Northern Mockingbirds are astute mimics, so it's most likely he has heard an owl at some point in his life.
I've seen caged Mockingbirds in Mexico mimic rusty hinges,
cackling hens and dogs barking.
There have been test done that show even electronic analysis could not tell the difference between the Mockingbird and the original sound.
Scientist have found that female Mockingbirds are attracted to males that can make the most different sounds.

That's amazing litholad!
mimicking rusty hinges! lol!
I wonder what function it serves them?
Whatever it is, I'm impressed! I knew we didn't have that type of owl around here.....but I was certain it was its call. Mystery solved!

Mockingbirds are amazing. One day I was out in my neighbors' back yard and we heard a red tail hawk. It was very, very close so we started looking around. Couldn't see anything bigger than regular birds. The sound was coming from the top of a huge oak tree. While we were staring at this mockingbird, it launched with the most convincing red tail skree. Perfection down to the trail off on the skree.....amazing.

the mockers in my neighborhood are always impersonating blue jays. I've also heard the sounds of red-tail hawk, house sparrow, killdeer, crow, robin and also what sounded like a car alarm. They are amazing!

Many birds are amazing mimics; I remember several Starlings back in the 1970s that used to sit on a wire outside my apartment window in Louisville and perfectly mimic the crashing, squeaking sounds of the cars that bounced across the pot-hole filled parking lot.

Many other birds "speak" that are generally not noticed. Our male Pekin duck, "Pillow," talked so clearly one of my kids' playmates ran home and told her mom my duck was possessed. Our geese also spoke our names clearly and mimicked the dogs barking so perfectly I had to look at the dogs to make sure.

My hand-raised canary used to call me "Mom" as did most of my hand-raised baby parrots and finches. The adult birds-parrots, canaries, finches alike-called me "Rachel" but only if they needed something. I tried to make sure all feeders and sippers were full to make sure nobody had to call my name for assistance.

I no longer raise exotic birds but even now if a wild bird feeder is low, birds tap on the window, yell against the glass to make it amplify through the house, come to my car window to tell me about it, or even follow me up the mnt behind the house chirping in my face.

But everything communicates even if it can't "talk"; our frog splashes the side of the tank-HARD, when he wants another earthworm. The snake comes out and follows us up and down the side of the tank when he wants mice or his tank sprayed down. The mule stands in the corner by the gate and brays when he wants me to tether him out to graze. Whistle, the retired racking horse, also does so, but whinnies.

jsimran..........a car alarm? That's funny!

You're right rachel........every animal talks. We just have to decifer it!