Flights of Fancy in Avian Evolution
From mousebirds to terror birds, the class Aves has encompassed a remarkable diversity of species over the past 150 million years.
Heyday of the Mousebirds
Most people have never seen a mousebird. These small, social birds are found creeping through vegetation in their namesake mouse-like manner, sometimes clinging upside down using their dexterous grasping toes. Mousebirds are unknown to most because the six living species only occur in sub-Saharan Africa. We might thus assume mousebirds were always minor players in the story of avian evolution. Not so!
Fossils show that during the Eocene and Oligocene Epochs (56 million to 23 million years ago), mousebirds underwent a fascinating diversification that gave rise to a kaleidoscope of different forms. Among the species that flourished in North America and Europe during that golden age of mousebirds were the sharp-billed Chascacocolius cacicirostris, which specialized in prying open bark or hard fruits, the long-winged Celericolius acriala, which most likely chased down insects, and the parrot-skulled Oligocolius psittacocephalon, known from a skeleton packed with seeds. These forms had all vanished by 25 million years ago, possibly losing out to the wildly successful songbirds. They are only one example of many evolutionary radiations that were followed by diversity collapse.