I was very excited to find several Monarch caterpillars munching my milkweed plants this year. I then discovered a pest chowing down one of my guests. I removed the pest and watched my plants carefully. Unfortunately they began to disappear one by one til there were none left. I think it might have been birds, since they disappeared without a trace as opposed to insect pests. Does anyone have any thoughts about what may have happened and how I can prevent a reoccurrence next year?

Monarch butterflies and caterpillars have a reputation for tasting very bad. They have been known to make birds vomit after eating either of them. So to say some bird ate them would be hard to imagine.
In one of your images is see a smaller beetle that appears to be attacking a caterpillar. That surprises me because I thought that nothing bothered them. Maybe some one else will know more.

In fact other butterflies mimic the Monarch hoping to escape from being eaten because a predator would like twice because of the bad rep. The Viceroy Butterfly is one such example.

Lots of insects eat Monarch caterpillars: ants, spiders, wasps, beetles, praying mantids.....

As a lover of Monarch butterflies, you must know how essential it is to have Milkweed (also known as Asclepias) plants as they act as a host plant for these beautiful creatures. Now even though Monarchs prefer some of the varieties of Milkweed plants over others, they will happily lay eggs in most of them.

Lakegirl

Is it possible they have gone off to the next phase of development? They looked pretty big. I have found chrysalis on fences and neighboring plants. They know how to hide.

I appreciate all the replies I received. I was unable to find a definitive cause for either the complete disappearance or shriveled remains left behind. I brought some survivors into my kitchen ,while waiting for delivery of butterfly enclosures. I had great results and am looking forward to more survivors this coming year. One early observation.....the caterpillars are voracious eaters and prolific poopers!

Our plants had 14 caterpillars that dwindled down to three. I assumed it was the birds.

It is quite common for monarch caterpillars to "leave the area" when they are ready to form a chrysalis. There really isn't any incentive to stay. it's better that they find adequate cover, from predators or from bad weather.

They can be very sneaky at all stages- I think best bet is just keep planting more milkweed and other flowers for insects. If they're feeding wild birds.. well, then, you're helping wild birds