Tritrophic experiments with plants, herbivorous insects and birds – Part 9: Study about the attraction of birds to pines induced by pine sawfly oviposition

I tell in this post some further details of our recently published article “Insectivorous birds are attracted by plant traits induced by insect egg deposition”. It was published in Journal of Chemical Ecology (website link and free read-only PDF). This study was part of my postdoc project at Freie Universität Berlin. I did it together with Prof. Monika Hilker, Prof. Silke Kipper and Sven Kleier working in there, and with Dr Carita Lindstedt from University of Jyväskylä.

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 continued from our earlier study where we showed that insectivorous birds were attracted to Scots pines with systemic induced defence caused by herbivory of pine sawfly larvae. Actually this study was a step backward because now we studied the attraction of insectivorous birds to Scots pines that had systemic induced defence caused by the oviposition of pine sawflies. So, before there were any larvae or herbivory.

The pine branches were organized in pairs. On a lower twig of each branch we put a perforated plastic bag. In the branch that would get the systemic induced defence due to oviposition we put three adult male and female pine sawflies inside the bag for three days. The intact control branch didn’t have any insects inside the bag. As we were interested in the systemic induced defence of pines, we cut off the twig with the plastic 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 eggs or damaged needles. They could only sense the systemic cues of the pines.

Then the two branches were placed in the bird study booth. My earlier blog post tells details of this booth. I released the bird to the booth and I followed its behaviour with a video camera for 10 minutes (see snapshot of the video on top of this post). 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 70 individuals) was in captivity for only ca. 40 minutes.

From the video data we saw that after calming down the birds significantly more often first jumped on or flew to the systemically oviposition-induced branch (see figure below). I explain the methods of observing the behaviour in this blog post. The treatment of the branches was made blind to the observer, so I didn’t know which branch was oviposition-induced 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.

Figure1_firstchoice

A Total number of individual birds that chose first a systemically egg-induced (black bar, n = 46) or a control (white bar, n = 22) branch after calming down. **: χ2 = 8.47, P = 0.0036. B Number of individual birds that chose first an egg-induced or a control branch offered each day of the experiment.

 

We were interested if the bird could use its vision and/or olfaction to recognize the systemically oviposition-induced branch. Therefore, we measured from branches treated in the same way as in the bird experiment the light reflectance of the needles and collected oviposition-induced plant volatiles (OIPVs) emitted by the branches. The needles of the control branches reflected significantly more light than the oviposition-induced branches. The avian discrimination threshold model suggested that the birds’ ability to discriminate between control and oviposition-induced branches was poor in terms of hue and luminance. But maybe in bright light conditions the discrimination could have been possible. The branches emitted 29 different OIPVs, and one compound, (E)-β-farnesene, was emitted significantly more from the oviposition-induced branches. This compound is known to attract egg parasitoids of the pine sawfly, so we were pleased that we collected that compound also now.

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

 

© Elina Mäntylä (elkuma@utu.fi), 24 November 2018

 

PS1. We did this study in 2013, and it was published in 2018. There were several reasons for this delay. But I had told about the results of the study already in presentations at congresses of European Ornithologists’ Union 2015, Nordic Oikos 2016 and European Conference of Behavioural Biology 2016.

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

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, Carita Lindstedt (CL) did the discrimination threshold models, EM performed statistical analyses, all authors wrote the manuscript.

 

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