I have often mentioned that bird fossils are hard to come by, as compared to other vertebrates and hard-shelled invertebrates. It’s mainly because they are lightweight and thin-skinned. The earliest birds or birdlike creatures were reptile-like and were heavier boned than later bird forms and they left a decent fossil record, but later versions, as they became lighter in weight and densely feathered, didn’t form fossils as readily.
The typical bird today, being thin-skinned and light in weight, tends to decompose readily. A small bird falling to the ground in death will become unrecognizable in a few days due to bacterial decomposition and scavenging by invertebrates such as centipedes, beetles, and maggots. Even larger birds like a road-killed owl don’t last long. Compare that to a run-over squirrel which might lie by the roadside for weeks. But animals falling to the ground don’t typically fossilize.
An animal falling into a lake or pond or water-filled quarry might become a fossil. A dinosaur, raccoon, or mouse, expiring in water, sinks rather rapidly. Birds don’t. Their waterproof feathers and light weight allow them to float, exposing them to the ravages of the weather, scavengers, and decomposers.
Fossils are created in different ways, but most are formed when a plant or animal dies in a watery environment and is buried in mud and silt. Soft tissues quickly decompose leaving the hard bones or shells behind. Over time sediment builds over the top and hardens into rock. Permineralization occurs when dissolved minerals carried by ground water fill the cellular spaces of plants and animals. The dissolved minerals crystalize and produce rocks in the shape of the organism. This most common type of fossil preservation include teeth, bones, shells and wood. This is why most bird fossils are of early birds without well-developed feathers and large body sizes – those most likely to sink.
For many years, a fossil bird named Archeopteryx lithographica, found in a Bavarian quarry in 1861 and now resides in the London Museum of Natural History, was considered the first bird. Living about 150 million years ago, it clearly is a bird as it shows distinct outlines of feathers. Many earlier fossils have been found since and good arguments have been made to establish the origin of birds as much earlier, perhaps as much as 250 million years ago.
New avian fossils are found regularly, revealing more secrets of bird evolution. In 2020 a fossil called a “wonderchicken” was discovered. The bird had the face of a chicken and the butt of a duck, hence the name. Discovered near the Dutch town of Maastricht, in famous fossil beds that formed between 66.8 million and 66.7 million years ago, the bird lived just before the mass extinction that killed off the dinosaurs. And then there is the hummingbird-sized dinosaur from Myanmar about 99 million years ago I’ll let you read about,
There are certainly many more bird fossils to be found but they are rarities and finding them is a tedious process.