Its bone structure and probably its appearance were similar to those of the large, leggy, long-necked and bent-beaked flamingos of today, and its plumage may have exhibited their characteristic rosy hue. It stood about 1.5 meters high. But its reproductive habits—laying several eggs in a twig-nest built in a lacustrine environment—suggest those of a modern grebe, a small or medium-sized bird that, to a non-expert in ornithology, looks like a type of grayish duck. With the appearance of a flamingo and the behavior of a grebe, therefore, it was probably a now-extinct bird that laid five small eggs in a floating shelter with a fragile woody framework 18 million years ago in territory that is now Spain.
That long-ago winged creature built the oldest fossilized bird nest ever recorded in the scientific literature. The remains of the nest were found nearly intact in what once was a shallow saline lake, now buried under layer upon layer of sediment in the calcareous Ebro River Basin in the north of Spain. “It is the first known floating nest and the first evidence of a structure for sheltering avian eggs,” says biologist Luís Fábio Silveira, curator of ornithological collections for the Museum of Zoology at the University of São Paulo (MZ-USP), who co-authored a study of the Spanish fossil material, published on October 17 in the journal Plos One.
Recovered beside the nest, which protected semi-oblong eggs measuring up to 4.5 by 3 centimeters, were a few bone fragments from a foot (tarsus and metatarsus) and a well-preserved left tibiotarsus, or thigh, of the bird. “We conducted separate analyses of the tibiotarsus and the nest with the eggs, and we arrived at the same conclusion,” Silveira says. “They are from a paleoflamingo, a new, extinct genus and species of that family of birds.” Swiss paleontologist Gerald Grellet-Tinner, a specialist in reptilian and avian eggs at the Regional Center for Scientific Research and Technology Transfer (CRILAR) in Argentina and the Field Museum in Chicago, also has no doubt that the nest was made by a primitive flamingo. “Eggs are biomineralized tissues that have the same functional morphology and phylogenetic value as the bones of a skeleton,” notes Grellet-Tinner, another of the study’s co-authors. “From that perspective, an eggshell is a fingerprint that provides specific information about a species.” The microstructure of the eggs was analyzed using five different electron microscopy techniques so as to increase the reliability of the results.
The twigs and the remaining plant material used to make the nest were also identified, but more generically in this case. The twigs were from a plant of the huge family of Fabaceae, leguminous plants that range from foods such as beans and soybeans to trees such as cherry and brazilwood. The environment where the fossils were recovered—water with a high saline content—is also associated with flamingos, which live along seashores or in river estuaries. Grebes, on the other hand, submerge to catch fish and prefer a freshwater environment, although they can also be found in saline streams.
The nest containing the eggs and the tibiotarsus were discovered by Spanish paleontologists in 2003 as they performed field work in the so-called Tudela Formation prior to construction of a dam designed to prevent flooding along the Ebro River. That semi-arid region had already yielded fossilized crocodiles, turtles, snakes and ostracods, a type of crustacean that measures only a few millimeters in size. The Iberian researchers were not avian specialists, so they endeavored to surround themselves with scholars who study such animals and made the excavated material available to them. They called upon Silveira, who did all of the bone structure analysis by comparing the bones found in the Ebro to osteological material from the avian collections at MZ-USP and the Museum of Natural History (MHNT) in Taubaté, state of São Paulo. They also contacted Grellet-Tinner, who was responsible for studying the nest and the eggs.
Between 12 and 29 million years ago, there existed a now-extinct bird genus, Palaelodus, that is sometimes presented as having an anatomy and lifestyle halfway between the morphology and behavior of flamingos and grebes. The researchers believe that the new fossil does not belong to that extinct genus. At most, it is a relative that may have been contemporaneous with Palaelodus. This does not mean that the paleonest from the Ebro lacks importance from an evolutionary standpoint. On the contrary, scientists classify it as additional support for a theory that has gained ground in recent years: that although flamingos and grebes presently exhibit very different morphology and behavior, they are actually sister groups.
Studies of the anatomy and genetics of these birds suggest that in the distant past, before diverging into two distinct families of winged animals, they had a common ancestor more than 20 million years ago during the Miocene geological epoch. The new fossils recently described in the pages of Plos One further support that idea. “This article opens doors to much evolutionary speculation on these groups of birds,” notes paleontologist Herculano Alvarenga, director of the Taubaté Natural History Museum, who is an avian fossil specialist.
The new species of paleoflamingo, not yet given a scientific name, appears to indicate that the first examples of this family of birds had reproductive and nest-building habits similar to those of grebes in the past. It is possible that such practices hark back to the hypothetical common ancestor of the two avian families. Generally speaking, that behavior consisted of laying several small eggs in a nest covered with twigs, and it is still practiced today among the 22 living species of grebes, but it has disappeared among the six living species of flamingos.
In that context, the Spanish fossil is likely to be a vestige from a distant time when the nests of grebes and flamingos exhibited similar structure. The living species of flamingos build mud shelters for their future nestlings and do not use any covering of twigs around that structure. They generally lay a single large egg, much larger than the fossilized eggs from the Ebro Basin.
One of the first challenges for Silveira and Grellet-Tinner was to determine if the five eggs protected by a circular twig structure, which apparently floated along the shore of the ancient lake, were part of a constructed nest that has remained miraculously preserved for millions of years. There was a remote possibility that each egg had a different origin and that their proximity to one another inside the wooden nest happened by chance. But all the evidence gathered by the researchers overturned that hypothesis. The five eggs were identical, of the same type, and the context in which the nest was found indicated that the twig structure was not the result of a fortuitous movement of nature. The discovery beside the nest of bones from a bird further supported that theory. “To find (fossilized) eggs is rare. To find nests is rarer still. But to find eggs in a nest and be able to establish what group they belong to is extremely rare and interesting,” says Alexander Kellner, a paleontologist from the National Museum of the Federal University of Rio de Janeiro (UFRJ).
It is impossible to know for sure whether the eggs came from the bird that made the nest, but that is a plausible hypothesis. After all, the Brazilian and the Swiss researchers conducted their analyses independently and learned of each other’s findings after completing the work, and they both concluded that the tibiotarsus and the eggs were from some primitive type of flamingo that no longer exists on Earth. “The eggs that were found apparently belonged to a unique type of bird,” Silveira says. For a reason that will never be known, the animal possibly died next to the nest. Nor can we say whether the bone belonged to a female or a male bird. Male flamingos do not lay eggs, but they may incubate them in their mate’s nest.
Unfortunately, there are no other fossilized nests like the one recovered in the Ebro Basin. Any comparison of that type will hinge on a chance discovery of a second twig structure with bird eggs—a rather improbable occurrence given the fragility of that type of construction, specialists say. But who is to say: history as it happened on Spanish shores may someday repeat itself in another corner of the globe.
GRELLET-TINNER, G. et al. The first occurrence in the fossil record of an aquatic avian twig-nest with phoenicopteriformes eggs: evolutionary implications. Plos One. Published on-line. 17 Oct. 2012.