Low light reflectance may explain the attraction of birds to defoliated trees

Plants use volatile organic compounds to attract invertebrate predators and parasitoids of their herbivore pests. Recently, it has been suggested that plants, either through visual or olfactory cues, may also “cry for help” from vertebrate predators such as birds. We show that in a laboratory choice test, passerine birds (Parus major and Cyanistes caeruleus) were attracted to the intact branches of trees (Betula pendula) suffering from foliar damage caused by herbivore larvae (Epirrita autumnata) in nontest branches. Species, age, or sex of the experimental bird or lighting (ultraviolet [UV] or non-UV) did not affect the preference. However, the birds made a clear choice between the treatments when the trees came from a forest patch receiving more sunlight, whereas no obvious choice was observed when the trees came from a shadier forest patch. Results of the choice test were supported by the spectral reflectance of tree leaves. In the sunnier forest patch, control trees reflected more visible light than the herbivore trees, whereas no such difference was found in the shadier forest patch trees. We suggest that avian predators use their vision within visible wavelengths to find insect-rich plants even when they do not see the prey items or damaged leaves.