Did you notice any milk or butterfly eggs on the ground? Butterflies (and other animals) have finite lifespans, and when they ascend, they'll leave something behind.
When my daughter's butterflies died, they left a bounty of butterfly eggs. When mine died, nothing was left behind -- it's possible that they left milk and my piggies ate it, but since my piggy feeder was almost full, the piggies shouldn't have been "hungry."
Please have them leave eggs every time, if they're going to leave anything, and make them inedible to pigs?
Pigs don't eat milk, but it does seem to be a little random what parting gift an ascending butterfly might leave you -- a butterfly egg, a few milk, or nothing.