A tiny bird fossil, named Eocypselus rowei, offers clues to the precursors of hummingbird and swift wings.
Specimen of Eocypselus rowei (Daniel T. Ksepka et al)
First collected in southwestern Wyoming in a fossil site known as the Green River Formation, Eocypselus rowei lived about 50 million years ago. It was a small creature about 12 cm long (from head to tail), and weighed less than an ounce.
Paleontologists led by Dr Daniel Ksepka of the National Evolutionary Synthesis Center in Durham, North Carolina, spotted the fossil while working at the Field Museum of Natural History in Chicago.
The bird was named in honor of John W. Rowe, Chairman of the Field Museum’s Board of Trustees.
The fossil is unusual in having exceptionally well-preserved feathers, which allowed the researchers to reconstruct the size and shape of the bird’s wings in ways not possible with bones alone. Feathers account for more than half of the bird’s total wing length.
To find out where the fossil fit in the bird family tree, the paleontologists compared Eocypselus rowei to extinct and modern day species. The analysis suggests that the bird was an evolutionary precursor to the group that includes today’s swifts and hummingbirds.
Given the differences in wing shape between these two closely related groups of birds, the researchers have puzzled over how swift and hummingbird flight came to be. Finding fossil relatives like this specimen is the key to figuring that out.
“This fossil bird represents the closest we’ve gotten to the point where swifts and hummingbirds went their separate ways,” said Dr Ksepka, who with co-authors report the results in the journal Proceedings of the Royal Society B.
Hummingbirds have short wings relative to their bodies, which makes them good at hovering in mid-air. Swifts have super-long wings for gliding and high-speed flight. But the wings of Eocypselus rowei were somewhere in between.
“Based on its wing shape, it probably wasn’t a hoverer, like a hummingbird, and it probably wasn’t as efficient at fast flight as a swift,” Dr Ksepka explained.
The shape of the bird’s wings, coupled with its tiny size, suggest that the ancestors of today’s swifts and hummingbirds got small before each group’s unique flight behavior came to be.
“Hummingbirds came from small-bodied ancestors, but the ability to hover didn’t come to be until later,” the scientist said.
Closer study of the feathers under a scanning electron microscope revealed that carbon residues in the fossils – once thought to be traces of bacteria that fed on feathers – are fossilized melanosomes, tiny cell structures containing melanin pigments that give birds and other animals their color.
“The findings suggest that the ancient bird was probably black and may have had a glossy or iridescent sheen, like swifts living today. Based on its beak shape it probably ate insects.”
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Bibliographic information: Daniel T. Ksepka et al. 2013. Fossil evidence of wing shape in a stem relative of swifts and hummingbirds (Aves, Pan-Apodiformes). Proc. R. Soc. B, vol. 280, no. 1761; doi: 10.1098/rspb.2013.0580
