Appearances are deceiving, even for bird poop.
Pretend you’re an insect that birds like to eat. Your life depends on evading detection from these master predators! But birds have keen eyesight, and recent research has even shown that birds that hunt insects have evolved the largest eye size to body ratio of bird species. How do you survive?!
Small Bird Dropping Moth "dripping" from Wild Rye grass
Insects have evolved a few strategies of their own. Mimicry, the act of closely resembling something to deceive, is one of those strategies. You might resemble a feared stinging bee, or a poisonous bug marked by red/ black coloration. Maybe even a plant branch or leaf. But one of the more ingenious approaches insects have evolved is resembling the poop of their predators! Have you ever seen birds playing in their poop? Pet dogs, yes. But probably not birds.
One of these is the Pale Gray Bird-dropping moth
One of these is a Tortricid family moth (with no common name)
What is bird poop? When fresh, it can be squiggly and has dark areas and white goo. What’s with that? Birds, different than mammals, only have one body opening for their waste. So it’s actually a combination of stool and pee – all mixed together. The stool area can be various shades of greens and browns, and even have speckles. The white parts have been believed to be uric acid which birds produce instead of urine like mammals. But scientists recently determined that conventional wisdom might be wrong. Regardless, the light-colored areas are the pee part. The combination is diverse in appearance, providing mimics opportunities to specialize in different aspects such as fresh, dry, speckled, splats, and even smears.
Can you tell the real poop from the Giant and Eastern Tiger Swallowtail caterpillar imposters?
Lepidoptera are poop mimic masters! The majority of bird poop insect mimics in our area are moths and butterflies. Considering they are a favorite bird food, this makes sense. The early smaller stages (instars) of swallowtail caterpillars are renowned imitators. This includes our common Black, Eastern Tiger, and Spicebush Swallowtails. Our migrant Giant Swallowtail mimicry is so amazing, it has its own common name – Bird Poop Caterpillar. In their final mature stage, these now larger instar caterpillars change to a different coloration. While the reason is uncertain, I venture a guess that birds aren’t tricked by a larger-than-life poop! Many moth caterpillar species mimic poop as well. In addition to coloration, they rely on resting with a bent posture that resembles the shape of poop that is often folded on itself.
Poop or Swallowtail caterpillar mimic?
Funerary Dagger Caterpillar using a bent posture to mimic poop
Does this strategy work? Researchers in Japan created fake caterpillars to test color and posture while observing bird behavior. Their fake green colored caterpillars didn’t fare well against the birds. Those colored black and white to resemble poop did much better, especially when placed in a bent posture. Their continuing work is attempting to answer new questions, like why swallowtails are successful without a bent posture.
Red-spotted Purple Pupa and Small Bird Dropping Moth mimic the tapered end of poop
Beyond caterpillars. The pupa of the Red-spotted Purple butterfly resembles a tapered poop. Adult Small Bird-Dropping moths are white on one end, have a dot in the middle, and are mottled in various colors of browns and greens on the other end, resembling a common poop configuration (see picture). This common moth, and a few of its relatives like the Olive-shaded Bird Dropping moth, can be found in our area if you inspect suspect poop on leaves.
Bolas Spiders mimic fresh poop splat
Beyond Lepids. There are other bug species that mimic bird poop too. Spiders are the most noteworthy, although some may be trying to trick different predators than birds. In our area, some orb-weaver spiders resemble splats, enhanced by their using disc-type silk designs. The Bolas Spiders boldly sit exposed on the tops of leaves all day with a glisteny appearance and shape that resembles a fresh splat. Scientists have studied orb-weaver spiders’ success in tricking wasp predators, and according to the research, they’re pretty convincing.
Were you fooled? All real poops in the photos are at the bottom or right of the imitators. Now it’s your turn. When you see bird droppings this year, examine them a little closer to find your own mimic.
Karen Campbell, a member of Lehigh Valley Audubon and FocusOnNatives.com, provided the content and images for this article.