The Monarch caterpillars are gone. In the end, there were three in total, one plumper than the others. I am hoping that the biggest one managed to make its way to another leaf under which to pupate. But the situation looks grim for the other two. Not wanting to name any culprits, I will simply say that most birds do not eat Monarch caterpillars because the caterpillars ingest toxins from the Butterfly Weed/milkweed leaves they eat, and as a result, the caterpillars taste terrible. Thus, smart birds only make that gastronomic mistake once.
But there is a bird that likes insects for another reason: to use them as mini brooms with which to sweep the entrance to its nest cavity. That bird is called a White-Breasted Nuthatch.
Among its other eccentricities, the Nuthatch likes to hang out on trees face down.
Now, I’m not pointing fingers, but in contemplating the loss of the caterpillars, I noticed a small bird that seemed immune to the forces of gravity on the tree trunk right above the Butterfly Weed (previously home to the caterpillars). Furthermore, it remained immobile for long stretches of time, staring straight down at the Butterfly Weed in rapt concentration.The clues were piling up.
Appearance at the scene of the crime? Check. Motive? Check. Opportunity? Check.
Conclusion (based on total conjecture): having recently swept its nest with a couple Monarch caterpillars located very conveniently under the bird feeder and tree, the Nuthatch was so pleased with its efforts that it is now looking for a few more of those excellent brooms.
Lesson: Take the caterpillars indoor next year.