Angiosperms, or flowering plants, first appear in the fossil record about 160 million years ago. A new paper in Biology Letters, by Gerald Mayr and Volker Wilde (reference below and—I think—a free download) shows that by about 50 million years ago, birds had already evolved to take advantage of this new food source.
Mayr and Wild report a new bird fossil from the famous Messel formation of Germany. The specimen, Pumiliornis tessellatus, is remarkably well preserved as a complete skeleton and is dated at roughly 47 million years, in the middle Eocene. It is not a member of any of the three modern groups of birds that independently evolved the ability to eat nectar and pollen: hummingbirds, lorikeets + hanging parrots, and some groups of the Passeriformes (“perching birds,” whose nectar-and-pollen eaters include sunbirds, honeycreepers, etc.).
The remarkable thing about this specimen, as shown in the photo below, is that there is a clump of pollen grains near the femur—right where the stomach would be in a living bird. Although there are also a few insect parts (perhaps accidentally ingested along with the pollen), the number of grains, their clumping, and their position suggests that this bird was in fact eating pollen. Notice the wonderful feather impressions in the fossil below:
Here are some scanning electron microscope (SEM) photos of the fossilized pollen:
The other clue that this bird didn’t accidentally eat pollen, and was adapted to a flower-feeding lifestyle, comes from its appearance. It has a long beak and enlarged nasal openings characteristic of modern birds that sip nectar, and it has “zygodactyl” feet, meaning that the fourth toe could be turned backwards—a trait of perching birds that climb branches and flower stems. The #1 toe is the one you should look at in the photo below:
Now it’s not clear if pollen was the primary object of this bird’s diet, was ingested accidentally while drinking nectar, or if the bird ate both pollen and nectar. What is pretty clear is that by the middle Eocene, when this bird lived, birds had already evolved to use as food flowering plants that had been around for over 100 million years.
Although the authors were unable to identify the plant that produced this pollen, they suggest that it was already itself evolutionarily adapted to pollination by animals rather than wind:
Although pollen size does not allow discrimination of insect and bird pollination, the large size of the grains and the fact that some are still clumping (figure 2c) indicate direct ingestion from a plant adapted for animal rather than wind pollination.
This pushes back the earliest known bird/nectar/pollen interaction by 17 million years, as heretofore the earliest such specimen dated at about 30 mya.
Gerald Mayr and Volker Wilde. 2014. Eocene fossil is earliest evidence of flower-visiting by birds. Biol Lett 2014 10: 20140223