The role of ecological and geographical drivers of lineage diversification in the Squirrel cuckoo Piaya cayana in Mexico: a mitochondrial DNA perspective

The Squirrel Cuckoo ( Piaya cayana ) is a widely distributed neotropical species with 14 recognized subspecies. Two parapatric subspecies are distributed in Mexico. P. c. mexicana is endemic to the seasonally dry tropical forests of western Mexico, and P. c. thermophila is found in humid forests from eastern Mexico to western Colombia. The 2 taxa have a small area of overlap on the Isthmus of Tehuantepec, but there is pronounced phenotypic differentiation, and very few reported intermediate individuals between these forms, leading to debate about the taxonomic status of the Mesoamerican taxa. Based on two mitochondrial genes, we used phylogenetic, phylogeographic, morphological, and ecological modelling analyses to describe the evolutionary relationships and paleo-distributional patterns of P. cayana in Mexico. Divergence time estimates suggest that the split between P. c. mexicana and P. c. thermophila occurred during the Pleistocene, about 1.24 Mya. Morphometrics of Mexican subspecies indicate that tail length and the white tail tips are significantly longer in P. c. mexicana , while P. c. thermophila has a longer and wider bill. Ecological niche analyses indicated that niche similarity between both lineages was lower than expected by chance, while low values on cross-prediction tests suggested that the two lineages have inhabited different environmental spaces since at least the Late Pleistocene. The ecological niche difference may also be associated with a steep humidity gradient, suggesting a significant contemporary environmental barrier. Overall, our results indicate that P. c. mexicana and P. c. thermophila have divergent evolutionary histories; therefore, the current taxonomic status of the Piaya populations in Mexico reflects neither their evolutionary relationships nor their apparent divergence. Our results suggest a major split in the polytypic P. cayana coinciding with the Andes, suggesting that the western endemic P. c. mexicana and P. c. thermophila are best treated as separate species-level taxa .