The historical biogeography of the New World blackbirds (Icteridae)

The New World blackbirds (Icteridae) are a widely distributed, ecologically diverse avian family whose biogeographic history remains poorly understood. Competing hypotheses have proposed Neotropical or Nearctic origins, while the relative roles of dispersal, in situ speciation, and range evolution as drivers of diversification have remained unresolved. Here, the historical biogeography of Icteridae is reconstructed by integrating phylogenetic bioregionalization, probabilistic ancestral range estimation, biogeographic stochastic mapping (BSM), and Geographic Hidden State Speciation and Extinction (GeoHiSSE) modeling. Six phylogenetically distinct biogeographic regions were identified, and 18 models of range evolution were compared across three dispersal hypotheses. The best-supported model strongly supported a Nearctic origin, with subsequent stepwise southward colonization beginning with Mesoamerica (~7.53 Ma), followed by Amazonia (~6.06 Ma), the Andes (~5.84 Ma), Southern South America (~4.83 Ma), and finally Patagonia (~2.68 Ma). The earliest dispersal from North to South America predates the establishment of the Isthmus of Panama, coinciding instead with a documented dispersal wave of the Great American Biotic Interchange (GABI) and open-habitat expansion in northern South America. Dispersal was the dominant mode of range evolution, while in situ speciation was the primary cladogenetic process. The best-fit GeoHiSSE model revealed that geographic range alone does not fully explain diversification rate heterogeneity, with observed geographic states and unmeasured hidden rate classes jointly shaping macroevolutionary dynamics. Model-averaged speciation rates were comparable across regions. Together, these results support a Nearctic origin and stepwise southward colonization of the Americas by Icteridae, while demonstrating that lineage splitting is governed primarily by unmeasured biological factors within bioregions rather than geography per se . Six distinct evolutionary regions were identified for Icteridae using phylogenetic bioregionalization. Strong support was found for a Nearctic origin of Icteridae, with subsequent stepwise southward colonization that predates the establishment of the Isthmus of Panama, rather than a South American origin, as proposed by Ernst Mayr in 1946. The expansion of Icteridae appears to be connected to the historical expansion of C4 grasses in the Americas. Speciation in Icteridae is not driven by geographical features; instead, it might be linked to other biological or ecological traits of the lineage, such as ecological specialization or sexual selection. Icteridae appears to follow the pattern of its parent clade (Emberizoidea) of within-region in situ speciation.