There are about 10,000 species of birds extant in the world today, but over 200 million years or so there were many more. Scientists estimate that there might have been 123,000 species of birds over evolutionary time. They evolved and then went extinct. Most of the extinctions we know about were due to human causes. The Dodo and Passenger Pigeon got overhunted, the Maori killed off the Moas, and the Ivory-billed Woodpecker and Carolina Parakeet were done in mainly by habitat destruction. Other birds met their fate as people moved across the world and started changing habitats and hunting birds for food. But before people did that, what happened to all the other birds that disappeared?
The process of evolution picks the best fit individuals to survive and produce offspring and selects out those that are less fit. As parts of the earth changed due to geologic and climate factors changed, the birds that were well adapted to the environment may not have been as well adapted to the new environment, so they did out and others took their place. Global warming and cooling, ice ages, glaciers, volcanic eruptions, meteors, subsidence or increase of coasts and islands, the evolution of predators and their movement, changes in food supply, etc. all had their effects. Birds also flew to uninhabited land masses with new environments that required special adaptations.
Look at New Zealand. Across the world, 90% of birds are terrestrial. On New Zealand, 1/3 are terrestrial, 1/3 are wetland birds, and 1/3 are seabirds. Birds that landed there faced different challenges. And they met them in various ways. Besides the flightless moas, there is the Kakapo, the only alpine parrot, and the kea, the only flightless parrot. And of course the Kiwi. Flightlessness commonly evolved on islands because there were no land predators and it was easier to make a living as a ground dweller, not having to use energy to fly around.
Barcelona’s Ecological Applications and Forest Research Centre measured the brain sizes of 11,554 museum specimens of birds, including representatives of 110
island-dwelling species and 1,821 continental species. Island birds across the board exhibited bigger brain sizes than their mainland relatives. Why would this be? Well, one explanation is that the island birds have to be well adapted to their particular environment because they can’t move easily to a new one. If another species invades the first species’ niche, the first species will move or die off unless it can outcompete its competitor. So if it has a bigger brain with, presumably, more cognitive power, it can more easily exploit its habitat. Or maybe the brain can simply try new things that evolution will eventually adapt. One example is the Kakapo. Kakapo chicks feed almost entirely on the seeds of the rimu tree. The mating cycle of the bird is tightly linked with the fruiting of the rimu tree, and in abundant fruiting years, a greater number of kākāpō chicks are often born.
In sum, there are (and were) so many birds because evolution is constant. Evolution keeps throwing new features out there, some of which will be an advantage but most of which will be deleterious. Over a virtually infinite amount of time, lots of new birds keep cropping up while the obsolete ones disappear. Like the Tesla and the Edsel.