My Viceroys are hatching. Now I know what to look for hatchlings. They eat the whole end of the leaf and leave the vein at the tip. Their mother lays them on the very end of the leaf tip on the bottom. They are eating willow, but will also eat poplars and cottonwoods (Populus).
This is a species search. They are everywhere.
If you want to really see this species of butterfly and many others and you can do it safely, put out some fruit to rot. They love it when it gets really rancid so don't throw it away.