Tritrophic experiments with plants, herbivorous insects and birds – Part 6: Study about the attraction of birds to pines damaged by pine sawfly larvae

I will in this post tell about our latest publication. The title of that article is “The attraction of insectivorous tit species to herbivore-damaged Scots pines” and it was published in Journal of Ornithology (free read-only link). This study was part of my postdoc project at Freie Universität Berlin in Germany, and I did it together with Sven Kleier, Prof. Silke Kipper and Prof. Monika Hilker.

I have already during my PhD thesis studied attraction of insectivorous birds to herbivore-damaged birches and I got some good results in those (Mäntylä et al. 2004, 2008). Now I wanted to try the same thing with coniferous trees, since they are rather different from deciduous trees. Great tits and blue tits forage in coniferous forests, so they must be able to search signs of insects in those trees. Therefore, our study species were Scots pines (Pinus sylvestris), pine sawfly larvae (Diprion pini), great tits (Parus major) and blue tits (Cyanistes caeruleus).

We couldn’t use pines growing in forest but instead cut branches from trees and used those. The pine sawflies we got from the lab stock of Prof. Hilker’s group. I captured the birds with mist nets from the neighbouring Botanical Garden and from the garden of the Applied Zoology/Animal Ecology research group.

pine_branch

Scots pine branch with mesh bag (Photo by Elina Mäntylä)

The pine branches were organized in pairs. One branch would have larvae eating needles and one would be an intact control. We were interested in the systemic induced defence of pines. In the experiment the birds did not see or smell any larvae, damaged needles or faeces. So, we placed a textile mesh bag over the lowest twig of both branches (see photo on right). In the herbivore branch there were 30 pine sawfly larvae inside the bag eating needles and in the control branch the bag was left empty. The larvae ate needles for three days and then we cut off the twig with the mesh bag from both the herbivore and control branch.

Then the two branches were placed in the bird study booth. My previous blog post tells details of this booth. It was random which branch was on which side. I released the bird to the booth and a video camera recorded its behaviour for 15 minutes (see photos on top of this post and below). Then the bird was captured again, and I ringed (every bird was used only once) and measured it before release back to nature. Every bird (in total 100 individuals) was in captivity for ca. one hour.

greattit_booth

Great tit in a study booth (snapshot from video by Elina Mäntylä)

 

From the video data we saw that after calming down the birds significantly more often first jumped or flew to 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 and which control when I was looking the videos and collecting the data. Since there was no food available for the birds on the branches, they quite quickly lost their interest and just jumped from one branch to another.

figure_fc

(a) The total number of individuals per branch pair that chose a systemically herbivore-induced Scots pine branch (black bar, n = 62) or a control branch (white bar, n = 31) (***p = 0.001). (b) The same data separately for every branch pair (a new branch pair was used for every study day).

 

We were interested if the bird could use its vision and/or olfaction to recognize the systemically herbivore-damaged branch. Therefore, we had branch pairs treated in the same way as in the bird experiment and measured the light reflectance of the needles and collected herbivore-induced plant volatiles (HIPVs) emitted by the branches. The needles of the control branches reflected a bit more light than the herbivore-damaged branches but this difference was not significant. We used only seven branch pairs, so the sample size was rather small. This will need more studying later. The branches emitted 29 different HIPVs, and 21 of them were emitted significantly more from the herbivore branches. So, it is possible that the birds could smell the difference between the herbivore-induced and control branches.

To summarize, we showed for the first time that insectivorous birds are attracted systemically herbivore-damaged coniferous trees. And we found some support for both olfaction and vision being possible ways birds to recognize the herbivore-damaged trees.

 

© Elina Mäntylä (elkuma@utu.fi), 15 November 2016

 

PS. This study was done in autumn 2012, and it was finally published in November 2016. We had some problems finding a suitable journal. As this study combines avian, entomological, plant, behavioural and chemical ecology, it doesn’t fit in the profile of that many journals. But we are very pleased that Journal of Ornithology wanted to publish this. If someone wanted to start a journal with a name “Multitrophic interactions involving birds + some chemistry”, I would have couple of manuscripts to submit there. 😉

PS2. The original version of the manuscript had the author contributions listed but that was not included in the published version. So let’s post it here:

Elina Mäntylä (EM), Sven Kleier (SKL), Silke Kipper (SKI) and Monika Hilker (MH) were involved in designing the experiments, SKI helped to acquire necessary licenses, EM performed the experiments, SKL collected and analysed volatile and spectrophotometer data, EM performed statistical analyses, all authors wrote the manuscript.

 

 

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