Experimental trees and their induction
In our experiment, we aimed to test responses of naïve chicks of great
tits (Parus major ) to an evolutionarily familiar odour of
herbivore-damaged Scotch elm, Ulmus glabra Huds.
(Ulmaceae ), and evolutionarily novel odour of herbivore-damaged
Cattley guava Psidium cattleyanum Sabine (Myrtaceae ). We
used saplings (1.5-m-tall) of Cattley guava as a novel plant species,
which did not have any evolutionary contact with our experimental bird
species. Genus Psidium was distributed throughout tropical
America and Caribbean region. However, they were adopted as a crop in
subtropical and tropical Asia, tropical Africa and Oceania. SeveralPsidium cultivars are also commercially grown in southwestern
Europe and Greece since the middle of the 20thcentury. Scotch elm (1.5-m-tall) was used as a sapling which naturally
occurs in the great tit range of distribution, and birds thus have
evolutionary contact with them. Saplings were planted in 20l pots into a
standard agricultural soil two months prior to the beginning of the
experiment. Control (N = 10) and experimental (induced) saplings (N =
10) were placed into two greenhouses just prior start of the experiment,
so the control saplings can’t receive chemical signals from induced
saplings. All saplings were watered weekly.
To prepare the induced version of a sapling (i.e. those which will
produce herbivore-induced volatile compounds), we slightly scratched 10
leaves by razor and applied saliva of Locusta migratoria on them.
After 30 minutes, we removed the damaged leaves. In this way, plants
started to produce herbivore induced volatile compounds, but the
“herbivorous” damage was not visible to the birds. For training as
well as experiments, new saplings were prepared this way every 4 hours,
i.e. for every 9th trial in experiment. In total, we
used 30 individuals of each plant species.
We predicted that untrained, naïve birds, will not show any preferences
for any saplings, with no respect whether the sapling provides
olfactorial signal of insect infestation or not, and without respect
whether the species is evolutionarily familiar or not. On the other
hand, we predicted that birds trained to find food on saplings of
experimental species will prefer that species also in pair-wise
preference tests. Finally, we predicted that birds will more easily
learn the volatile compounds of the tree species, which is
evolutionarily familiar, and naturally occurs in their distributional
range, than the volatile compounds of a completely novel, evolutionarily
unfamiliar, volatile compounds.