Imagine, you have been dreaming all day about that delicious leftover pizza in your fridge from last weekend, only to open the box and see fuzzy green spots. You will probably throw the pizza away immediately, and maybe even gag at the sight of the mold. This reaction is caused by the green spots that inform you the pizza has gone bad. Similarly to how humans use visual signals to determine if food is expired, birds also rely on visual signals to determine what they can safely eat.

Visual signals are so important in the battle between predators and prey that certain species of prey have developed specific coloration signals to deter predators. For example, caterpillars are a popular dinner choice for many birds, but before picking a caterpillar to eat, birds must use visual signals to determine which caterpillar will make the best (and safest) dinner. Aposematic coloration is a visual signal displayed by certain species of prey to prevent predation. This coloration is typically bright yellow, red, or orange and tells potential predators, “Stay away! I am poisonous!” Aposematic coloration signals are beneficial for both the prey and the predator, but only if they are interpreted accurately.

To determine if prey coloration influences the predation choices of Mediterranean woodland birds, scientists J.A. Hernandez-Agüero, V. Polo, M. García, D. Simon, I. Ruiz-Tapiador, and L. Cayuela, performed an experimental study. At eleven sites in the Mediterranean woodlands, the researchers placed three green, brown, and yellow man-made, clay caterpillars in five separate trees, for a total of 45 clay caterpillars at each site. Over a period of 10 months, researchers visited the sites monthly, recorded the amount of damage on each clay caterpillar, and took note of which colors had the most damage. If necessary, they repaired or replaced the damaged caterpillars. Greater amounts of damage to a caterpillar correlates to higher bird attack rates and suggests it was a popular prey choice. Less damage to a caterpillar correlates to lower bird attack rates and suggests it was a less popular prey choice. After collecting data, researchers categorized the caterpillar damage by color to determine if a particular color had higher attack rates.

After evaluating the results, the researchers found that only two color groups varied in attack rates. The yellow and green caterpillars had the lowest rates of attack while the brown caterpillars had the highest attack rate. The difference in attack rates between yellow and green caterpillars was not statistically significant. The lower attack rates on yellow caterpillars suggest that the birds were able to interpret the aposematic signal telling them to “Stay away!” and the lower attack rates on green caterpillars suggests cryptic (camouflage) coloration hides the caterpillars from the birds. Brown caterpillars, on the other hand, seem to make the perfect dinner for birds, and had a significantly higher attack rate than yellow and green caterpillars.

Understanding how birds interpret visual signals, like coloration, provides insight on bird predation. Similarly to how the look of moldy pizza influences us to make different dinner choices, birds use visual signals like aposematic coloration to steer clear of potentially poisonous prey.

Hernandez-Agüero J.A., V Polo, M García, D Simon, I Ruiz-Tapiador, L Cayuela. 2020. Effects of prey colour on bird predation: an experiment in Mediterranean woodlands. Animal Behaviour. 170: 89-97