Birds, creatures belonging to class Aves of the Animal Kingdom have several characteristics that help us easily distinguish them from the other class of animals. Some of these characteristics are not visible to the naked eye such as the skeletal system and endothermy, while some of them are prominent and unique to them such as feathers and beaks.
Below we have discussed the 6 major or defining characteristics of the class Aves, or simply birds.
1. Feathers
Feathers are the most prominent and defining characteristic of birds or the class Aves, in general. Feathers are just highly modified scales made of protein beta-keratin, the same protein which is responsible for the formation of hair, nails, hooves, and horns in other classes of animals. From the soft and fluffed-up feathers of a swan to the bright colours on the feathers of the King Bird-of-Paradise, there are numerous variations since feathers have surprisingly diverse forms.
The feathers of all birds are arranged in a branching structure with the calamus, the rachis, and the barbs and barbules. It is the interlocking of these structures and the modifications in them that change the type of feather you’ll be looking at on different species of birds, all serving different functions.
Functions of Feathers in Birds:
If you think the feathers on a bird are merely meant for flight and warmth, this is where we burst the bubble. From flight to ‘snowshoeing’ (feathers on a bird’s foot to protect from snow and cold) to distracting and insulating, feathers have numerous functions, some of which we have discussed below.
Flight: It’s a no-brainer that the primary function of the feathers is to aid in flight in Aves. There are certain muscles attached to the base of each feather, which allows the bird to move the feathers when needed.
It is the primary and the secondary wing feathers that allow the bird to fly higher. Also known as remiges, the primary and secondary feathers are attached to the bones underneath with the help of sturdy ligaments that aids withstanding the demands of taking up to the sky.
Display: Now, let’s talk about awe-striking colours! A number of bird species have brilliantly coloured feathers that, in general terms, are used to ‘show-off,’ to woo a partner during the mating season. Significantly, it is the male that has a bright display of feathers to attract the females.
For example, peacocks display their mesmerizing bluish-green fan on the upper tail to woo their partners. Another example is the Eastern Kingbird, which shows off its vibrant yellow contour feathers on the top of its head to attract females.
Some birds, such as the Wood Ducks and the King Bird-of-Paradise, have feathers for courtship displays. Some other birds, such as the Blue Jays, have a blue crest on top of their head that they raise during aggressive interactions or to warn enemies.
Camouflage: While for some birds, feathers are an excellent medium of display, some birds use their feathers to blend in with their environment, AKA camouflage.
The Snowy Owl has completely white feathers to blend in with the wintery frost and snow. Owls, in general, have brown feathers to help them blend with their natural landscape and habitat, i.e., trees. The Common Potoo has feathers that mimic the tree branches, therefore serving as the perfect camouflage.
Some birds, such as the Dark-eyed Junco, use their brightly coloured feathers to flash against their predators, then quickly wrap them in, turning in the opposite direction to get away.
Insulation: Insulation is an important factor for survival, especially for young birds that lose heat faster than adults. Mostly when the young chicks hatch, they have a fuzzy coat of natal down to keep their body warm and later, this natal down gets replaced by an adult down and an outer coat of contour feathers.
These feather coats help in insulation and keep them warm during cold and cold during the hot months. To survive in sub-zero weather, the birds create a layer of air and feathers by fluffing their feathers, and this insulation helps them to survive in freezing weather.
Waterproofing: The contour feathers arranged in an overlapping pattern on the bird’s body make their body waterproof, and the water rolls right off their body. The feathers are maintained through grooming and preening by the birds to ensure that the waterproof coat is in good condition.
The interlocking structures of the feathers prevent waterlogging, but any disruption might cause the bird to be helpless and waterlogged. Maintaining the feather coat is critical for the survival of the birds like ducks, swans and other birds spending most of the time in the water.
2. Wings
It may sound confusing at first when we count wings to be one of the major characteristics of birds since not all birds fly. However, here’s the thing – all birds have wings, some of which use it for flight, and some do not and can not. Wings, however, are not the distinguishing characteristics of birds since mammals such as bats and insects such as bees and butterflies, too, have wings.
Wings are nothing but the bird’s hands, or forelimbs in particular, that have been strongly transformed. Some of the bones in hand are merged with each other, and the others are removed, therefore making the formation of wings possible.
Wings are specially designed to help in flight, and the differences in the shape provide a biased advantage to each species of the class Aves such as speed, energy use, etc. The shape of the wing also determines the capability to fly and maneuverability, and the shape itself is defined by two parameters that are aspect ratio and the length of the wing.
Different types of wings have different functions such as the flippers in penguins help them to move across the land, and propel through the water, albatrosses have wide wings to soar high, and small birds, usually living in thick forests, such as songbirds, have elliptical wings that allow them to swiftly and gracefully move through cluttered space.
3. Beak
Beaks are another major characteristic of birds. Beaks or bills in some birds are made of bony cores with keratin layering the jaws since birds do not have true teeth. The Beaks are the most common characteristic occurring in all birds, ranging from broad and flat to sharp and pointed.
Each type of beak has a different function, and as species change, so does the shape of the beak in response to the habitat and the environment that the species live in. The major functions of the beak are to aid in diet, feeding offspring, defence, grooming and preening feathers, mating, and building nests.
