I suspect that Teresa is right and the robin will be sorry. But apparently, some birds can and do eat monarchs without any ill effects. I've read that broad-winged hawks will congregate along monarch migrating routes just to feast on them. Bluejays, on the other hand, are sickened almost instantly and learn never to try them again. I don't know about robins.
We have been led to believe that monarchs are so toxic that nothing will eat them, but they actually have many predators at every stage of life. I was hiking through a nature preserve last year and came upon a burrow of some kind of critter, and there were the remains -- wings, that is -- of scores of monarchs just outside it. The ground was littered with them. It was kind of shocking, but hey, that's nature.