Grebes are familiar diving birds belonging to the Order Podicipediformes.  Although there are only 22 species in the order, grebes are found on all continents except Antarctica and on many islands.

All grebes nest in freshwater habitats although some spend the winter in marine waters. In Maine, the only breeding grebe we have is the diminutive Pied-billed Grebe. These birds migrate south for the winter. However, we have grebes throughout the year. Red-necked Grebes and Horned Grebes migrate to Maine coastal areas in the fall and spend the winter with us.

The Podiciped- prefix in the order name translates as “butt-foot”, an allusion to the rearward position of the feet. Loons have a similar morphology with the feet set far back on the body. This arrangement makes for efficient foot-propelled swimming but awkward walking on land.

Older field guides placed grebes next to the loons. However, DNA analysis shows that loons are only distantly related to grebes. In fact, the closest relatives to the grebes are, surprisingly, the flamingoes.

The paddles of loons and grebes are quite different. In loons, the three forward toes are fully webbed. In grebes, each of the three anterior toes has a flattened lobe to increase the surface area. The toes are connected only at the base by a narrow web. This same sort of toe is seen in the unrelated coots.

Grebes feed on small fish, crustaceans and freshwater insects. Note that all of these prey items have non-nutritious hard parts that pose a challenge for digestion.

All birds have feathers but grebes are the only birds that eat and then regurgitate their own feathers. The phenomenon of feather-eating has been known for over 500 years. In 1580, an Aztec author wrote that the diet of Eared Grebes was mostly feathers with occasional fish. This peculiarity was not appreciated in Europe until 1781 and even then, scarcely studied until the early 20th century. It became clear that all grebes eat feathers.

But why eat feathers? Feathers have virtually no caloric value. Think of trying to live on a diet of fingernails. There has been no shortage of explanations of feather-eating, mostly unconvincing speculations.

In a recent article in the Wilson Journal of Ornithology, Joseph Jehl provides a definitive study of the functional significance of feather-eating in grebes.

To appreciate Jehl’s work, we need to first consider the digestive system of a bird. Food passes from the mouth, through the esophagus to the anterior stomach or proventriculus. This stomach produces lots of hydrochloric acids and enzymes, facilitating the chemical break-down of food. The food next passes to the gizzard or muscular stomach. With the aid of hard ridges and sometimes the presence of grit, the gizzard mechanically grinds the food. The gizzard plays the role of teeth. The gizzard is well developed in birds like ducks, grouse and seed-eating songbirds but poorly developed in predatory birds. The food then passes into the intestines for absorption.

Jehl found that grebes pluck flank feathers and swallow them. These feathers have a peculiar twisted, open-veined appearance. The feathers are ingested and pass into the gizzard. Most are accumulated there, forming a dense mass.

Jehl’s breakthrough in understanding feather-eating came from his discovery that Eared Grebes regurgitate the feathers nightly in up to six different pellets. The pellets consisted of eroded feathers and the hard parts of shrimp and insect skeletons.

Because insects and crustaceans are difficult to digest, Jehl suggests the main function of feather-eating is to delay the movement of food into the intestine until chemical digestion is complete. The gizzard plays no role in mechanical digestion. Each day the bird regurgitates the indigestible remains of its prey along with the feathers. In the morning, it is time to pluck some more feathers and start the process again.

[Originally published on December 8, 2017]