Birds have flown the coop

"Birds have flown the coop." he said, opening a bedroom door.
"Birds?" I said.
"My children." he said.
"Your children went abroad?" I asked.
"No, they are here in US." He said.
I remember this conversation although about 20 years have past. He was MIT professor of mechanical engineering and was kind enough to invite me over to his house for dinner. On our way to his house in his car he asked me about my kids and I told him how cute small kids were. Soon we got to his house and went in his house where his wife welcomed me. Then he showed me around in his house, a big one. As he opened one bed room, he suddenly talked about "birds."
As you may know already, he meant by birds children, by coop his home. He meant "My children have already got independent and no longer here." I was not sure if this was a kind of idiomatic expression because there was none in my dictionary. (I should have asked him.) But in my flight back to Japan, I was surprised to find the same expression in an article of a magazine. So I knew it was an idiom. Since then I have used this a few times maybe.
I once heard an idiomatic expression "par for the course" where "common" should be used. I guess this comes from a golf term. As I always tried to use the words I'd newly learned, I said in a chat at a party something like "That's par for the course with children." "Pardon?" he asked me. So I quickly rephrased. "that's common to children."
If we can't get something across to someone, one of the possible reasons is that he or she does not expect that we know such difficult or rarely used expressions. In other words it sounds unnatural for a person with low-level English to use such high-level English (Another possibility is he or she does not know such idioms, especially when he or she is young). For a certain period of time, I avoided using idioms. But as I learned English more, I gradually came to know when and to whom we can or should not use such expressions. So it is a matter of getting used to.
When I think back, I didn't have to avoid using unnatural expressions. It may be a bit funny but who cares. Use whatever you learned and make mistakes and laugh it off. That's the royal road to success if there is one.
By the way my bird still seems to enjoy staying in the coop.