The formation of the mountain ranges of Costa Rica and western Panamá, as well as the cold climatic conditions that prevailed during the upper Pleistocene, played a crucial role in determining the bird species composition of the highlands in this region. Glacial conditions favored dispersal movements of bird species from the Andes, and from the Neartic region. Subsequent inter-glacial conditions reduced the connectivity between neotropical highlands (e.g., Talamanca-Andes), and between neotropical highlands and Neartic temperate region, isolating recently established populations from the ancestral populations, and promoting speciation. Within Costa Rica, the highland vegetation and the birds that occupied this vegetation possibly had a continuous distribution throughout all mountain ranges during glacial periods. This vegetation retreated to the summit of the mountains during inter-glacial periods, fragmenting the original continuous bird populations and forming “sky islands”, which decrease in size from Talamanca mountains towards the northwestern ranges. The sizes of such islands of available habitat determine the number of highland birds present in each mountain range.
Keywords: highland birds, biogeography, climate, endemism, diglossa, catharus, piranga, Costa Rica