In the Science journal there is a remarkable report (DOI: 10.1126/science.1252243) by Lee et al. on the evolution of birds from dinosauri. A lineage is depicted which comprises 12 different independent steps in size reduction. These steps occurred mostly long before the first flying animal hatched. A Perspective article (DOI: 10.1126/science.1257633) by M.J. Benton puts this paper in its context: 20 years ago the evolution of birds was believed to occur within 10 Millions of years. This article summarizes the research done since: Around 50 Millions of year were necessary to the steps to reduce the animal to the size of Archeopteryx and further to that e.g. of a swallow. Some of the intermediate species had feathers, some were “paragliders”, but only Archaeopteryx developed free-flying.
Benton argues that the minaturization was caused by an environmental change i.e. that they tried to live on trees. The gain when living in a tree environment would be safety from predators and a wealth of additional food. Possible.
The authors sampled 120 therapods and early birds. That by itself is a tremendous achievement and makes their analysis fairly sound. Excellent work!