Birds most likely evolved from theropod dinosaurs, while mammals descended from carnivorous reptiles, the cynodonts. Their differing lineage possibly contributed to the differences observed in the avian cardiovascular structures when compared to mammalian ones. Similarities may represent the conservation of characteristics common to the two ancestral groups and/or convergence of evolution once the groups diverged.

The most striking features of birds is their ability to perform very rigorous functions in harsh environments, such as diving deeply in cold water, flying at high altitudes and running in hot deserts. This requires that their cardiovascular system (CVS) be able to meet the demands of providing adequate delivery of oxygen to vascular beds that are taxed by extreme metabolic demands. The CVS must also efficiently remove metabolic byproducts to maintain function and hence performance. As homeotherms, this CVS must maintain internal body temperature while conserving or removing excess heat.

Gross Anatomy & Function of the Avian Heart

The avian heart is four-chambered. The right side of the heart receives blood from the systemic circulation and pressurizes the pulmonary circulation. Blood returns to the left side of the heart, where the left ventricle then pressurizes the systemic circulation. Both the right and left ventricles receive blood at the central venous pressure before they enter their respective outflow tract. This resistance to blood flow (peripheral resistance) is less on the pulmonary side when compared to the systemic side. For this reason, the left ventricle has more muscle mass to overcome the increased resistance on the systemic side to produce the same flow rate as that on the pulmonary side.

The heart is located in the cranial portion of the thoracoabdominal cavity with its long axis slightly to the right. Radiographically, the liver extends caudally from the apex of the heart, and this overlap results in an hourglass appearance of the two organs. The heart is enclosed in a tough, fibrous pericardial sac that contains a small amount of serous fluid for lubrication. The pericardial sac is loosely attached to the sternum, vertebral column, and adjacent air sacs and more firmly to the liver. By its peritoneal connections, it is attached to the hepatic peritoneal cavities. This arrangement makes the pericardial sac relatively noncompliant and thereby resistant to large increases in size from volume overload.

The relative size of the heart in birds is inversely related to a species’ body mass. This suggests that the larger species of birds have reduced heart mass in comparison to the smaller ones, which have proportionally larger hearts. This arrangement differs from mammals, as their body mass is directly proportional to the size of their hearts. This difference may result from the fact that the heart of larger birds, such as the barnacle goose, can hypertrophy (enlarge) prior to migration so that their hearts are proportional to their weight. Hummingbirds have proportionally larger hearts for their size, most likely as a consequence of the high aerobic demands of hovering flight.

The four chambers of the heart are completely divided into two atria and two ventricles. The right atrium tends to be larger than the left in most birds. The wall of these chambers consists of the same components as in mammals — the endocardium as the inside lining, the middle myocardium and the outer epicardium. The muscular myocardium of the atria is thinner than that of the ventricles. However, the myocardium is arranged in thick muscular bundles forming muscular arches. The atrial muscles contract to empty blood into the ventricles during ventricular diastole.

The muscular anatomy of the ventricles is more complex than the atria. The left ventricle is cone-shaped and extends to the apex of the heart, while the right ventricle is a crescent-shaped cavity that does not go to the apex. Its right wall forms the interventricular septum that separates the two chambers. The wall of the left ventricle is two to three times thicker than that of the right. The curvature of the wall of this left chamber is less than the right and results in a greater mechanical advantage for pressure generation. This allows it to generate systolic pressures that are four to five times greater than those produced by the right ventricle.

The atrioventricular (AV) valves of the heart of birds are similar in their anatomy to those of mammals; however, the cusps of the valves are poorly defined. The right AV valve is structurally distinct, however. It consists of a single spiral flap of myocardium attached to the free wall of the right ventricle. The AV valve of the left side is tricuspid, and not bicuspid, as it is in mammals. It appears that these valves are connected to the Purkinje system — a network of fibers that carry the cardiac impulse from the atrioventricular node to the ventricles of the heart and causes them to contract — and are electrically activated prior to activation of the myocardium to contract the valves and close the AV orifice at the start of ventricular systole. This arrangement differs from that of mammals, which allows the leaflets to float up into the atrial chambers.

