Research from the University of Bristol has revealed that birds can learn to avoid plants that host toxic insects. While it was known that young birds easily learn to recognize the coloration and markings found on dangerous prey, the new study is the first to show that birds also remember the plants that harbor them.
“Our findings suggest that insect herbivores that specialize on easily recognizable host plants gain enhanced protection from predation, independent of their warning signal alone,” said study co-author Professor Nick Scott-Samuel.
To investigate, the scientists placed artificial, brightly colored cinnabar caterpillars alongside non-signalling fake caterpillars on two types of plants. The experts used ragwort, the natural host of cinnabar caterpillars, and bramble, a non-toxic plant that is not a natural host of the cinnabar. When foraging birds were abundant, both target types survived better on ragwort compared to bramble.
Study lead author Callum McLellan, a graduate student at the School of Biological Sciences, noted that Cinnabar caterpillars have a recognizable yellow and black appearance. “They also only live and feed on ragwort, which itself has distinctive yellow flowers,” said McLellan.
The researchers wanted to learn whether birds use these bright yellow flowers as a cue to avoid ragwort. They removed the spikes of flowers from the ragwort and pinned them onto the bramble.
This experiment revealed that only the non-signaling targets survived better on plants with ragwort flowers, compared to the same plant type without the flowers. Meanwhile, the cinnabar lookalikes survived equally across all of the plants.
“We have shown that birds learn that the ragwort flowers are a cue for danger, so can avoid going anywhere near toxic prey. It’s more efficient to avoid the whole plant than make decisions about individual caterpillars,” explained McLellan.
“Interestingly, any camouflaged caterpillars living on the same plant also benefit from birds’ learnt wariness of ragwort, despite being perfectly good to eat,” said Professor Innes Cuthill, who devised the study.
“Our results provide the opening to a brand-new discussion on how toxicity initially evolved in insect prey, and the conditions under which warning coloration is, or is not, favored.”
The study is published in the journal Current Biology.