I can't vouch for the quality of this site, but it says monarchs have been seen that high by aircraft. https://journeynorth.org/monarchs/resources/gallery/gall

I can't vouch for the quality of this site, but it says monarchs have been seen that high by aircraft. https://journeynorth.org/monarchs/resources/gallery/gallery-how-high-do-monarchs-fly https://journeynorth.org/monarchs/resources/gallery/gallery-...