Introduction

The ability to fly has enabled birds to occupy a wide diversity of habitats with great adaptations for feeding. This has led to a large number of about 9,700 extant species belonging to the class Aves divided into about 27 avian orders. The largest order of all is the Passeriformes with over 5712 species while the smallest is the Struthioniformes with one species, the ostrich. Whilst reptiles and mammals show incredible diversity, the constraints of flight means the basic bird design varies very little from species to species. In fact there are fewer morphological variations among all the bird species than among, for example, the mammalian order of Carnivores that contains only about 300 species.

Metabolism

Birds are endothermic meaning they have ability to maintain a relatively stable body temperature, irrespective of the ambient temperature. Their body temperature is about 3 degrees higher than mammals at around 40°C (+/- 1.5°C) so high metabolic rates are needed to maintain this and enable them to fly. Birds expend 20-30 times more energy than reptiles of similar body size so their circulatory and respiratory systems have evolved to rapidly provide energy and oxygen to cells.

Thermoregulation

Birds regulate their body temperature between 39-42°C with smaller birds like the passerines having higher body temperatures and large flightless birds like the ostrich falling within the mammalian range. They have very poor tolerance for high temperatures and 46°C is fatal. Unlike mammals, they have no brown fat but regulate their body temperature by the following behavioural and physiological means.

Plumage

Birds use their plumage both for heat loss and heat conservation. The contour feathers provide some insulation but it is the fluffy down feathers underneath which provide most thermal insulation. When cold, birds fluff these feathers to trap air pockets between the feathers and will shiver the pectoral muscles to produce heat. They can also reduce heat loss by 12% by tucking their head under their wing and by 40-50% by sitting down. To dissipate heat they can extend their wings from their body and elevate the scapula feathers to expose the bare skin (apterylae) of the back of the neck

Body Mass

Birds are extremely sensitive to draughts or poor ventilation as heat loss due to convection means they must increase their metabolic rate. This is particularly severe in small birds as the high ratio of surface area to body mass means body cooling is more rapid. Likewise feather plucking birds or young chicks are also very vulnerable and need extra nutritional support to avoid negative energy balance. Fat is a very poor thermal conductor so aquatic birds like penguins which inhabit cold climates have a large fat subcutaneous layer to insulate against the cold.

Evaporation

Birds which are overheated can use thermal panting or gular fluttering. Thermal panting increases evaporative loss from the upper respiratory tract and is a highly effective means of heat loss. Gular fluttering is when the bird vibrates the hyoid muscle and bones in the throat causing evaporation from the lining of the mouth and throat. When the bird is expending high energy while flying or running, heat can also be dissipated through the large surface area of the airsacs. Flying also exposes the thinly feathered ventral wing and dissipates heat by convection.

Blood Shunting

Birds do not have sweat glands but lose heat through their skin or via blood shunts. Some birds, like pigeons and doves, dilate a large vascular plexus on the back of their neck called the plexus venosus intracutaneous collaris. A large proportion of the blood from the left ventricle flows to the legs during stress to increase heat loss. In some long legged species the legs get three times as much blood per heartbeat as the pectoral muscles and twice as much as the brain. Some aquatic and wading birds have counter current arterio-venous retes in the proximal feathered part of the leg. These tibio-tarsal retes transfer heat from body core arteries to the colder venous vessels bringing blood from extremities. This enables blood to flow to the legs without detrimental heat loss.

Behaviour

When cold some birds select microclimates to reduce heat loss, like roosting in holes, or sheltering in trees. Small birds often huddle together to keep warm. They also adapt their behaviour in the heat of the day by shade seeking and bathing or soaring on thermals for cooler air.

Skeletal System

The avian skeleton has the following modifications to enable birds to fly. They have a lightweight fused skeleton and the avian forelimb is modified into a wing while the beak and neck are modified for food prehension. The manus is tapered and fused to hold the primary feathers. Many bones of the backbone and limbs are also fused to form a rigid and strong but light framework. This fused rib cage helps resist the twisting and bending of wings in flight while the rigid pectoral girdle acts like a wing strut. A fused tail vertebra (pygostyle) provides a short tail for steering and manoeuvrability. The sternum is keeled (carinate) to hold the muscles of flight. The supracoracoideus muscle lifts the wing by passing from their ventral attachment on the sternum through the triosseal foramen to insert on the dorsal humerus. This keeps all the heavy flight muscles along with the muscular gizzard situated ventrally at the bird's centre of gravity. The airsacs extend into the medullary cavity of the major bones like the humerus, coracoid, pelvis, sternum and vertebrae. They are most developed in the good fliers to help in weight reduction. The skull also consists of a honeycomb of air spaces with delicate spicules for support.

