Adapted from Jim Conrad's online book A Birding Trip through Mexico, This excerpt from "Above Xoconostle" in San Luis Potosí state
Next morning, while working my way up a canyon, a metallic snapping sound causes me to turn and see a Greater Pewee, called Coue's Flycatcher in some books, with a Monarch Butterfly in its beak. For five minutes the pewee batters the butterfly against the dead snag it's perched on. Finally one of the butterfly's hindwings flutters to the ground. At this point the Pewee rears back its head and with jerking motions swallows the butterfly's remains. The Monarch's black body slides down easily but it takes about fifteen seconds for the orange and black wings to disappear. The whole episode astonishes me.
For one thing, the poor pewee is about as skinny and haggard looking as a living bird can be. Maybe it has a severe case of intestinal parasites or some other affliction. Moreover, I've never seen a bird work so hard to swallow anything. The impression is very clear that here is a starving bird eating something that normally it would avoid.
Also, Monarch Butterflies are famous for being so bitter that birds avoid eating them. Their flesh is permeated with noxious alkaloids from the milkweed plants Monarch caterpillars eat.
Once the Monarch is swallowed, the pewee wipes its bill on the snag twelve times and then preens it feathers for twenty minutes. This excessive bill wiping and feather preening suggests to me that the bird is trying to recover from an unsavory and unsettling experience.
Right now hundreds of Monarch Butterflies are sailing by. Some sail so high that they only show up in binoculars, but others pass within a meter of the disinterested pewee. The pewee's preening session ends only when a clear-winged, soft-bodied Mayfly-like insect wings by, for this is something more typical of a pewee's diet. This insect vanishes into the pewee in a fraction of a second.
In review, I first saw migrating Monarchs on September 21, in Kentucky. Then on October 6 a smaller number than this appeared at Samalayuca. The date on which I am seeing these great numbers in Santa Catarina is October 23. Many of the butterflies are faded and have tattered wings. It's easy to believe that they have come long distances.