What is a Bird?






Gray-breasted Mountain Toucan

Andigena hypoglauca

How do we define a bird? What makes it different from other creatures? I cringe when I hear the phrase "birds and animals" as if birds were something other than an animal. But let's start there. There are five kingdoms of living things, one of them is Animalia, the animals. The Animal Kingdom breaks down further into 31 phyla, among them sponges, segmented worms, molluscs, echinoderms, arthropods, and finally chordata. Under chordata is the subphylum vertebrata that we are most familiar with and is comprised of five classes: fish, amphibians, reptiles, mammals, and of course, birds.

Each of these groups have their own set of characteristics that define them and set them off from other groups. So how do we define a bird? First, the most definitive and obvious character is feathers. All birds have feathers and only birds have feathers so birds cannot be confused with any other animal group. But there are lots of other features that define birds. They all lay eggs. This doesn't set them off from other animals but all birds lay eggs without exception and unlike other animals, bird eggs have a hard protective shell. And birds are warmblooded. more accurately, homeothermic, allowing them to live in terrestrial environments not accessible to reptiles and amphibians.

Almost all of the 11,000+ bird species fly  and only bats and insects share flying skills with birds. Birds are allowed to fly because they evolved a number of features that either reduce their weight or increase their metabolic energy. There are 60 species of flightless birds but they lost the ability to fly after evolving from flying ancestors.

The skeletal system has evolved a number of adaptations. Over evolutionary time birds have lost bones  or fused them - the tibia and fibula became the tibiofibula and the ilium, ischium, and pubis fused into the pelvic girdle. Abdominal ribs disappeared and the thoracic ribs developed uncinate processes to overlap the ribs and make the rib cage stronger.They lost their teeth and replaced them with a lightweight beak. The furcula, the wishbone, arose as a fusion of clavicles and serves as an attachment for flight muscles. The caudal vertebrae, the tail bones, have been fused into  the pygostyle from which the tail feathers grow. Many bones, especially those of the arm/wing are hollow but with an internal network of struts to both lighten and strengthen the bones. The atlas and axis, the first two cervical vertebrae just below the skull, are designed to let the bird swivel its head 180 degrees or more and preen themselves. The long neck contains 15-25 cervical vertebrae, allowing great flexibility. Most of all, the sternum has a keel to which the powerful flight muscles attach.

The ability to fly requires energy so their metabolism has adapted.  Birds tend to be smaller than mammals and thus use more energy; their body temperatures (37.7-43.5 C) are also somewhat higher than mammals' (36-39 C). But birds are generally more active than mammals and so need to eat more and digest their food efficiently.

The beak has evolved into a wide spectrum of forms, allowing birds to catch, grasp, snap, scoop, gnaw, tear, rip, and hold prey, It serves as the bird's only tool (with exceptions like parrots) to obtain food, build nests, defend territories, attract mates, move eggs, feed young, and thermoregulate.

We'll talk more about each of these adaptations and many more in future blogs.