This song sparrow may have tasted the monarch caterpillar, and then vomited it up, as the caterpillar has eaten the chemicals in the milkweed plant and will be unpalatable to most birds.
UPDATE: Thanks to help of a good friend, I realized I had mis-identified this bird. It is more probably a juvenile Rose-breasted grosbeak. Which makes sense because the grosbeak is one of the two bird species that are actually predators of the monarch. From my reading, I learned that it is possible they eat all except the abdomen of the caterpillar, as that is where the poison is. Amazing.
The last of the caterpillars are hatching, and that will be the end of the season for this year. I was happy to see many more caterpillars than the last few years.
In the close up of the caterpillar head, you can see the semicircular arrangement of its eyes, six eyes on each side, shiny black dots on one of the black bands.
This little beetle was quite evasive, dodging around behind the milkweed stem whenever my camera got close.