Owls are birds of prey, so they only eat meat. This can include insects, butterflies, snakes, fish, skunks, porcupines, rabbits, and whatever other small animals they can lift.
They can't chew, their tongues just work like a conveyor belt to pull things down to be digested, so they can't really eat a wide variety of things, and they don't have a crop to break down fruit/vegetable matter or seeds like other birds.
You will find people online of course saying they've seen an owl eat this or that. Owls are goofy like most animals and will chew on things to check them out when they are bored, but I don't see any accounts of them eating anything but meat for sustenance. All the rescues give them various birds and rodents and that's it. They don't even need to drink as they get all the water from their prey, though again, they will drink if they're hot or bored or while taking a bath.
Cats are purely carnivorous. And yet we've had cats who'd regularly eat:
lettuce
potato chips
popcorn
canteloupe
grass
Hershey's Kisses
The lettuce thing was a kitten phase; she stopped doing it as she got older. Potato chips and popcorn are "cheat" foods; most of the chips we eat have MSG, which is the "meat" flavor, and we put seasoning with MSG and nutritional yeast on our popcorn, so they probably think they're getting meat. I've read many cats like cantaloupe; something in it tastes like meat, and it's pretty common. Grass, well... I don't know. Supposedly they eat it as an emetic, but this one just gnoshes on it and never throws it up. We have to keep a small lawn in the house for him; he's just a weirdo.
The Hershey's Kisses thing was odd. I know chocolate is toxic for cats, and we'd try to keep it away from him, but he'd find the bowl wherever we put it, pull one with carefully unwrap it, and eat the chocolate. He'd eat one or two at a time, and never seemed to suffer any ill effects from it. We'd re-hide the bowl, and when we discovered the wrappers around the house we knew he'd found it again. It was only around Christmas; Kisses in a bowl at Christmas was something my wife was brought up with. Anyway, strange cat.
Oh! We had another who'd dig through the trash for fast food wrappers and eat them. The paper. I think that's because he was an older adoption and had lived on the streets and learned to live out of trash cans, though.
This has nothing to do with owls, but there's a pure carnivore parallel and it makes me wonder. Are there owl individuals who pick up a taste for something omnivorous somehow?
Ok, now I am never going to be able to look at a barn owl again without seeing this lol.
(Also, I'm working my way through the Ori video game series. Not sure how I feel about barn owls being used as the big bad... Barn owls seem pretty cool.)
To most little creatures, any owl might as well be one of the fellbeasts the Nazgul ride on.
I hadn't heard of Ori, but it looks cool, though all the pics and videos I'm looking at are too dark. I'll have to look into it some more. No Xbox, so I'll have to watch a play through.
I'm playing on Steam on my PC! And it's available on Switch as well. If you're ever curious to play. It's not the most innovative metroidvania ever made, but the art, setting, and story are fun.
And yeah, I guess I can totally see an owl being the big bad villain of any forest from the perspective of a creature like Ori haha