The hypothesis that dinosaurs sired birds was first thought possible in 1860 in the swamps of Germany, when Archaeopteryx, possibly the world’s most controversial fossil, was discovered. Its controversy is largely due to the time of its discovery. Archaeopteryx was dug up two years after Darwin’s “The Origin of Species” was published, and as the popularity of Darwin’s book grew, so did Archaeopteryx because it was evidence that Darwin’s theory of evolution by means of natural selection might be true. Bearing wings and feathers as well as many other bird-like traits, it is clear that Archaeopteryx predeceases modern day birds. However, this creature bears many reptilian features found in small dinosaurs, such as long legs, three-toed feel, and an extended spine which forms a tail. Therefore, Archaeopteryx a link between modern day birds and dinosaurs, and since this hypothesis became popular in the 1990’s, many new fossils have been discovered.

The information from this post was found on:

http://pbs.org/lifeofbirds/evolution/index.html
http://www.pbs.org/wgbh/evolution/library/03/4/l_034_01.html
http://www.geologyrocks.co.uk/tutorials/origin_and_early_evolution_birds