Abstract
The geography of the Americas is similar to the form of an hourglass. The uppermost bulbous end is composed of Mexico, the United States and Canada. The lower portion is South America. Joining these two is the narrowing neck of Central America.
Inverting an hourglass enables gravity to pull sand through the constricting collar and likewise there are natural pressures that would move animals and plants past the Isthmus of Panama. Obviously, birds will have a greater facility to transfer up or down than mammals. Migratory species sweep through Panama twice in a year. However, those with less well developed powers of flight, or those specialized for a particular habitat or diet, would find it more difficult. Some species have found it impossible to shift through Central America. Cranes and chickadees have still not entered the southern continent, whereas a few species of tinamous, cracids, hummingbirds, toucans, antbirds and cotingas have progressed past Panama in the opposite direction. This is certainly true for parrots.
If we think of the nectar-feeding lories, the nocturnal, grazing Kakapo, the inner-bark eating pygmy parrots; the broadtails and cockatoos of the Australian region and then compare these with the parrots from the Americas, the first group seems to have a far more obvious complexity. There may be several reasons for this. Isolation on islands is notorious, in the evolutionary sense, for rapid speciation. Another is that all of this area has suffered repeatedly from climatic change, and with this the changing vegetational patterns with each event would thrust the parrots into another evolutionary maelstrom. Perhaps, therefore, this rich, antipodean variety has taken a comparatively short geological time to accomplish. The "age" and history of parrot development is another story.
Nevertheless, the overall sameness of the New World forms suggests that they are relatively recent in a geological sense. This is but one reason why I am convinced that they immigrated there somewhere between fifteen and ten million years ago.
The world's climate was then warmer than it is today and the hardy trees such as pine, alder, and birch, and cold-resistant herbs and grasses grew to the extreme north. It was not just parrots but many animals, including such cumbersome creatures as the elephants, that crossed from Asia, over the Bering Straits, to become the first representatives of their kind in North America. Precisely as did the Asiatic people, some 45,000 years ago, the limited number of pioneering parrots who gained entry into such a fertile land would have rapidly spread their multiplying populations as far south as Terra del Fuego.
In the absence of competitors for hard-shelled seeds, in a "double continent'' providing so very many different habitats, vegetation and climates, the parrots would have experienced exactly the circumstances to give them a comparatively rapid speciation. Ultimately, this gave us the modern American species as is found today.
Long after the time of their entry, the several ferociously protracted Ice Ages must have wiped the "slate" of most of North America completely clear of parrots. As each cold spell ended, the intervening warm respites allowed parrots to expand upward and outward from the tropics. The dry deserts of northern Mexico and California were barriers and this made it difficult for the parrots to get into the States. Nevertheless, the Carolina Conure (Aratinga carolinensis) (it may have got in via Florida) established itself as far north as the state of New York. A few conures, some Amazons and the Thick-billed Parrot (Rbyncbopsitta pacbyrbyncba) came to inhabit Mexico and northern Central America. (The Thick-bill may, today, be not too dissimilar from its Asiatic "ancestors").
Some macaws were able to move northwards out of South America. Of those that did, the Green-winged (A. cbloroptera) and Severe Macaws (A. severa) became stuck at Panama. Two species, the Military (A. militaris) and Scarlet Macaws (A. macao), managed to get through. Once there, they had no competition from other macaws and could evolve into their "niches'.' Coming from the mountain slopes and moving into the humid tropical lowlands isolated some Military Macaws. This ecological difference enabled them to evolve larger size, paler color, and now a different pattern of behavior. This went to such an extreme that they became a different species: Buffon's Macaw (A. ambigua). Elsewhere, the Military Macaw also became somewhat less montane and has now diffused over much of Mexico. These now differ subspecifically from those to the south.
Likewise, the Scarlet Macaw of Mexico and Central America may also have gained entry only towards the end of the last Ice Age (some 12,000 years back). Originally, so few individual birds percolated through the constraints of the Isthmus that they were limited in their genetic diversity. Once through this geographical barrier, the inevitable inbreeding and subsequent evolution from the founder-immigrants ensured that they developed a different appearance from those Scarlet Macaws found on the mainland of South America.
The museum worker is very much inclined to subdivide species of birds into geographical races ( or subspecies). The usual reasons are that they have differences in coloration or size. It is strange that no one has subdivided the Scarlet Macaw.
While in Washington, D.C. for the 1990 AFA Conference, I went to further confirm this in the ornithological section of the Smithsonian. At once, I found that Joanne Abramson (of Raintree Macaws) had independently come to the same conclusion, for she was there carefully working through their skins.