Bird distribution: a question of history and the nature of habitat

By BILL CHAISSON
Of a Feather
One of my fascinations with birds — and with life forms in general — is their geographic distribution. Why are species found where they are? Why are there, wood warblers, tyrant flycatchers, and orioles in the New World, but not in the Old?

The answer is a combination of historical and ahistorical factors. Ahistorical means without respect to history, meaning what happened yesterday or a thousand years ago doesn’t have any bearing on how things are right now. Pelagic birds (fulmars, petrels etc.), for example, can fly anywhere they want, but their distribution is determined by the availability of enough food and safe nesting sites (i.e. cliffs).

But historical factors are often the puzzling component of plant and animal distributions. Why are rhododendrons, for example, found in the wild in eastern North America and eastern Asia, but not anywhere in between? Why are boa constrictors native to South America and some Indo-Pacific islands, but are not found in Africa? 

Before the revolution of ideas in geology and geophysics that was eventually called plate tectonics, biogeographers and evolutionary biologists were forced to come up with some far-fetched stories about “land bridges” between continents to explain various distribution patterns. George Gaylord Simpson, who worked for the American Museum of Natural History, wrote a number of popular science books in the 1950s and ’60s that introduced the various land bridge schemes to the general public. My ninth grade earth science teacher was an early adopter of the theory of plate tectonics and in 1974 he added it to our curriculum even though it wasn’t in our textbook yet. When faced with the authority of George Gaylord Simpson versus my earth science teacher, I balked a bit, not really understanding at first how wrong Simpson’s ideas were.

But continental drift, the idea that the Earth’s surface was made up of giant plates that were being produced by volcanic activity on one side and consumed by volcanic activity on the other, resulting in their movement relative to one another, explained a lot of strange plant and animal distributions. Birds, of course, have the advantage of flight and some strong fliers are able to move from one continent to another as long as they are not too far apart. 

This may explain why there are parrots in South America, Africa, and Australia. This is a so-called Gondwana distribution. Gondwana was the great southern continent that included all those modern dispersed land masses as well as Antarctica. The last continent drifted over the pole, became covered with ice sheets and has relatively little life on it now, but fossil evidence shows its more verdant past, including the remains of life forms now found on the other three continents.

Fossil evidence for parrots does indeed show them to have originated on Gondwana nearly 60 million years ago. They drifted in different directions with the continents, a phenomenon called vicarious biogeography because the plants and animals don’t have to actually go anywhere, the continent is moving for them.

The tyrant flycatchers (Tyrannidae) represent an opposite sort of history; they are found only in the Americas. In fact they are the most diverse family in every New World country except Canada and the United States. Birds make notoriously bad fossils because of their lightly built skeletons and tyrant flycatchers are smaller and more lightly built than parrots. However, the advent of molecular methods of estimating the age of a lineage has put the origin of the tyrant flycatchers at around 15 million years ago. This is much more recent than parrots and well after the breakup of Gondwana. 

Tyrant flycatchers are confined to the New World because the oceans had gotten too wide to fly across. Their adaptive radiation coincides with a significant change in the plant community: the appearance of grass. We mow grass and it continues to grow because the cells that lengthen the stems are at the base of the plant, not at the distal ends as in most plants. This was an adaptation to cope with grazing animals and the combination of those animals and the grasses created a whole new kind of habitat: the grassland, which includes prairies, savannas, and steppes. 

Tyrant flycatchers sit on exposed branches and sally out to snatch flying insects on the wing. They became more and more diverse as varying types of grassland habitat evolved in different climate regimes. 

In the northeastern United States we have five species of flycatchers in the genus  Empidonax. They all resemble each other closely, suggesting that it has not been very long since their lineages split apart. But they are associated with distinctly different types of semi-open habitat. The Acadian flycatcher is found in broadleaf (deciduous) trees near open water. The yellow-bellied flycatcher in spruce woods around bogs. The willow flycatcher is associated with low, brushy habitat near water. The alder flycatcher is found in the same type of habitat as the willow flycatcher. Finally, the least flycatcher lives near clearings in mature hardwood forests.

All of these species are 5.5 to 6 inches long, greenish on the head, back and tail, have darker brownish-green wings with two white wing bars, and a white eye ring. The differences in their appearances involves the varying washes of gray or yellow on their chests and bellies. The willow and alder flycatchers are almost identical in appearance and habitat preference but segregate themselves by having different calls. The alder also ranges much further north, but their ranges overlap in the northeastern U.S.

So, on the grand scale you see an entire family confined to two continents after they were isolated by plate tectonics. Then the family became incredibly diverse (>400 species) by adapting to different types of open or semi-open habitats. On the evidence of the Empidonax genus, they are still doing it. Evolution never rests.