<b>Behavioral co-option of plant secondary compounds by a cavity-nesting bird is an adaptation against competition and predation</b>

Interactions with competitors and predators can generate strong selection and favor the evolution of novel strategies for mitigating fitness costs. Adaptations to competition and predation risk often involve evolution of traits which directly reduce costs. Simultaneously, the evolution of behaviors that co-opt the functions of non-interacting, third-party species have also been observed. Here we investigate if the novel behavioral co-option of conifer resin in a cavity nesting bird can ameliorate interactions with nest predators and competitors. Red-breasted nuthatches (<i>Sitta canadensis</i>) collect resin from live trees and apply it around the entrances of their nest cavities in dead trees. We used field experiments and ancestral trait reconstruction to test <i>a priori </i>hypotheses about the evolution and adaptive value of this behavior. We rule out a major role of phylogenetic inertia and provide experimental evidence that application of conifer resin to nest cavities ameliorates interference competition from other cavity nesting species and reduces nest predation risk. We found that competitors brought less nesting material and were less likely to initiate nests in boxes treated with resin compared to paired controls. We also found that the application of resin to nest boxes placed near red squirrel middens reduced the likelihood that squirrels would chew and enlarge the entrance holes to obtain food from inside. Overall, our results showcase how behavioral co-option can have important consequences for fitness in nature.