Extinction & Evolution
Edentulism – the absence of teeth – evolved convergently among various vertebrates, including turtles, birds, and several lineages of mammals. Birds lost their teeth and went with a better beak ~116 MYA.
Numerous early dinosaur-birds diversified before the Chicxulub asteroid impact. Rather modern-looking birds began to appear over 100 MYA. Though most did not survive the aftermath of the Yucatán big bang, many living avian families flew among the dinosaurs.
The birds that did cross the K–Pg boundary were able to do so because they were small generalists, able to relocate and adjust to different environments. Being able to subside on seeds was probably critical.
The birds that made it quickly adapted to further their flight capabilities, as well as taking advantage of nutrient sources and nesting opportunities. The wings, tail, hips, legs, and feet refined, as did the core parts of the body that power flight.
The most momentous change birds underwent was losing their long bony tails during the early Cretaceous. This freed up their legs to become versatile and adaptable tools.
Avian flight allowed birds to readily populate otherwise remote or difficult biomes. Geographic isolation promoted speciation.
The islands and archipelagoes that dot the western Pacific and Indian oceans provided the most prolific sites for adaptive radiation. The forests of the Andes and Amazon basin were other sites that especially engendered rich diversity, owing to niche habitats that offered opportunities for specialization.
Though birds had long been consumers of plant parts, ~50 MYA flowers managed to entice birds into mutual relations. Bringing birds into the fold as pollinators gave plants an edge on their heavy reliance upon insects.
More than 10,000 avian species have lived since hominins came out of Africa. Extant species are a small fraction since Aurornis appeared.
A confounding facet in following paths of avian descent is convergent evolution. Birds not closely related adaptively evolved selfsame traits. Conversely, birds that look quite distinct may well be relatives.
Except for long claws on their back toes, the meadowlarks of the North American grasslands look like the longclaws of the African grasslands. These distantly related passerines are both ground dwellers of similar size, with streaked backs, yellow underparts, and a bold black V-shaped mark on their chests. The shared features reflect adaptation to a largely identical lifestyle in geographically distinct but identical habitats.
When Darwin passed through the Straits of Magellan in 1838, he was amazed to find birds that looked and behaved just like those he had seen in the North Atlantic. Darwin’s confusion was eminently understandable. Later analysis of the petrels and auks that Darwin saw showed such similar morphology that their classification is easily brought into question.
The same applies to hawks and owls, grebes and loons, swallows, and swifts. Convergent evolution in the numerous families of forest birds, which have been most prolific in speciation, can reduce lineage determinations to guesswork.
Biomolecular analyses offer some promise in filling the enormous gaps between modern birds and their Mesozoic ancestors but determining the lineages of birds is among the most difficult exercise among extant animal classes.
75% of modern bird species live in the tropics. Owing to sheer numbers much niche speciation has occurred there, as avian residence is long-standing. Dramatic climate changes over geological time have been greater in the higher latitudes, but proximity to the poles has little to do with species formation.
Diversification rate does not vary with latitude, though it does by longitude. Bird species abound in the Eastern Hemisphere, but Western Hemisphere birds have diversified faster than those in the east. No single lineage has driven this hemispheric pattern. Instead, scattered bursts of rapid speciation have transpired on a variety of evolutionary branches, likely owing to a variety of causes, including habitat isolation from climate changes.