There is no evidence that the West Indies were connected to a continent in the past, and thus the various native bird species are assumed to be descended from those that immigrated to the Caribbean at some point. Some small species would have encountered problems traversing large bodies of water, but parrots have flight strength and various behavioral characteristics that would facilitate "over-water" dispersion. Most Caribbean bird species originate from Central, North and South America. The Amazona species found in the Caribbean are divided in two groups: five mid-sized species found in the Greater Antilles and seven large species in the Lesser Antilles. All the Greater Antillean Amazons display characteristics leading to suppositions of relatedness, including predominantly green-toned color patterns and white rings around the eyes. Russello and Amato conclude that all Greater Antillean Amazona descend from Amazona albifrons with Amazona vittata, Amazona leucocephala, and Amazona ventralis constituting a complex, a cluster of species so closely related that they intergrade.