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.