Cardiovascular System

In order to be able to fly, birds have high oxygen demands so consequently the avian heart is 50-100% larger than mammals of same size. The size of the heart also depends on the amount of aerobic energy each species expends. For example a large bird like a swan will proportionally have a smaller heart for its size than a racing pigeon. Birds also have a far greater exercise capacity than humans--at maximum exercise the cardiac output in a flying budgie is seven times greater than a man or dog.

In order to pump large volumes of blood to the wings, head and flight muscles, birds have a much higher cardiac output than mammals. This is achieved by a high stroke volume, fast heart rates (150-350 resting) and slightly lower peripheral resistance. They also have stiffer arteries to improve blood flow and maintain a high blood pressure ranging from 108-250 mm Hg (compared to an average of 150 mm Hg in humans). The consequences of this high pressure can however mean that aortic rupture, heart failure and haemorrhage are a common cause of death in stressed avian patients.

Respiratory System

The avian respiratory system is unique as birds have small lungs, that have little change in volume when breathing, and air sacs, which act as bellows but do not participate in gas exchange. This segregation of ventilation and gas exchange helps to increase the total gas exchange surface area. The bellows system allows continuous gas flow as opposed to 'in and out' tidal flow of mammals. Birds have no diaphragm--instead the horizontal septum separates the lungs from the viscera. This septum plays no active role in respiration but passively helps to displace the viscera during breathing.

Flight and the ability to fly at altitude means that birds have much higher oxygen demands than mammals. Avian lungs are 10 times more efficient than mammalian lungs in capturing oxygen due to the following modifications:

 Thin blood-gas barrier: the air capillaries of the lung (equivalent to the mammalian alveoli) are finer and more numerous and the blood gas barrier is very thin. This is possible because, unlike mammalian lungs which have to expand and contract with every breath, the fixed avian lungs require little interstitial tissue for added strength.

 Cross-current blood flow: the blood flow is at right angles to air flow giving a cross current exchange system. This means that blood flow is always at right angles no matter which way the air is flowing. Cross current exchange allows more efficient absorption of oxygen without incurring high levels of carbon dioxide in the blood.

 One way air flow: the air flow through the lungs is unidirectional as opposed to 'in and out tidal flow' of mammals. The parabronchi being tubes and not dead end sacs like alveoli allows for continuous gas exchange in the avian lung and it may explain why birds can fly at high altitudes.

 Rigid lung: the fact that the lungs are rigid and play no role in ventilation means that there is 20% more area for gas exchange than in mammals.

Digestive System

The avian gastrointestinal tract is relatively short with low volume to keep the bird lightweight for flight. Consequently birds ingest small volumes frequently and extract energy and nutrients rapidly to sustain their high metabolic rate. Transit times ranging from as little as 16 minutes to 2 hours are found in passerines. Birds also have an extremely efficient digestive system passing remarkably small amounts of excreta in contrast to the amount of food eaten.

Birds have no teeth so no time is spent on chewing, so food passes rapidly to the crop for storage and passes to the gizzard for mechanical digestion. This heavy organ located at the bird's centre of gravity has taken on the role of mammalian molars in grinding down the unmasticated food. Herbivorous birds like the ostrich and chicken also have well developed caecae for food breakdown as well. The cloaca is the site for termination of the urogenital and digestive systems. It is usually a bell shaped dilation at the end of the rectum and consists of the coprodeum, urodeum and proctodeum. The Bursa of Fabricius is located in the dorsal wall.

Integument

Avian skin is very thin as it is protected by the plumage and helps to reduce weight. It is lightly attached to underlying muscle but firmly attached to bone. Feathers are keratinised epidermis, which are derived from specialised follicles in the dermis. They play a vital role in protecting, insulating and waterproofing the bird. They are also essential for flight and often courtship. During growth there is a healthy arterial and venous blood supply to the follicle, which degenerates when the feather matures. Immature feathers will thus bleed if broken and are called 'blood feathers'. The feathering or plumage of birds can weight 2-3 times than that of their bones. They are not attached to the skin evenly but are set in feather tracts (pterylae). Featherless regions (apteria) are also present and may be used for wing and leg movements and to provide space for these appendages to be tucked in.