Another plausible explanation for the hairy–downy mimetic complex is deception of third parties, as hypothesized by Wallace (1869), later defined specifically as type-D disjunct defensive antergic mimicry (Rainey & Grether, 2007).
Indeed, we found some evidence for this hypothesis; downy woodpeckers held a nonsignificantly higher position in the dominance hierarchy than would be expected based on their body mass and phylogenetic position alone. These results demonstrate that downy woodpeckers win aggressive contests at a higher rate than expected (even after accounting for the tendency towards increased dominance in woodpeckers; see Miller et al., 2017a). Thus, species that are smaller-bodied and/or less aggressive than hairy woodpeckers (but of larger or comparable size to downy woodpeckers) are more likely to cede ground when challenged or chased by a downy woodpecker. This is similar to other cases of aggressive mimicry, where the mimic gains access to resources due to mimicry (Cheney, 2010, Nelson and Jackson, 2009).
We propose that reduced aggression from third-party species may be a more plausible hypothesis for avian mimicry than reduced aggression from the model. In general, competition is typically most intense between conspecifics (Anderson and Whiteman, 2015a, Anderson and Whiteman, 2015b, Hu and Tessier, 1995), and individuals typically need to identify conspecifics to breed successfully. Individuals should therefore be less prone to error when identifying conspecifics compared to identifying individuals of other species; it is therefore expected that hairy woodpeckers are adept at identifying whether or not another individual is a conspecific. Furthermore, vocalizations, well known species-recognition signs across birds, are given regularly by downy and hairy woodpeckers and are quite dissimilar (The Birds of North America, 2015). It is possible that other species besides hairy woodpeckers would be more readily fooled by hairy-mimicking downy woodpeckers, given the reduction in selective pressure to identify heterospecifics that are neither major predators, prey, nor close resource competitors. For example, downy woodpeckers often form flocks with white-breasted nuthatches, tufted titmice, Baeolophus bicolor, and black-capped chickadees, Poecile atricapillus. By occasionally fooling these nonmodel species into relinquishing resources (e.g. high-value invertebrates), the sum of these successful deceptions may provide the selective pressure that drives mimicry in this complex. Indeed, another study found that mimicking dangerous species can influence the feeding behaviour of target species. Specifically, Davies and Welbergen (2008) demonstrate that the Accipiter mimicry by Cuculinae cuckoos influences the behaviour of other sympatric species by depressing the feeding behaviour of smaller species.
