I had to work for this one!
The first part was easy: "there once was an owl who bought jeans." The second part was much harder!
Saw there was alt text on the original: jeu de mot entre hibou et le jeans qui boudine => qui serre
All the translation things I ran it in did not help much. I knew I needed to get that last word that was a pun of owl and something. Boudine was coming up as "tight", "sausage", or "bullseye" and none of those felt right. Then there is "boudiné" which seems to mean "curly".
Most of these could have worked, given the picture, as the jeans looked rolled up, so it could be the cuffs curling, the owl's bottom looks a bit scrunched in so they could be too tight, but it didn't feel funny enough to be either of those.
I caught one stray translation at the bottom of one dictionary page that didn't even reference this particular meaning, but there was an example that said "Ce costume, il me boudine? / 'Do I look fat in this outfit?" Now this sounds like a winner! The dictionaries kept saying "podgy" was a translation that meant curly, but podgy sounds like pudgy, whoch means a little heavy looking, and looking at the drawing again, where the jeans scrunch the owl, it looks like when jeans are just a little too small and you get the stomach to come over the top giving the "muffin top" look.
So my final translation is: "there was once an owl that wore jeans, but they made my belly look hoo big" to work in an owl pun.
Did I get it?