Happy as a Lark
I have heard it many times and was curious about its origin. "Happy as a Lark"
We humans tend to anthropomorphise animals, attributing human emotions and motivations to them. It's a harmless hobby. We attribute to larks, which have a beautiful melodious song, happiness... since humans sing when happy. The bird is issuing mating calls and staking out territory when biologists un-romanticize it... but "horny and territorial as a lark" just doesn't cut it as a phrase.