If you ever hold a bird in your hand, it is hard to believe that something that small and lightweight could have a complex skeletal structure. Some birds weigh no more than a coin, and even bigger raptors feel like light bundles of feathers.
Birds are vertebrates. Vertebrates have an internal skeleton to support and protect their vital organs with a spinal column and softer tissues on the outside. Pneumatized bones, keeled sternums, and extra neck vertebrae are vital differences that birds have from mammals.
If you watch footage of a wryneck contorting its body, or an owl twisting its head, you would be forgiven for thinking that it has no neck bones. So what is going on underneath those feathers? Do they have skeletons in any way like our own, and does this make them vertebrates?
Why are Birds Vertebrates?
Birds are vertebrates because they share the same skeletal features as other vertebrate species. Invertebrates, such as insect and arachnid species, have an exoskeleton around their body as protection.
One of the primary differences between an invertebrate and a vertebrate is that invertebrates don’t have a backbone. This is where the name comes from – the vertebrate of the internal spinal column. Mammals and birds do have this. We have a flexible spine running from the base of the skull down into the pelvic region. Other mammals have the same, with theirs often including a tail. When you look at the skeleton of a bird, you see similarities in their form and many differences.
The neck or cervical area of a bird’s spine is much longer than our and most mammals’ necks. There are typically between 11 and 25 vertebrae in a bird’s neck. This variation depends on the species and how they use their necks for hunting. The more vertebrae they have, the greater their flexibility. Longer necked birds like the anhinga can bend and move their necks fluidly to attack prey from different angles and strike with ease.
Greater flexibility also allows birds to groom with their beaks, reaching hard-to-reach places to preen and maintain their feathers. You can also see head-bobbing movements in some shorebirds. This belief allows them to gain more information on the distance or angle of something before making a judgment. Meanwhile, hovering birds like kestrels use the flexibility in their necks to stabilize their heads in the wind and fix their gaze on their target.
Trunk and Lower Spine
Beyond the neck, there are further similarities between bird spinal columns and those of mammals. A trunk section leads down into the synsacrum, which contains several fused vertebrae and the caudal area at the base near the tail. Here, the synsacrum is fused to the pelvic area. The caudal area has between five and ten vertebrae depending on the species. Similar to the coccyx, the stability of the structure and the connected muscles helps to control movement in flight.
Vertebrates, like birds, have a more complex internal skeleton than insects’ exoskeleton. Invertebrates don’t have any internal skeletal structure at all. All the protection they need comes from the exoskeleton outside. These structures vary greatly depending on the animal. Some have pretty flexible and thin casings, while others, such as beetles, have thicker structures to protect them from harm. The exoskeleton grows and molts with time as layers break down and replenish.
With vertebrates, the bones need to grow healthy and strong within the body, causing different growth. This happens to birds as they develop in the nest and become ready to fly. Depending on the species, the skeleton can have some impressive adaptations.
Birds develop a strong internal structure around the vital organs with a ribcage and other features found in mammalian species. Some of these are much smaller than you would imagine. The size of some birds is an illusion created by a density of feathers. There is a notable size difference when comparing an owl species’ skeleton to another bird.
Ribcage and Breastbone
Another important adaptation is that the bones in the skeleton are much lighter. Birds have pneumatized bones, which essentially makes them hollow with a series of crisscrossing struts. This makes the bones as light as possible but also strong.
Lighter bones help them get off the ground easily and aid with flight, but the strength means they are no more brittle. Birds that don’t fly as much and rely more on their diving abilities tend to have denser bones, making them less buoyant.
There are also some interesting differences in the internal skeleton structure, such as fused collarbones and a keeled sternum. By combining the collarbones, there is greater strength and support for the bird where necessary. The same is true for fusions around the pelvis and in the bones in the wings. The lighter weight balances out any density created by these fusions.
The sternum is the breast bone. We have one at the front of the ribcage, but it isn’t anywhere near as pronounced as those in birds. The shape and size of this breastbone allow for the attachment of stronger muscles for flight or swimming. As a result, this keel is not present in flightless birds, whose power comes from their legs, and is much bigger in birds that swim a lot.
The skeleton of birds shows that these vertebrates are ideally suited to life as winged creatures. The bones’ lighter structure makes up for the fact that these birds still require a strong, heavy internal skeleton for stability and mobility. What could have been a hindrance has become a brilliant evolutionary adaptation with these pneumatized bones, keeled sternums, and the extra neck vertebrae for flexibility.
Bryan Harding is a member of the American Society of Mammalogists and a member of the American Birding Association. Bryan is especially fond of mammals and has studied and worked with them around the world. Bryan serves as owner, writer, and publisher of North American Nature.