Covariation between morphological traits and ant-following behavior in Neotropical birds

Abstract Some species rely on others to obtain part, or in extreme cases, all of their food. For example, ant-following birds exploit insects that are disrupted by swarms of army ants. Such behavioral specialization is often linked to the evolution of morphological traits that enhance resource exploitation. In this study, I investigated whether morphological traits covary with ant-following behavior. I predicted that traits than enhance competitive ability, like a heavier weight and signals of dominance, should be more prevalent in species that rely more on ants to obtain food. Other morphological traits were also expected to diverge reflecting the unique lifestyle of ant-following birds. I focused on Neotropical families that include obligate ant-following species, as well as others that follow ants frequently, occasionally, or not at all—thus providing a gradient of ant-following behavior. Using a phylogenetic framework that incorporated primary lifestyle and foraging behavior, I identified several morphological changes in ant-following species. Compared to non-following species, obligate species were generally heavier with a significantly reduced wing area due to shorter wings and secondaries. They also had shorter tails and narrower, shorter bills. Additionally, bare periorbital areas, which are thought to act as signals in competitive interactions, were more common among obligate species than in other groups. Facultative ant-following species exhibited intermediate phenotypes and species that rarely follow ant swarms showed no significant morphological differences from those that never do. Overall, the findings support the hypothesis that morphological traits covaried with ant-following behavior in Neotropical birds.