I had a monarch fly through my yard right after I planted some flowers.
Of course, before the squirrels ate the milkweed I was trying to grow, I didn’t see a single monarch. Milkweed is supposed to be their favorite food.
They are migrating right now.
No, it's their only food, for caterpillars anyway. The adults won't lay their eggs on anything else.
The caterpillars feasting on the bitter milkweed sap is supposedly what gives the adults their unpleasant taste, which is why the usual predators want nothing to do with them.
I used a habitat in August/September to help house Monarch caterpillars during a cold spell. I had 35 caterpillars and 31 successfully emerged from their chrysalises and flew away. It was a fascinating experience.