Researcher collecting secretions from an oak (Quercus sp.) tree infested by caterpillars. This was part of a study into the ways plants may defend themselves from herbivores. In this study, chemicals secreted from an infested and a healthy oak were placed in an enclosure with a captive Eurasian blue tit (Cyanistes caeruleus). The blue tit preferentially flew towards the sample collected from the infested tree. This may indicate that infested trees can detect predation by caterpillars and secrete volatile chemicals to attract birds in response.