out of that small picture, including the part where the tidying is done where others can see.
From the word’s history
Wiktionary treats this preen as a variant of prune, shaped by an older English preen meaning 'a pin', and traces it back through Middle English to Old French proignier, 'to trim the feathers with the beak'. The modern core image is still that Old French phrase, almost word for word.
依照附上的維基詞典資料，這個 preen 是 prune（修剪）的變體，受到另一個舊字 preen（別針）影響，再往前可以追到古法文 proignier，意思就是「用嘴整理羽毛」。今天的核心畫面，幾乎就是那句古法文本身。
Wiktionary, "preen" (etymology 2)
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