Tritrophic experiments with plants, herbivorous insects and birds – Part 10: Birds can both see and smell systemically herbivore-damaged pines

During my postdoc at Freie Universität Berlin, I did several experiments to study are insectivorous birds attracted to herbivore-damaged pines. Now the last part of those studies is published in Ecology and Evolution: “Insectivorous birds can see and smell systemically herbivore‐induced pines” (website of the article with free open access). I did this study together with Prof. Monika Hilker and Prof. Silke Kipper. We did two separate experiments. In the vision experiment the birds could see, but not smell, the pine branches. In the olfaction experiment the birds could smell, but not see, the branches.

We used as study species great tits (Parus major), blue tits (Cyanistes caeruleus), pine sawflies (Diprion pini) and Scots pines (Pinus sylvestris).  I captured the birds from the garden of the Applied Zoology/Animal Ecology research group (and released them unharmed right after the short test). The building where we had the study booth for the behavioural tests was also in that same garden, so only very short distance to carry the pine branches and birds.

This study is a continuation of our earlier study where we showed that insectivorous birds were attracted to systemically herbivore-damaged Scots pines (Mäntylä et al. 2017). Then we showed the attraction but could not specify what senses birds could use.

The pine branches were organized in pairs. On a lower twig of each branch we put a textile bag. In the branch that would get the systemic induced defence due to herbivory we put 30 pine sawfly larvae inside the bag for three days. The intact control branch didn’t have any larvae inside the bag. As we were interested in the systemic induced defence of pines, we cut off the twig with the textile bag after three days from both branches. The remaining part of the branches was used in the behavioural tests and thus the birds did not see or smell any sawfly larvae, faeces or damaged needles. They could only sense the systemic cues of the pines.

Then the two branches were placed in the bird study booth. In the vision experiment, the branches were inside transparent, air-tight PMMA cylinders, and in the olfaction experiment the cylinders were white (non-transparent) and the lid was of tight mesh where air could get through but you could not see what was inside the cylinder. In both experiments I released the bird to the booth and followed its behaviour with a video camera for 10 minutes. Then the bird was captured again, and I ringed (every bird was used only once) and measured it before release back to nature.

From the video data we saw that in both experiments after calming down the birds significantly more often first jumped or flew to the cylinder with the systemically herbivore-induced branch (see figure below). The treatment of the branches was made blind to the observer, so I didn’t know which branch was herbivore-induced and which control when I was watching the videos and collecting the data. Since there was no food available for the birds, they quite quickly lost their interest and just jumped from one cylinder to another.

First choice of the birds in the vision and olfacion experiments.

In that previous study we had collected the herbivore-induced plant volatiles (HIPVs) and there was a significant difference in their emissions between the herbivore-induced and control treatments. We then also measured the light reflectance of the needles. There was a difference between the treatments, but it was not significant. Now based on the results of this study, we can say that those differences in visual and olfactory cues are clearly big enough for the birds to see and smell systemically herbivore-damaged pines.

PS. We did this study in 2012-13, and it was published in 2020. There were several reasons for the delay. I told about some results of these experiments already at the EOU 2017 conference.

© Elina Mäntylä (elkuma@utu.fi), 22 September 2020

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