Repeated, irreversible evolution of ant-following behavior across Neotropical avian families

Ecological specialization is a result of the interplay between ecological and evolutionary processes. One iconic ecological specialization of the Neotropics involves birds that follow army ant swarms in feeding groups. Prior work has focused on a single avian family, the Neotropical antbirds (Thamnophilidae), but over a century of fieldwork has now revealed that ant-following occurs in hundreds of distantly related birds. To understand the relative contributions of shared ancestry and ecological specialization in the evolution of ant-following, we compiled a database of all Neotropical ant-following birds (n = 472 species) and their degree of specialization on army ants, and tested if (1) ant-following becomes increasingly specialized through evolutionary time and (2) ecomorphological functional traits predict ant-following behavior. Ancestral state reconstruction revealed that specialized ant-following evolved independently in 8 clades and 4 families of Neotropical birds (Antbirds: Thamnophilidae, Ovenbirds: Furnariidae, Tanagers: Thraupidae, and Cuckoos: Cuculidae). Ant-following behavior was highly conserved phylogenetically (Pagel's λ = 0.97), and specialized clades evolved from less specialized ancestors, with few evolutionary reversals. In contrast, ecomorphological traits poorly predicted the level of ant-following specialization across species. Our results suggest increasing specialization on army ants is governed by niche conservatism, not ecological specialization.