Birds are vertebrates (backboned animals) that are covered in feathers. They have many things in common with other vertebrates but there are also many things that make them unique. Like mammals, birds have a four chambered heart and are warm blooded (maintain a constant body temperature) which enables them to live in a variety of different climates. Like reptiles, birds lay eggs. Birds are best known for their ability to fly but not all birds fly, for example the ostrich or penguin. Birds can vary in size from a small hummingbird to the large ostrich.
There are about 9, 000 different species of birds that are divided into about 24 different orders as seen below.
Birds have many features that they all share, however, as birds are highly adaptive animals, the different families have many differences between them. Though some birds do not fly, their features do reflect that all birds came from a flying ancestor.
Feathered friends
One thing all birds have in common, and share with no other animal, is that they are covered in feathers. The feather is made of
keratin just like our fingernails and the scales of reptiles. Feathers play 3 extremely important roles for birds:
- They are instrumental in flying
- They are used in temperature regulation
- Different shapes, colors and patterns serve different functions for different birds. Colors can help with camouflage or help to find a mate, some feathers ward off predators whilst others help to funnel sound.
All of the feathers on a bird are known as its
plumage and the number of feathers can vary from bird to bird. Smaller birds will obviously have less feathers than larger birds but the waterfowl also have many more feathers than other birds and birds living in cold climates have more feathers during the winter months than in the summer.
Feathers however are not perfect. They have to be constantly cleaned and re-aligned by the bird so that they function the way that they are supposed to. Feathers can get damaged very easily and so birds periodically replace their feathers through a process called a
molt. Some birds will go through a full molt over a short period of time which is referred to as a
complete molt. Other birds only molt part of their plumage and this is referred to as a
partial molt. Molts also allow birds to change their appearance for breeding season or to help camouflage after a change in season, ie summer vs. winter.
Beaks
Birds do not have teeth. Teeth are heavy, a beak is much lighter and is a very effective tool for getting food. The upper and lower parts of the beak can move independently enabling them to open their mouths extremely wide in most cases. Beaks come in a variety of sizes and shapes depending on the type of food that the bird eats. Most birds are very specialized feeders, only eating certain items which the beaks is highly adapted to obtain.
Different digestion methods
As many birds are so specific in what they eat, their digestive tracts vary accordingly. Many birds have an extra carrying pouch at the base of the throat so that they can fill it up and fly somewhere safe to slowly digest their food, sending small amounts to the stomach at a time. This pouch is known as the
crop. It is mostly present in seed eating birds and is not usually present in the insectivores or nectar feeders. The stomach in all birds consists of 2 chambers: the
proventriculus and the
gizzard. The proventriculus is the first chamber and it secretes the acids to digest food. It is best developed in the birds that digest whole animals, like birds of prey. Next food is moved into the gizzard which has very powerful, tough muscles designed to grind up and digest tough foods. The gizzard is more developed in birds that eat tough foods like the seed eaters.
Fly like a bird
All birds have modified fore limbs called
wings. Wings come in a variety of sizes and shapes depending on the birds style of flight. Birds that have large wings compared to their body size can fly long distances with little effort, whereas birds that have short wings compared to body size need to flap constantly to stay in the air and as this takes a lot of effort, they do not fly long distances.
The skeleton of a bird is also well adapted for flight. The bones are not solid but are instead filled with air spaces. There are three places of the skeleton that are fused together - the thorax, pelvis and the outer wing. In other vertebrates they occur separately. This fusion increases the rigidity of the skeleton during flight which produces major mechanical stresses and it is even stronger in birds that dive into water from high up in the air.
Flying requires a lot of energy and this energy comes from an extremely fast metabolism. A birds high metabolic rate is produced from a high body temperature of about 40-42° C (104-108° F) which provides an environment that supports rapid chemical reactions but also increases nerve impulse responses and muscle strength. To provide enough oxygen for the muscles, breathing in birds is always rapid (450 times a minute in pigeons compared to 30 times a minute in a running human) and blood pressure is also higher than in other vertebrates.
Bird senses
Birds have keen senses. Sight and sound are the most important to birds and this is made apparent by the fact that birds are often brightly colored and produce complex vocalizations.
The most superior sense in nearly all birds is vision. On average birds can see at least 3 times better than humans and some birds of prey can locate small prey more than a mile away. In all diurnal (day active) animals, the retinas in the back of the eye are densely packed with color receptor cells, called
cones. A typical bird eye has 5 times the amount of cones than in a human but the bird cones are also enhanced by pigmented oil droplets which are thought to help sharpen the image and also reduce glare.
Hearing is the next most important sense in birds, most important in the nocturnal owls. Even though they do not have visible ears, they do in fact have ears. The ear openings lie just behind the eyes and are covered in special feathers called
ear coverts. These feathers protect the ear but offer little resistance to sound. The internal structure of the ear is simpler in birds than in mammals as they have one bone instead of three. As a group, birds hear a broader range of sounds than humans but individual species are specialized in hearing a specific range.
