Birds select for higher mimetic accuracy, but imperfect mimics benefit from chromatic resemblance to their models.

Spiders have a high incidence of ant mimicry, exhibiting morphological, chromatic, and behavioral mimetic traits, but their mimetic accuracy varies greatly across taxa. Passerine birds may potentially select for accurate mimicry, but studies focused on their responses to ant mimics are surprisingly very scarce. In this study, we tested great tits (Parus major) as predators using photographs of ant-mimicking spiders, ants, and non-mimetic spiders to investigate the importance of morphological and chromatic traits for generalization from ant models to ant-mimicking spiders. Birds were divided into 2 groups, each trained to discriminate between non-mimetic spiders and either monochromatic (black) or dichromatic (black-reddish) ants, before being tested with mimics. Once birds from both groups learned to categorize correctly non-mimetic spiders and ants, they subsequently avoided morphologically accurate mimics, but only the birds trained to avoid dichromatic ants avoided morphologically inaccurate dichromatic mimics (e.g., Micaria sociabilis). Our results suggest that morphological and chromatic traits both play important roles in ant mimicry and chromatic traits can compensate for inaccurate morphological mimicry. Furthermore, classification of mimetic accuracy by birds corresponded roughly to the classification by humans.