How do I mount a flat object on a rounded roof tile?
My pidgeon friends assure me your tawdry trick will fool noone. They laugh at your owl.

That said, you can attach the owl by creating using wax to make a dam, attach some wax to the decoy to temporarily hold it inside the dam, then pour a strong epoxy into the dam around it. If done correctly, a big block of epoxy will harden onto the roof, permanently attaching the owl.

Mortar can be used as an alternative to epoxy.