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

Plants use volatile organic compounds to attract invertebratepredators and parasitoids of their herbivore pests. Recently,it has been suggested that plants, either through visual orolfactory cues, may also "cry for help" from vertebrate predatorssuch as birds. We show that in a laboratory choice test, passerinebirds (Parus major and Cyanistes caeruleus) were attracted tothe intact branches of trees (Betula pendula) suffering fromfoliar damage caused by herbivore larvae (Epirrita autumnata)in nontest branches. Species, age, or sex of the experimentalbird or lighting (ultraviolet UV] or non-UV) did not affectthe preference. However, the birds made a clear choice betweenthe treatments when the trees came from a forest patch receivingmore sunlight, whereas no obvious choice was observed when thetrees came from a shadier forest patch. Results of the choicetest were supported by the spectral reflectance of tree leaves.In the sunnier forest patch, control trees reflected more visiblelight than the herbivore trees, whereas no such difference wasfound in the shadier forest patch trees. We suggest that avianpredators use their vision within visible wavelengths to findinsect-rich plants even when they do not see the prey itemsor damaged leaves.