Researchers from the Experimental Station of Arid Zones (EEZA-CSIC) and the Netherlands Institute of Ecology (NIOO) have discovered that birds that feed on insects are attracted to trees infested with butterfly caterpillars (Lepidoptera), and the mechanism responsible for this behaviour.
“When faced with a caterpillar attack, a defence mechanism is activated in the plant which involves releasing volatile compounds that predatory birds use to find their prey,” Luisa Amo de Paz, lead author of the study and researcher at the EEZA-CSIC, tells SINC.
“This phenomenon had been studied in arthropod predators, but scarcely in insectivorous birds, even though they are one of the most important predators of insects,” she continues.
To discover this mechanism, scientists carried out various experiments with great tits (Parus major). They allowed the birds to choose between a tree infested with Lepidoptera caterpillars and another uninfested tree. In the experiments they removed any chemical residues from the caterpillars in order to be able to conclude that the birds are attracted by chemical signals emitted by the tree, and not by any signal from the caterpillars.
“This phenomenon had been studied in arthropod predators, but scarcely in insectivorous birds”
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“Our results showed that great tits are able to distinguish between trees infested with caterpillars and uninfested trees, since we observed that the birds visited the infested tree first, and also visited the tree with caterpillars more frequently than the uninfested tree,” the researcher notes.
The birds were attracted to the infested trees even when we removed the caterpillars and leafs they had damaged just before the experiment, which proves that the birds receive a signal from the infested tree.
The infested and uninfested trees differ in both the emission of volatile compounds and the colour of the leaves. Both sight and smell could be involved in the birds´ discrimination.
“However, we conducted a second experiment to discover which type of signal the birds were using. In this experiment we offered the tits both signals separately, and we observed that the birds were equally attracted to the infested trees when they could only smell them, but not when they could only see them,” Amo de Paz points out.
This suggests that birds can smell which tree is infested thanks to differences in the chemical compounds emitted by plants.
A mutual benefit for birds and plants
According to the researcher, this confers a benefit to the plant, since the insectivorous birds are large predators and help them to get rid of insects.
For the bird, it is also beneficial to use chemical signals from infested plants, since they provide information about the presence of their food. This is especially important in breeding periods, when birds must find food not only for themselves, but also for their chicks.
“This proof of insectivorous birds´ ability to use plants´ chemical signals is very important, given that the predation rates of these animals are much higher than those of arthropod predators. Moreover, it highlights the need to consider insectivorous birds in biological pest control,” Amo de Paz concludes.