Dr. Heppner used very sensitive recording equipment to record the low-frequency
sounds made by burrowing earthworms. He found that robins ignored the
Dr. Heppner concluded that robins don't seem to notice the smell of worms
at all. He observed:"Robins nonchalantly ate foods smelling like
rotten eggs, decaying meats, rancid butter, and the absolutely worst smell
of all bad smells: mercaptoacetic acid."
Dr. Heppner didn't even consider the possibility that robins use taste
to find worms. (Robins capture worms before tasting them, and would have
to taste a LOT of dirt to pick out worms using their sense of taste!)
Heppner wondered if robins could feel the vibrations worms make and sense
them that way. He drilled worm-like holes in the ground and placed dead
worms in them. The robins peeked in the holes, found the dead worms, and
ate them readily! Therefore, he concluded that robins do not rely on their
sense of touch to hunt worms.
SIGHT (the conclusion!)
Dr. Heppner supected sight was the most important sense robins use to
find worms. He drilled holes that looked exactly like worm holes. Robins
ignored the holes UNLESS a worm was inside the hole within visual range.
Whether that worm was alive and normal, alive but coated with a bad-smelling
odor, or dead, the robins found the worms and ate them. He concluded that
sight is the key sense robins use to find earthworms.