Behavioural co-option of plant secondary compounds by a cavity-nesting bird is an adaptation against competition and predation

1. Interactions with competitors and predators can generate strong selection and favour the evolution of novel strategies for mitigating fitness costs. Adaptations to mitigate competition and predation risk often involve evolution of traits which directly reduce costs. Simultaneously, the evolution of behaviours that co-opt the functional traits of non-interacting, third-party species have also been observed but remain poorly studied (e.g. anointing behaviour, tri-trophic interactions). 

2. Here we investigate if the novel behavioural co-option of conifer resin in a cavity-nesting bird can ameliorate interactions with nest predators and nest site competitors. Red-breasted nuthatches (Sitta canadensis) collect resin from live trees and apply it around the entrances of their nest cavities in dead trees. 

3. Using paired nest boxes, we mimicked the nuthatch behaviour by apply resin around the hole of one and not the paired box in two field experiments to test a priori hypotheses about the evolution and adaptive value of this behaviour. The first experiment baited the paired boxes with food to attract mammalian predators (e.g. red squirrels Tamiasciurus hudsonicus). The second experiment left boxes available for breeding by cavity-nesting birds that compete with nuthatches for nest sites (e.g. house wrens Troglodytes aedon). We also used ancestral trait reconstruction to test whether resin use is a non-adaptive derivation of mud-plastering behaviour observed in numerous congeners. 

4. Experimental nest boxes with resin applied to the entrance were less likely to have bait removed and less likely to have nests built or initiated than paired nest boxes without resin. Ancestral trait reconstruction suggested that resin use was more likely (>77%) to have evolved in ancestors that did not exhibit mud-plastering behaviour. 

5. Our results do not support 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. 

6. Overall, our results showcase the ecological function of behavioural co-option and its important consequences for fitness in nature. Read the free Plain Language Summary for this article on the Journal blog.