Several studies have shown that insectivorous birds are attracted to herbivore-damaged trees even when they cannot see or smell the actual herbivores or their faeces. However, it often remained an open question whether birds are attracted by herbivore-induced changes in leaf odour or in leaf light reflectance or by both types of changes. Our study addressed this question by investigating the response of great tits (Parus major) and blue tits (Cyanistes caeruleus) to Scots pine (Pinus sylvestris) damaged by pine sawfly larvae (Diprion pini). We released the birds individually to a study booth, where they were simultaneously offered a systemically herbivore-induced and a non-infested control pine branch. In the first experiment, the birds could see the branches, but not smell them, because each branch was kept inside a transparent, air-tight cylinder. In the second experiment, the birds could smell the branches, but not see them, because each branch was placed inside a non-transparent cylinder with a mesh lid. The results show that the birds were more attracted to the herbivore-induced branch in both experiments. Hence, either type of the tested cues, the herbivore-induced visual plant cue alone as well as the olfactory cues per se, is attractive to the birds.

Birds were tested individually in a booth with two branches inside cylinders. One cylinder had a systemically herbivore-induced branch and one had an intact control branch. In the vision experiment birds could see, but not smell the branches. In the olfaction experiment birds could smell, but not see the branches.

In the analyses we used the "raw" values of first choice, time spent on cylinders and number of visits on the cylinders.

Birds that did not calm down were left out from the analyses (two individuals in vision experiment and four in the olfaction experiment).