What is a "bird"?
The fossil record indicates that birds evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs; their closest living relatives are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages. But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics.
Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Many species annually migrate great distances. Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous (referring to social living arrangement, distinct from genetic monogamy), usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are polygynous (arrangement of one male with many females) or, rarely, polyandrous (arrangement of one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilised, though unfertilised eggs do not produce offspring.
Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds (poultry and game) being important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertiliser. Birds prominently figure throughout human culture. About 120 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.
One typically learns in school that a bird is a bipedal, warm-blooded animal, a vertebrate, covered in feathers and lays eggs. Some can fly, but not all of them. And, until we started to think about extinct animals, this was enough. It was pretty clear what was a bird, or a mammal, or a reptile, when you only looked at things that were alive today.
But when you start to consider extinct organisms, the picture becomes significantly less clear. In school, typically the answer for what the first bird was, is Archaeopteryx. Archaeopteryx was a small, bipedal dinosaur, covered in feathers, with clear wing feathers, and was probably warm blooded. Problem solved!
Except as we learn more and more about dinosaurs and how they evolved, it becomes increasingly clear that Archaeopteryx was not unique, and it was not the first dinosaur to show these traits. Wing feathers are actually present in such dinosaurs as Velociraptor and Oviraptor, animals we don t typically call birds. These animals were also bipedal and warm blooded. They couldn t fly, though, so perhaps that s what sets Archaeopteryx apart, it s ability to fly.
But Archaeopteryx couldn t even fly. It did not have the musculature necessary to power its own flight, the most it could do was glide! And given that flight is an optional characteristic for birds anyway, it seems rather unfair to Ostriches and Emus as well as Archaeopteryx to make flight a necessary characteristic of being a bird.
Frankly, scientists have trouble deciding at what point an animal is considered a bird. They know this point is somewhere in the group of animals that are more closely related to birds than they are to any other modern animal – this group is called Avemetatarsalia, and includes all dinosaurs (including modern birds,) pterosaurs, and close relatives. We know all these animals are more closely related to living birds than to any other group of animals through the process of cladistics. Paleontologists and biologists look at the different characteristics of animals, and test what is the simplest explanation for how those characteristics evolved, and thus how the animals themselves evolved. Crocodiles, the closest living relatives to birds, are used as an external marker – if anything is as closely related to crocodiles as it is to birds, then they aren’t in Avemetatarsalia.