Different Types of Beaks and their functions:
- Hooked beaks: The hooked beaks are ideal for biting the skull and tearing the body of the prey into small pieces to swallow. Birds like Owls and birds of prey, including golden eagles, and hawks use their beaks to catch and kill their prey.
- Cone-shaped beaks: Cone-shaped beaks, also known as “Cracker,” are short, thick, and robust beaks with conical bills, which help the birds to crack seeds and nuts easily. Sparrows, Pigeons, Peacocks and Finches are some common examples of birds with cone-shaped beaks.
- Short, curved beaks: Curved beaks are used by birds for climbing, peeling off fruit skin, and manipulating and cracking objects like nuts and seeds. Birds like parrots and macaws have short, curved beaks.
- Straight, thin beaks: The insectivorous birds like woodpecker or robin have thin and straight beaks, which helps them to catch insects, peck through the woods to find bugs.
- Long, thin, needle-like beaks: Tailorbird, hummingbird and other nectar-feeder birds use their beaks to suck nectar from the flowers.
- Wide, flat beaks: Birds with filter-feeding systems like Flamingos, swans, and ducks have flat beaks which appear broad and a bit similar to the shape of a spoon. With these flat beaks, the birds collect dirt or water around the ponds or river beds to collect the worms or plants to feed on.
- Spatulate beaks: More Like spoon-shaped, spatulate beaks are wider at the end as compared to the base. Spoonbills, Northern shoveler or Mallard, are some of the birds that have spatulate beaks, and these birds feed on insects, crustaceans and plant matter and catch them by using snap shut.
- Large, long, and strong beaks: Birds feeding on fishes have these large and strong beaks, which helps them to catch their prey, and the curved shape of the beaks prevents them from escaping. Pelicans, albatrosses and seagulls are some good examples of birds with large, long, and strong beaks.
- Crossbill beaks: The red crossbills have the crossbill beak; the crossed tips of their beak helps in removing the conifer seeds from the cones.
- Multifunctional beaks: A Toco Toucan’s beak is a multifunctional beak, which they use for skinning fruits, grooming, attracting mates, frightening predators and also keeping them cool in the tropical days.
4. Oviparity/ Eggs
Birds are oviparous animals, i.e., they lay eggs instead of giving birth to young ones (called viviparity). Oviparity is one of the main characteristics of birds. However, it is not a distinguishable characteristic since amphibians, fish and insects lay eggs too.
A bird egg usually has a hard calcium carbonate shell and a layer of mucus, while the inside has an embryo that later develops into the bird chick. The embryo receives nutrition from the yolk and the albumin during the hatching period.
Although most eggs are white owing to the CaCO3 shells, some eggs, usually the passerines’, are pigmented owing to biliverdin that gives eggs a greenish or bluish hue. Another pigment that leads to coloured eggs is protoporphyrin (red and brown eggs).
In most bird species, both male and female parents take care of the eggs and the hatchlings, devoted to protecting them against predators, extreme weather conditions, and other harms.
Two major reasons for oviparity in birds:
- Mammals and other viviparous animals have a heavy body that is adapted to carrying the fetus within the womb of the female. In oviparous animals, especially birds, the embryos develop outside the female body since the female body is not heavy enough to carry the embryo around or fly with the fetus in the womb.
- If within the mother’s womb, the bird embryo can not receive adequate oxygen since the eggshells and oviducts of birds are not structured to support the long-term development of the embryo within the womb of the bird. The constricted airflow in a bird’s oviduct can kill the embryo before it has even properly developed. Thus, birds are oviparous animals that have adapted to oviparity to develop healthier chicks and continue the survival of their class.
5. Light and Rigid Skeleton
Birds have a unique skeletal structure, specially designed to aid in flight. The skeletal structure of Aves is light and rigid enough to help them take off and land down without any difficulties and withstand the stress experienced in the meantime.
In general, birds have sternums that are large enough to act as powerful and rigid attachment points for the muscles under the wings that allow wing movement. Birds have fewer bones than mammals, as a result of several bones merging into one in certain parts, and some completely reduced from the body. Most birds have 13-35 highly elastic bones that allow easy flight movements.
The lesser the bones there are in the body, the hollow the skeleton is, therefore allowing absorption for extra oxygen needed for flight. In flightless birds, the skeletal structure is only slightly different. For example, most birds with the capability to fly have struts and hollow bones, while birds that can not fly have solid bones.
When it comes to the foot, most birds have only three to four toes, while the ostrich has only two.
6. Endothermic Metabolism
Endothermic or warm-blooded animals maintain a constant body temperature that is independent of the environment. It is an important adaptation that helps adult and juvenile birds to survive extreme temperatures by maintaining a constant body temperature. When the heat loss exceeds heat generation, these animals shiver to raise their body temperature, and the metabolism process also increases to warm up the body.
This is the reason birds shiver even when it’s not too chilly outside or just the right temperature for us. Mechanisms like panting or perspiring help in increasing heat loss when heat generation exceeds heat loss in birds. As endotherms, birds require a lot of food in order to produce heat and survive extremely low temperatures.
