Where have all my caterpillars gone?

This morning we had up to 20 caterpillars of various sizes, esp 2 big ones almost ready to pupate, but when we came home this afternoon we could only find 2 little babies! It has been a hot afternoon at about 26 degrees celcius would this make a difference? Or just what has happened?

Karen says:
It could have been a couple of things. First of all, I have watched out my window as the birds (Cardinals) "chow down" on my monarch caterpillars. I know that the toxins from the milkweed are suppose to make the caterpillars taste terrible and make the birds sick, but my cardinals seem to love them!

The other thing it could be is that you said the caterpillars looked ready to pupate. Usually the caterpillars do leave the milkweed and wander off to pupate, so maybe that's what happened.