Does anyone actually use "as a lark" to mean the action was done while being a bird? Common usage these days is for "as a lark" to be effectively synonymous with "on a lark"; thus it would be more useful/helpful for people to be familiar with this variation of the common idiom.

"On a lark" is something I (a native English speaker, from London, which is in England, after which the language is named) have never heard or seen written -- anywhere -- before your ill-considered and wholly-incorrect posts on this thread.

Shush.

Not sure why new information has provoked such a personal response from you. As a native English speaker from London, have you heard of “as a lark”?

Usually in British English (Yorkshire, might vary by region), I just hear "It is/was/will be a lark", so I'm not sure if either is particularly common. However, I've definitely come across people using "as a lark", whereas I can't recall hearing "on a lark". I've seen it used in text, but I'm not sure how many were from British English speakers.