English: Bird nest (likely a California gull nest) encrusted by calcareous tufa from the shores of Mono Lake, eastern California, USA (Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA).

Mono Lake, on the edge of eastern California's Sierra Nevada Mountains, is an unusual lacustrine environment. It is hypersaline and quite alkaline (pH is about 10). Modern Mono Lake is not natural - its size and chemistry have been strongly influenced by human activity, principally by the diversion of water from the inflowing Owens River to the Los Angeles urban area. About three dozen species of birds use Mono Lake as a resting site during migration, as a source of food (brine shrimp), and as a nesting site.

Bird nests along the shores of Mono Lake can be subject to inorganically- & biogenically-induced encrustation by calcite and aragonite (CaCO3 - calcium carbonate) when lake levels rise after storm/flood/runoff/snowmelt events. The specimen shown is a bird nest having two eggs encrusted by calcium carbonate. The hollow tubes surrounding the eggs represent molds of the stems of shore plants. Rocks like this are given the horrible name “calcareous tufa”, which is a friable precursor to travertine.

Some might call this a fossil bird nest, but it is almost certainly Holocene in age. It is now lithified, so it could be called a subfossil bird nest. It is probably a California gull nest.

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Birds are small to large, warm-blooded, egg-laying, feathered, bipedal vertebrates capable of powered flight (although some are secondarily flightless). Many scientists characterize birds as dinosaurs, but this is consequence of the physical structure of evolutionary diagrams. Birds aren’t dinosaurs. They’re birds. The logic & rationale that some use to justify statements such as “birds are dinosaurs” is the same logic & rationale that results in saying “vertebrates are echinoderms”. Well, no one says the latter. No one should say the former, either.

However, birds are evolutionarily derived from theropod dinosaurs. Birds first appeared in the Triassic or Jurassic, depending on which avian paleontologist you ask. They inhabit a wide variety of terrestrial and surface marine environments, and exhibit considerable variation in behaviors and diets.