Mystery bird: Sungrebe, Heliornis fulica

Sungrebe, Heliornis fulica Boddaert, 1783 (formerly, Heliopais fulica; protonym, Colymbus fulica), also known as the South American finfoot or as the American finfoot, photographed at Rio Três Irmãos, Mato Grosso state, Brazil.

Image: Nick Athanas/Tropical Birding (with permission). [velociraptorize].
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Question: This Brasilian mystery bird has a remarkable anatomical feature that no other bird possesses. What is that special feature and what does it allow these birds to do? Can you identify this bird's taxonomic family and its species?

Response: This is a sungrebe, Heliornis fulica, the only species placed into the genus, Heliornis. Its taxonomic family, Heliornithidae, contains just two other species, the African finfoot, Podica senegalensis, and the masked finfoot, Heliopais personata (notice that all three genera are monotypic).

Sungrebes are small, shy waterbirds that prefer to swim in slow-flowing streams and secluded waterways, sometimes partly submerged, like a grebe. They are very aquatic, but rarely fly. This species was originally placed into the (now-suppressed) genus, Colymbus, within the now disused taxonomic order, Colymbiformes (loons and grebes -- read more about this taxonomic battle in this free PDF from 1952). Sungrebes have grebe-like lobed feet -- a result of convergent evolution and adaptation for aquatic life. They feed primarily from the surface of the water, taking crustaceans, amphibians, and invertebrates fallen into the water.

The individual in the photograph is an adult female, as indicated by her rufous (or buff) coloured cheeks (males have white cheeks). This poorly-known species is remarkable because the male has specialized pockets under each wing that he uses to carry his young, even in flight:

M. Alvarez del Toro, who observed a nesting pair in Mexico, discovered that the male has a shallow pocket under each wing into which the two young can fit. The pocket is formed by a pleat of skin, and made more secure by the feathers on the side of the body just below. The heads of the chicks could be seen from below as the bird flew. Alvarez del Toro collected the bird in order to examine it and confirm the unlikely discovery. Subsequently, he found it confirmed also by a report published by Prince Maximilian of Wied 138 years earlier but apparently ignored, forgotten or not believed.

This adaptation is unique among birds: in no other species is there any mechanism whereby altricial young can be transported. Of course, the precocial young of some swans and grebes may hitch rides on their swimming parents' backs, and a male jaçana can transport his chicks about holding them between his wings and body, but neither of these cases applies when the adults are in flight....

The transport system of the Sungrebe raises numerous further questions. How do the chicks get into the pocket? Are they put in by the male? Does he feed them in there? Do they stay inside, or get in and out? Why does the female not have similar pockets?

(Bertrand, B. C. R. (1996). "Family Heliornithidae (Finfoots)" in del Hoyo, J., Elliott, A., & Sargatal, J., eds. Handbook of the Birds of the World: Hoatzin to Auks. Vol. 3. Lynx Edicions, Barcelona, ISBN 84-87334-20-2 [Amazon UK; Amazon US]).

Here is a very short video of a male sungrebe (uploaded 16 June 2011):

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