Birds have a wide range of diets that allow them to survive in different habitats around the world. While many birds eat seeds, plants, insects, fish and other small creatures, some have evolved the ability to swallow and digest entire animals, bones and all. The ability to digest bones allows these avian scavengers access to nutrients locked inside bones that other animals cannot access.
Birds That Can Digest Bones
There are a few groups of birds that are specially adapted to digesting bones:
Vultures
Vultures are the most well known bone eaters. There are over 30 species of these large, predatory birds that inhabit ecosystems in Africa, Europe, Asia, and the Americas. When feeding, vultures swallow small bones whole and digest them in their extremely acidic stomachs. Their gastric acids have a pH estimated to be around 1, which dissolves connective tissue and assists in releasing marrow inside bones. Some key vulture species include:
- Turkey Vulture – Found in the Americas, turkey vultures have very acidic stomachs with pH around 1.
- King Vulture – The largest vulture found in Central and South America. Their thick skin and immune system allows them to ingest rotting carcasses.
- Lappet-faced Vulture – One of the larger old world vultures found in Africa and the Middle East. They are able to crack open large bones.
- Bearded Vulture – This unusual vulture inhabits mountainous regions from southern Europe to east Asia. They specialize in eating bones only.
Owls
While not as specialized as vultures, some species of owls also digest small rodent bones. Their gastric juices have a pH of about 2.
- Great Horned Owl – Powerful talons allow them to crush bones of prey like rabbits and small rodents before swallowing them.
- Snowy Owl – Their acidic stomachs help break down the bones of lemmings, their primary food source.
- Northern Hawk Owl – Found in the artic forests, they feed on small mammals like voles. They digest the bones to access marrow.
Seabirds
Many seabirds have evolved the ability to swallow fish bones. Their digestive systems are adapted to their fish heavy diet. Examples include:
- Penguins – They feed on fish like sardines, swallowing them whole. The bones are digested in their acidic guts.
- Puffins – Feed on small fish like herring or sandlance that they are able to swallow whole.
- Boobies – These large seabirds dive from the air to catch large fish, swallowing them bones and all.
- Albatross – Scavenges for dead animals at sea, including boney remains floating on the water’s surface.
Other Birds
While vultures, owls and seabirds are the most specialized bone digesters, other birds also show adaptations for swallowing small bones, including:
- Storks
- Herons
- Pelicans
- Cormorants
- Swallows
Adaptations for Bone Digestion
Birds that regularly digest bones have evolved a number of physical and physiological adaptations that allow them to make use of this unusual food source.
Acidic Digestive System
The most important adaptation is an extremely acidic stomach pH. Vultures and other scavengers have gastric acid many times more acidic than other animals, allowing them to dissolve bone material that is indigestible to most species.
Protective Lining
In order to withstand this highly acidic environment themselves, bone-digesting birds also have a thick protective lining in their stomach and esophagus. Vultures in particular have a keratinized layer that protects their digestive tract from their eigenen acidic secretions.
Powerful Enzymes
Scavenging birds produce powerful digestive enzymes that further help break down tissue, cartilage and bones. Enzymes like cathepsins play an important role in bone degradation.
Strong Immune System
Due to their scavenger diet, these birds also need a strong immune system to help fight off potential pathogens from rotting carcasses. Their immune defenses are adapted to their unusual eating habits.
Cracking Bills
Vultures, condors and other large birds have thick, hooked bills used for tearing flesh and also cracking bones to access the nutritious marrow inside. This gives them access to additional nutrients.
Retention
Some vultures have the ability to retain food in their crop for long periods while digestion occurs. This allows more time to fully dissolve bone material before it passes to the intestines.
Benefits of Eating Bones
For birds that have adapted the ability to safely digest them, bones provide a number of nutritional and other benefits:
- Calcium source – Bones provide a bioavailable source of calcium for eggshell formation and bone health.
- Marrow – The fat and protein filled marrow provides an energy dense food source when prey is scarce.
- Consistent food – Bones last longer than flesh after death, providing a more reliable food source for scavengers.
- Hygiene – Digesting bones helps clean up carcasses and prevent spread of disease.
Accessing these difficult to digest bone nutrients allows these specialized bird groups to thrive in lean habitats where carrion is the primary food source.
How Bones Are Digested
The digestion of bones by birds involves mechanical and chemical processes in the gastrointestinal tract:
Ingestion
Birds like vultures swallow small bones whole. Larger bones are cracked or broken into pieces by the beak first to allow swallowing.
Physical Breakdown
Muscular contractions in the gizzard grind the bones into smaller fragments. Gizzard stones may assist this mechanical breakdown.
Chemical Digestion
In the proventriculus and stomach, highly acidic gastric juicesrich in hydrochloric acid, pepsin and other proteolytic enzymes begin dissolving the bone material.
Absorption
The dissolved bone components like calcium, phosphorus and protein are absorbed in the small intestine to be utilized by the body.
Egestion
Any remaining bone fragments that resisted chemical digestion pass out of the digestive tract through the cloaca as part of the feces.
Why Birds Don’t Typically Develop Metabolic Bone Disease from High Calcium Diet
The consumption of bones provides a substantial amount of calcium to birds that digest them. Most animals eating such a high calcium diet would develop metabolic bone disorders like nutritional secondary hyperparathyroidism. However, adapted bone-eating birds have mechanisms that allow them to safely process the extra calcium:
- They maintain calcium balance by excreting excess calcium through the kidneys.
- Parathyroid hormone and vitamin D levels are modulated to optimize calcium absorption and prevent toxic accumulation in soft tissues.
- Their gastrointestinal tract is designed for high calcium throughput.
- Bone resorption and mineralization rates in the skeleton are optimized for their calcium rich diet.
These adaptations along with an appropriate ratio of calcium to phosphorus from the bones keeps the calcium levels in balance. This allows birds to access the nutritional benefits of bone consumption without the detrimental effects seen in most species.
Interesting Facts
- Vultures have been observed digesting bones from animals as large as kangaroos and moose.
- The turkey vulture has the lowest stomach pH of any animal at 1.
- Owls cough up pellets containing the non-digestible fur and bones of their prey.
- Some vultures can go weeks between meals thanks to their slow digestion.
- The extinct Haast’s eagle from New Zealand is believed to have been able to digest moa bones based on its large size and strong talons.
- Bones from lead shot game animals can cause lead poisoning when ingested by vultures and other raptors.
Conclusion
The ability to digest bones provides definitive advantages for birds in many different ecosystems. By evolving extremely acidic digestive systems and other physical adaptations, vultures, owls, seabirds and other groups are able to gain additional nutrients from bones that most animals are unable to access. This allows them to thrive on more scarce, sporadic food sources than other predators. Understanding these unique adaptations provides insight into the diverse feeding strategies that evolved among birds to allow them to colonize habitats all across the globe.