This past summer, WC was in the Pahshimeroi Valley in eastern Idaho, a guest at a ranch there. On a foggy, grey morning, a very young Western Wood Pewee appeared on the patio. Pewees are insectivores, but this youngster was willing to experiment. These photos were taken in very low light, through a double-pane glass door. They aren’t very good. But they tell a story.
Note the foam starting to appear around the edges of the young bird’s bill. WC assumes that’s a reaction of the bird’s saliva to something excreted by the slug. The slug was still moving, so the Pewee did what an insectivore does.
Three or four good whacks later, the Pewee checked to see if the slug was dead. It was still moving. Note there is still more foam around the Pewee’s bill.
It’s not like a slug has a central nervous system, so the Pewee whacked it even harder.
This time the Pewee gave the slug a full 30 seconds of thorough whacking.
The foam didn’t seem to harm the Pewee, but it was hard for the bird to clean the stuff off. Published literature says that some birds eat slugs with evident relish. Western Wood Pewees, not so much.