watch birds与watch for bird：及物与否的纠结

对现代英语，watch的典型用法是及物动词，如

【例句1】A large crowd watched the game on Saturday. (一大群人在星期六观看了比赛。)[牛津高级学者辞典]

【例句2】I'd stayed up late to watch the film. (我熬夜看电影。)[柯林斯高级学者辞典]

【例句3】I had dinner and watched TV for a couple of hours.(我吃过晚饭，看了几个小时的电视。)[剑桥高级学者辞典]

【例句4】We sat and watched the sunset.(我们坐着看日落。)[朗文当代英语词典]

显然，to watch birds含义是“看鸟、观察鸟”，其birds是及物动词watch的宾语。

不过watch在古英语里并非是及物动词。watch源自古英语wæccan，其含义为be awake, 即处于清醒状态。再往前可追到原始日耳曼语的*wakjan和原始印欧语词根 *weg-，后者含义是to be strong, be lively，更是状态描述。及物含义的observe是15c中期才有的含义。

因为它本来就是不及物，所以watch for自然可以有。不及物的watch含义是“处于机警的状态”，介词for接“等候出现的目标”。所以to watch for birds的含义是“机警地等待鸟出现”(即鸟还没来)。与此对比，to watch birds的birds已经在视野里出现了。

按现代watch的主流用法，watch for就有点别扭，所以被主流词典列为phrasal verb(短语动词)或习惯用法。例句如

【例句5】The cat was on the wall, watching for birds. (猫在墙上, 等着鸟来。)[牛津高级学者辞典]

【例句6】We'll watch for any developments. (我们将密切等候事态的发展。)[牛津高级学者辞典]

【例句7】We'll be watching for any developments. （我们正密切等候事态的发展。）[柯林斯高级学者辞典]

【例句8】He watched for signs of activity in the house. （他注视着屋里是否有动静。） [牛津高级学者辞典]

【例句9】She stepped outside to watch for the cab. （她走到外面等候出租车。）[朗文当代英语词典]

机警等候某物的主体往往躲藏在角落向外望，所以有时用watch out for，如

【例句10】What problems should I watch out for when buying an old house? （买老房子时应该提防哪些问题？） [朗文当代英语词典]

【例句11】The cashiers were asked to watch out for forged banknotes. （收银员被要求盯紧假钞）[牛津高级学者辞典]

对现代英语watch out有“小心”之义，所以watch out for也有“小心注意”之义，如

【例句12】Watch out for the stairs—they're steep. （小心楼梯——它们很陡。）[朗文当代英语词典]

【例句13】Watch out for his latest movie, out next month. （请留意他下个月上映的最新电影。） [剑桥高级学者辞典]

【例句14】He called out to them to watch out for the unexploded mine.（他大声叫他们当心未爆的地雷。）[柯林斯高级学者辞典]

故而to watch out for birds可能是“等候鸟到来”，也可能是“提防有危险的鸟”。