Feeding Ecology, Evolutionary Allometry, and Convergence in the Lower Jaw of New World Blackbirds and Allied Passerine Birds (Emberizoidea)

New World Blackbirds (Icteridae) are a diverse group of Neotropical passerines and an important component of the continental radiation of nine-primaried songbirds (Emberizoidea). Icterids and other emberizoids are chiefly omnivorous, but with variable predilection for invertebrates, fruits, and seeds. The icterids also exhibit a peculiar "gaping" behavior, whereby they insert their beaks into soil, fruits or bark and then open them inside the substrate. Cowbirds (Molothrus) stand out among the icterids for not being gapers and, with the exception of the Giant Cowbird, for a marked tendency towards a granivorous diet. To explore the signal of trophic ecology in the lower jaw of icterids and other emberizoids, we applied three-dimensional geometric morphometrics on Microcomputed tomography data and phylogenetic comparative methods. Phylomorphospaces were constructed and possible correlates with diet, gaping, and evolutionary allometry were assessed through phylogenetic ANOVA. Convergence between seed-eater species was also estimated by recently developed measures (Ct1). Results show a significant signal of trophic ecology in the mandible, with seed-eater species occupying a narrow domain of morphospace, partially overwhelming the signal of evolutionary allometry. The cowbird Molothrus ater stands out for clearly diverging from most icterids, showing significant convergence with other seed-eater emberizoids. Mandibular morphology, often neglected in avian ecomorphological studies, is informative of the trophic ecology of these birds.