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Twig Mimic Suggests Extreme Bird Selection

I am fond of making the point that toxic/poisonous insects are brightly colored to warn potential bird predators, and others such as some moths have eye spots to scare off birds. This indicates how important intense avian predation is in selecting the colors and patterns of insect prey.

But the opposite should also be true, but we do not think about it so much- namely that tasty insects should become cryptic and well camouflaged. Now we probably do not observe this phenomenon much for the obvious reason that these camouflaged bugs are as hard for us to see as they are for birds. The attached photo illustrates this principle for a caterpillar of unknown species (perhaps a zale moth?) that I found by chance today on the twig of a black birch. The photo was taken with a flash and makes the caterpillar a bit more conspicuous than it was in natural light. Either way however it matches the twig closely and accentuates this effect by the way it presses against the twig and places its head at the junction of two twigs.

The impact of the tireless and detailed examination of vegetation by birds is that a lot of insects are found and eaten, but certainly not all of them. The best hidden and lucky ones live to reproduce and perpetuate their genes. Indeed an experiment showed that trees from which birds were excluded had a lot more herbivory than those "cleaned" by birds.

So how many of you have seen these remarkable, well-hidden caterpillars? Surely there are a great number of them present so let's keep a sharp eye out for them and do our part to protect our insectivous feathered friends (and their arthropod counterparts) by using sustainable management practices.

Bill Dunson
Galax, VA & Englewood, FL

click here for lager view of the image