A South American bird called the hoatzin may have some of the characteristics of the very early birds. The young hoatzin has claws on the front of its wings like the Archaeopteryx, which it uses to scramble about in the branches. The adult birds are very clumsy flyers. Unlike all other birds, the hoatzin does not have a specialized gizzard in which food is ground up. Instead it grinds its food inside a large muscular crop. This feature is regarded as being very primitive.
Why is there so much argument about the origin of birds?
The difficulty in explaining the evolution of birds is that there are so few fossils. Bird bones are very fragile and honeycombed, with small air spaces. They crumble away very easily after death, and consequently bird fossils are extremely rare.
The only fossils of ancient birds are those where the animals fell into shallow freshwater lagoons and were covered very quickly with mud, which prevented decay or scattering of the remains by predators or scavengers. These special conditions appeared only occasionally, however. Until more types of bird fossil are found it will be difficult for scientists to fill in the evolutionary gaps and find out more about the origin of birds.
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