The other senses, particularly taste and smell, are secondary to most birds. Taste is particularly poorly developed and though they do have taste buds, they posses very few (averaging about at about 35 taste buds in birds compared to the 10,000 in humans!). The few taste buds that they do posses are in the rear of the tongue may be to help make a last minute discussion whether or not the bird should swallow the food item. In the past it was believed that taste and smell was not an important senses to birds but increasingly scientists are finding that smell is quite developed in birds like the vultures and some nocturnal birds.
A final sense, and one that humans seem to lack entirely, is the ability to detect the Earths magnetic field. It is thought to aid in navigation but it is still unclear exactly how birds do this.
Producing sound
Birds use sound to declare territory, find a mate, family members, the rest of their flock or to deter predators. This sound is produced using the
syrinx which is comparable to the larynx in humans. However, the single larynx in humans is located in one position high up in the trachea, whereas the syrinx is located further down where the trachea branches to form the two bronchial tubes and so there are essentially two separate syrinxes each capable of producing its own independent sound. One can take in small pockets of air whilst the other emits sound making the birds song seem endless.
Exactly how birds came about is highly controversial among scientists. One thing that is certain is that birds descended from reptiles around 200 million years ago which was proven after the discovery of the fossils of an animal that had the traits of modern birds as well as those of reptiles. This animal is known as
Archaeopteryx. However no-one is sure which group of reptiles birds come from. Many believe that birds may have evolved from a bipedal (2 legged) dinosaur whereas some believe that birds and dinosaurs share the same ancestor. Recent evidence is suggesting that birds came from two-legged, running dinosaurs called
Theropods. When the extinction of dinosaurs came to be, so was the end of many of these early birds. Apparently one lineage of birds survived to produce the very successful groups of birds that we have today.
Another question that has yet to be answered is how birds developed flight in the first place. Some believe that the ancestors were arboreal (tree dwellers) and developed flight through primitive gliding activities, similar to the flying squirrels of today. This theory is referred to as the
trees down theory. The next theory suggests that the ancestors were terrestrial (ground dwellers) and chased prey from the ground where the leaps and bounds eventually became a glide. This is referred to as the
Ground up theory. The recent evidence seems to support the fact that some ground dwelling dinosaurs did have feathers but both theories may have some truth to them as the ancestors may have been both ground and tree dwelling.
Birds have been of economic and ecological importance to humans for thousands of years. People have eaten them or their eggs, used their feathers as ornamentation or in pillows and many other uses. The earliest domesticated bird is probably the chicken which was domesticated from the Jungle fowl maybe about 3000 BC in Southeast Asia. Since then, all sorts of birds have been kept as pets, but birds have also been used as environmental indicators. In the 19th century, coal miners brought caged canaries into the mines with them and as birds are very sensitive to toxic fumes, if they stopped singing then that indicated that poisonous gases had escaped into the air.
Throughout time man has hunted birds. Studies of bones on some Pacific islands has shown that man has caused the extinction of many hundreds of bird species in that region. Birds that live on small islands are particularly vulnerable as many have evolved without predators and are often flightless making an easy target for humans as well as the animals they bring with them (ie. Dogs, cats, rats etc). The most famous case of where humans have over hunted such a bird is in the case of the Dodo, a flightless, pigeon-like bird from Mauritius which was over hunted in the 1600s. Another famous case is the Passenger pigeon of North America which was hunted to extinction in the beginning part of this centry.
There have also been birds that have gone extinct in the Bahamas because of human activities, like the Brace's Emerald Hummingbird (Chlorostilbon bracei) from New Providence, went extinct by the end of the 19th century mainly due to habitat loss. Currently, the Bahama parrot, particularly the Abaconian population is at risk of becoming extinct because of the animals that people have brought to that island and the Caribbean Flamingo nearly became extinct in the Bahamas in the 1940s from over hunting and habitat disturbances. However, both the Bahama parrot and the Caribbean flamingo are now doing a lot better because of the conservation measures put in place to protect them.
There are more than 300 different species of birds that have been spotted in the Bahamas. During the fall and winter months the number of birds in the Bahamas is much higher than other months as many species of birds over-winter in the Bahamas from colder regions. Three species are endemic (or restricted) to the Bahamas, the Bahama woodstar, Bahama Swallow and the Bahama yellowthroat and about 24 subspecies of other birds are also unique to the Bahamas (from Hallett, Bruce. 2006. Birds of the Bahamas and Turks & Caicos islands). The different families that can be found in the Bahamas include:
Ardastra has many different types of birds in its collection. Some of which are free to walk around and mingle with people. The various types of birds that live in Ardastra include:
