RESUMO
Detailed osteological descriptions of the craniomandibular complex of passerine birds are lacking for most species, limiting our understanding of their diversity and evolution. Cowbirds (genus Molothrus) are a small but widespread group of New World nine-primaried songbirds, well-known for their unique brooding parasitic behavior. However, detailed osteological data for cowbirds and other Icteridae are currently scarce and several features of their skulls remain undescribed or poorly known. To address this issue, a detailed comparative osteology of cowbird skulls is presented here for the first time based on data from x-ray microcomputed tomography, dry skeletal data, and multivariate analyses of linear morphometric data. Cowbird skulls offer some functional insights, with many finch-like features probably related to a seed-rich diet that distinguishes them from most other icterids. In addition, features previously overlooked in earlier studies might provide valuable phylogenetic information at different levels of passerine phylogeny (Passerida, Emberizoidea, Icteridae, and Agelaiinae), including some of the otic region and nasal septum. Comparisons among cowbirds show that there is substantial cranial variation within the genus, with M. oryzivorus being the most divergent cowbird species. Within the genus, distantly related species share similar overall skull morphology and proportions, but detailed osteological data allow species identification even in cases of strong convergence. Further efforts are warranted to furnish baseline data for future studies of this iconic group of Neotropical birds and to fully integrate it into phylogenetic comparative frameworks.
Assuntos
Crânio , Microtomografia por Raio-X , Animais , Crânio/anatomia & histologia , Filogenia , Masculino , Osteologia , Feminino , Aves Canoras/anatomia & histologia , Evolução Biológica , Passeriformes/anatomia & histologiaRESUMO
Cowbirds are a successful group of obligate brood parasites in the Neotropical passerine family Icteridae that offer an interesting model to explore the factors behind the evolution of the bird craniomandibular complex. The Giant Cowbird, Molothrus oryzivorus, stands out from its congeners, among other features, in diet (feeds mostly on fruit, nectar, and arthropods, instead on seeds), its larger body size, and longer, more robust beak with a much broader bony casque than in other cowbirds. In turn, Giant Cowbirds show a remarkable resemblance in these features to the distantly related caciques and oropendolas (some are its breeding hosts). However, the causes behind the latter resemblance and the distinctiveness among cowbirds have not yet been elucidated. We aim to explore the factors involved in the diverging morphology of the Giant Cowbird from its congeners and the convergence with caciques and oropendolas, surveying their skull and lower jaw under an explicit evolutionary framework. Using geometric morphometrics and comparative methods, we assessed the signal of phylogeny, convergence, feeding ecology, and size in skull shape. Our results indicated that evolution of the craniomandibular complex of icterids in general, and of the beak morphology in the Giant Cowbird in particular, are shaped by multiple factors, with phylogeny being largely overridden by changes in size (evolutionary allometry), primarily, and feeding ecology, secondarily. However, the evolution of a broad bony casque in the Giant Cowbird, otherwise a hallmark of caciques and oropendolas, does not appear to have primarily been ruled by evolutionary allometry. Instead, taking into account the unique extreme convergence between Giant Cowbirds and some of its caciques hosts, it might be consequence of selective regimes associated with parasite-host interactions acting on top of other evolutionary processes. This suggests chick mimicry as a reasonable explanation for this peculiar morphology that would require further investigation.
Assuntos
Parasitos , Passeriformes , Animais , Bico , Filogenia , CrânioRESUMO
The Unicolored Blackbird Agelasticus cyanopus (Vieillot, 1819) is a marsh bird with four allopatric subspecies restricted to lowlands in South America east of the Andes. I conducted a taxonomic revision of the species based on analysis of external morphological characters of 288 study skins, including all types available. My revision shows that: 1) Leistes unicolor Swainson, 1838, is a senior synonym of A. c. xenicus (Parkes, 1966) and, therefore, the correct name of the taxon should be A. c. unicolor (Swainson, 1838); 2) the range of A. c. unicolor (Swainson, 1838) is much wider than previously thought, extending from the mouth of the Rio Amazonas to the state of São Paulo, in southeastern Brazil, where it intergrades with A. c. atroolivaceus (zu Wied-Neuwied, 1831); 3) A. c. atroolivaceus extends its range well beyond the coast of Rio de Janeiro, reaching the coast of São Paulo, the central part of Minas Gerais, Bahia and Espírito Santo; and 4) specimens attributed to A. c. beniensis are highly variable, so this name must be considered a subjective junior synonym of the nominotypical taxon. Under the Biological Species Concept, two broadly parapatric species should be recognized, A. cyanopus and A. atroolivaceus (including unicolor as a subspecies). Under the Phylogenetic Species Concept or the General Lineage Concept of Species, the best taxonomic treatment is to recognize three species: A. cyanopus, A. atroolivaceus, and A. unicolor.