Bird bones are extremely lightweight and fragile compared to the bones of other vertebrates, which makes them less likely to become fossilized. There are several key reasons why bird bones tend not to fossilize:
Lightweight and pneumatic bones
Bird skeletons are highly adapted for flight. Their bones are hollow, thin-walled, and reinforced with internal struts for strength and pneumaticity. This makes them lightweight for flight, but also fragile and prone to destruction before fossilization can occur. The hollow bones collapse easily under pressure when buried.
Delicate skeletal structure
Birds have extremely thin and delicate bones in their skeletons. The bone walls can be as thin as 200 micrometers. This allows their skeletons to be as light as possible for flight. However, it also means the bones are fragile and fragment more easily compared to thicker-walled bones in mammals and reptiles. They disintegrate more readily after death.
Small body size
Most birds have a relatively small body size compared to many mammals and reptiles. This means they have less structural support to maintain skeletal integrity. Their lightweight bones are crushed more easily when buried. Larger vertebrates have stronger architectural support from thicker bones and denser musculature to resist compression when buried.
High porosity
Bird bones have a high degree of porosity, with some studies finding up to 70% air space inside some bird bone samples. This high porosity further reduces the bone strength and density. The high air space allows more pathways for degradation and microbial attack when buried.
Mineral composition
Bird bones are primarily composed of calcium phosphate, mainly in the form of hydroxyapatite. This is a highly soluble mineral that readily dissolves under acidic conditions in many burial environments. Acids from plant roots, microbes, and groundwater can dissolve bird bones before fossilization occurs.
Collagen content
Bird bones contain very little collagen, the elastic protein fibers that make up much of bone matrix. The collagen provides strength and structural stability in vertebrate bones. With less collagen, bird bones lose integrity more quickly after death when collagen decays.
Fast scaffolding turnover
Living birds have a high rate of bone remodeling through processes like osteoclast/osteoblast activity. This causes a rapid turnover of bone matrix. The bone scaffolding is continuously replaced with new tissue. This means the bones contain less mineralized matrix and have had less time to stabilize with hydroxyapatite crystals. Less matrix stabilization causes them to decompose faster when buried.
Small fossilization window
For fossilization to occur, bones need to be buried quickly before destruction. But birds have a relatively small time window for this to occur before the lightweight bones are destroyed. Larger vertebrates persist longer after death, allowing more potential time for protective burial. But light bird bones are quickly scattered and degraded when exposed.
Habitat
Most birds live in trees, forests, or other aerial environments. When they die, their bodies most often fall to the forest floor or some other exposed terrestrial setting. These areas do not promote rapid burial and fossilization like river sedimentation or caves might. The longer bones are exposed, the more degradation occurs.
Scavenging
Bird carcasses are very prone to scavenging by predators, insects, and microbes. Scavengers quickly scatter the lightweight bones at the surface. The bones become widely dispersed and fragmented, making rapid burial unlikely. Heavier reptile/mammal bones better withstand scavenging long enough for burial.
Stomach acid erosion
Bird bones that are eaten by predators and pass through the stomach undergo acid erosion that further degrades bone structure. Acid etching and fragmentation make the bones even less structurally sound and accelerate decomposition when deposited as feces.
Fragile fossils
The few bird fossils that do form are very delicate and fragmentary. They lack dense mineralization and are prone to destruction through natural weathering processes once a fossil bed is exposed by erosion. The fragile fossils crumble away more readily than robust reptile/mammal bones.
Small fossil fragments
When bird fossils are found, they most often consist of only tiny fragmentary remains. Complete bird fossils are exceptionally rare because the entire bone structure typically disintegrated before fossilization. The lightweight and hollow bones collapse into small fragments under pressure during burial and compaction.
Rapid decay of progenitor birds
Birds evolved from feathered theropod dinosaurs around 150 million years ago during the Jurassic. The dinosaur progenitors of birds also had highly pneumatic and lightweight bones. This means any bones from proto-birds that died prior to the Jurassic would have been unlikely to fossilize well.
Lack of skeletal pneumaticity in early birds
Primitive Mesozoic birds didn’t have extensive skeletal pneumaticity and bone hollowing. But they still had very thin walled and fragile bones. This combination led to poor preservation potential until pneumaticity evolved later in birds, making the bones even more fragile and less dense.
Smaller evolutionary time window
Birds have existed for around 150 million years. By comparison, mammals have a fossil record spanning over 300 million years. Birds have had less time to potentially enter the fossil record with their delicate bones that rapidly decay after death.
Lack of durable eggshells
Reptiles and many early mammals had durable eggs encased in mineralized eggshells. These fossilize well and can preserve bones. But bird egg shells are thinner and more porous, which allows rapid gas and water diffusion during embryonic growth. But the more fragile shells are less likely to provide a protective environment for bone fossilization.
Cartilage, not bone
Some bird anatomical features are composed more of cartilage than bone, such as the syrinx voicebox. Since cartilage does not readily fossilize, these structures are unlikely to be preserved even if some bones remain.
Lack of teeth
Birds lack teeth, which are bony structures that fossilize well due to their hardness and durability. The absence of teeth means there are fewer hard skeletal structures available to potentially fossilize compared to toothed animals.
Delayed hardening of bones
Ossification of bones occurs later in birds compared to other vertebrates. The cartilaginous precursor stage is more prolonged. More time as pliable cartilage allows bones to fragment and compress more before hardening into a rigid structure that can resist decomposition and destruction.
The rare exceptions
On very rare occasions, bird fossils do form. But usually only when conditions are ideal for extremely rapid burial and mineralization of the delicate bones, such as in ancient sinkhole death traps. But at most fossil sites worldwide, bird bones simply do not survive long enough for permineralization and fossilization.
Conclusion
In summary, bird bones are lightweight, pneumatic, porous, and fragile compared to other vertebrates. This allows living birds to fly, but also causes their bones to decompose rapidly after death. They are quickly destroyed by scavengers and environmental exposure. The small fragile bones seldom survive long enough for fossilization to occur. Only under very unique conditions will delicate bird remains be preserved as fossils before disintegrating completely.
