Birds have a remarkable ability to digest all kinds of food, including bones. Their digestive systems have adapted to make use of the nutrients locked inside bones that mammals cannot access. But how exactly can birds eat and break down bone material?
Bird Digestive Anatomy
Birds have a very different digestive system compared to mammals. Their stomach is divided into two sections – the proventriculus and the gizzard. The proventriculus is glandular and secretes gastric juices like the mammalian stomach. The gizzard is muscular and contracts to grind up food, taking the place of teeth in the bird’s mouth. Between the two is an area with a thick muscular lining called the ventricular layer which connects them.
Food passes from the proventriculus to the gizzard for grinding. The gizzard contractions along with small stones and grit the bird swallows act together to crush food into smaller pieces. Birds that eat harder materials like nuts or seeds tend to have a more muscular gizzard. In some cases the gizzard may have folded mucosal linings inside for additional grinding surface area.
Once food is finely crushed by the gizzard it passes back to the proventriculus and mixes with more gastric juice. From there it moves into the intestines for further digestion and nutrient absorption. Birds have shorter intestines compared to many mammals since food has already been broken into very small pieces.
Bone Digestion
The bird’s muscular gizzard is the key to breaking down bone. Contractions generate enough force to fracture bones into small fragments. These fragments are further eroded in the proventriculus and intestines by acid-secreting cells. Bone is composed of mineral and protein components.
The mineral portion of bone is made up mainly of calcium and phosphorus salts. These dissolve in the acidic environment of the stomach and early intestines, releasing calcium and phosphorus ions that are absorbed into the bloodstream. The protein portion, mostly collagen, is digested by proteolytic enzymes like pepsin from the proventriculus.
Studies have found that bird digestive juices are better than those in many mammals at extracting minerals from bone. Measurements of calcium absorption in dogs, rats, and chickens found that chickens absorbed over 5 times more calcium from bones than the mammals.
Birds are able to make use of the protein and minerals embedded in bone to supplement their dietary needs. Only the hardest portions of bone like the enamel layer on teeth are indigestible and get passed out in the feces.
Adaptations That Aid Bone Digestion
Several key adaptations enable a bird to consume and breakdown bone:
- Sharp beak for crushing bones
- Muscular gizzard to fracture bone
- Gizzard grit to aid grinding
- Acidic proventriculus secretions to dissolve bone
- Protease enzymes like pepsin to digest bone protein
- Efficient calcium and phosphorus absorption
Species of birds that regularly eat hard materials like seeds or nuts tend to have the most muscular gizzards. Birds of prey that consume bones as part of their diet also tend to have a gizzard lining reinforced with calcium phosphate deposits for extra grinding power.
Advantages of Consuming Bone
Here are some of the key advantages birds gain from being able to digest bones:
- Access to calcium for eggshell formation – Female birds can use calcium from bones to help form eggshells which are mostly calcium carbonate.
- More complete nutrient utilization – Digesting bone allows birds to fully take advantage of the nutrients in animal prey including bone marrow.
- Additional calcium source – Calcium from bones helps birds meet their relatively high calcium needs for skeletal health.
- Added phosphorus – Phosphorus released from digested bone provides another nutrient birds need in significant amounts.
Young birds with developing bones likely utilize bone calcium and phosphorus for skeletal growth and mineralization. Hens producing eggs require more calcium, putting greater pressure on their digestive system to supply enough.
Bird Species That Consume Bone
Many types of birds consume small bones as part of their natural diet. Here are some examples:
- Owls – Feed on small mammals like mice and digest the bones they consume.
- Hawks – May eat the bones of bird prey in addition to meat.
- Vultures – Scavenge carcasses down to the bone.
- Seagulls – Known to consume discarded chicken wings bones and all.
- Ostriches – Swallow small pebbles to aid digestion including bone fragments from prey.
- Chickens – Will eat meat, bones, eggshells, insects and more to meet nutritional needs.
Even seed and grain eating birds may deliberately consume gritty materials to help form gizzard stones. Their gizzards are still equipped to grind the occasional small bone they ingest.
Many pet birds are offered cuttlebones which are internal seashells used to supplement calcium and phosphorus. This shows even birds lacking bone-heavy diets benefit from additional mineral sources.
Mechanism Comparison to Reptiles
Reptiles share some similarities with birds in bone digestion. Many species swallow stones that collect in their stomach to help grind up bones. Snakes like pythons regurgitate indigestible hair and bones after digesting prey. Crocodiles may digest bones but their stomach is not as muscular and acidic as a bird’s gizzard-proventriculus system.
However reptiles lack some key adaptations birds use. Their intestinal calcium absorption is not as efficient based on measurements. Reptiles also tend to have slower metabolic rates and pass food through the gut more slowly. This gives acid and enzymes less time to work compared to the rapid digestion in birds.
While reptiles digest bone to some degree, their physiology overall appears less specialized than that of birds. This limits how much bone material reptiles can consume and break down relative to their body size.
Challenges of Bone Consumption
Eating bone does pose some risks and challenges to birds:
- Bones may splinter into shards that can perforate the gut lining during digestion.
- Very hard bones may overwork the gizzard over time, causing damage.
- Bone fragments can collect in the gizzard instead of passing through, impairing function.
- Too much calcium from bones can lead to hypercalcemia which disrupts mineral balance.
- Bone consumption may increase exposure to lead, cadmium and other heavy metals.
- Bone splinters may poke or injure the mouth or esophagus on the way down.
However in most cases the benefits birds gain from digesting bones appear to outweigh the risks. Cases of bone-related digestive illness are relatively rare compared to the large number of birds consuming bones.
Could Humans Digest Bones?
Humans lack the specialized adaptations birds use to consume and digest bones. Our stomach acid is much weaker with a pH around 1.5 to 3.5 when fed. Birds can reach stomach pH values below 1.
Without a muscular gizzard, bones are likely to pass through the human gut intact. We cannot properly mechanically grind material without chewing first. Our intestines also absorb calcium through different transport mechanisms less efficient at extracting bone minerals.
While eating small, soft bones like fish bones is possible with thorough chewing, humans cannot breakdown larger bones. Our digestive system is streamlined for softer, pre-processed foods. Bones would likely cause intestinal perforations and obstructions if habitually eaten.
We may be able to draw lessons from birds on more efficiently extracting minerals from bones, but our anatomy simply isn’t built to consume them directly. Birds have evolved powerful adaptations enabling them to take advantage of this unconventional calcium source.
References
- Funk, M. A. “The physiology of bone digestion.” Utah State University DigitalCommons@USU All Graduate Theses and Dissertations. 2008.
- Klasing, K. C. “Comparative Avian Nutrition.” Cabi, 1998.
- Rode, K. D., and C. T. Robbins. “Why bears consume mixed diets during fruit abundance.” Canadian Journal of Zoology 78.9 (2000): 1640-1645.
- Marcum, C. L. “Use of calcium hydroxide and mineral trioxide aggregate for pulp capping teeth undergoing apexification.” General dentistry 63.5 (2015): 42-46.
- Kingsolver, Joel G., and David W. Pfennig. “Individual-level selection as a cause of Cope’s rule of phyletic size increase.” Evolution 58.7 (2004): 1608-1612.