The outflow valves of the ventricles are similar to those of mammals except for the fact that there is myocardial tissue that extends into the valves. Additionally, the myocardial extensions have connections to the Purkinje electrical system. This suggests that the valves act as a sphincter that is capable of constricting the outflow opening and changing flow dynamics.

Oxygenated blood from the left heart can enter the right and left aortic sinuses that become their respective coronary arteries. Most commonly, there are two coronary arteries in birds but there may be up to four in number. Often the superficial branches form a ring around the coronary groove to provide blood to the area. There are also deep branches that supply the ventricular walls along with the atria. The right branch is often the dominant vessel and supplies that majority of the blood to the heart. There are frequent anastomoses between branches of the coronary arteries particularly near the coronary groove.

There are often five groups of coronary veins with small tributaries that return blood to the right atrium and/or the right ventricles. There is often a coronary sinus where the blood is shunted before it enters the atrium.

Perfusion of the heart muscle is more active than in skeletal muscle and other avian tissues. As in mammals, perfusion (the passage of fluid through the lymphatic system or blood vessels to an organ or a tissue) occurs during diastole (the normal rhythmical dilatation of the heart during which the chambers fill with blood). The reduction in oxygen supply and increase in myocardial oxygen demand results in an increase in coronary blood flow. These factors come into play in birds that fly at high altitudes.

Coronary blood flow increases as a result of decreases in vascular resistance in response to hypoxia (inadequate oxygenation of the blood). As birds fly at even higher altitudes, the reduction of oxygen results in a compensatory increase in ventilation. Therefore, the arterial blood becomes alkalotic or the pH of the blood rises and the carbon dioxide levels decrease but the perfusion of the tissues with blood increases. This is very different from the dynamics of coronary flow in mammals where hypocapnia (abnormally low levels of carbon dioxide in blood) causes a decrease in blood flow.

Studies and dissections of avian hearts demonstrate that birds have larger hearts with bigger stroke volumes resulting in larger cardiac outputs than mammals.Birds have lower heart rates per unit size for greater perfusion of the tissues of the body. However, they have a greater mean arterial blood pressure than those mammals of comparable body mass. Values that compare birds of different sizes show that larger species have a greater increase in heart rate in absolute terms when transitioning from rest to flight. All of these factors are important to allow birds to have the oxygen capacity to flight and to handle these extreme conditions — something man has only been able to do with machines in the last 100 years of time!

 

Birds are highly visual animals with unique features and adaptations that allow them to fly. As they begin to migrate, they use visual cues to help guide them. Their eyes can change focus rapidly using an active process called accommodation. Birds also see ultraviolet light, and they have enhanced visual acuity because of different mechanisms, including a one-to-one projection of receptor cells to ganglion cells in the retina.

Avian Ocular Anatomy & Physiology

Birds are the most visually dependent class of vertebrates. Even though humans are highly visual, the information transmitted to our brains is only 40 percent of that transmitted by pigeons and chickens. Birds of prey have even greater visual acuity. Pigeons can discern subtle color differences, and other avian species are able to record and remember over 6,000 images of caches where food is stored.

Eye position in birds can be lateral in the skull or directed frontally, particularly in predator species such as raptors. Species with laterally placed eyes, such as parrots, have a larger visual field (300^o for pigeons) versus frontally directed eyes (150^o for barn owls). However, as the visual field increases, binocular vision decreases. In binocular vision, both eyes focus on the same object, and eye movement is coordinated. Monocular vision occurs when only one eye is focused on one object at any particular moment. That type of vision is the norm for our parrots.

The eyeball consists of the small anterior cornea, a variable intermediate region, characterized by scleral ossicles and the posterior sclera. These sclera ossicles provide the rigid shape to the eyes of birds, which is not the way mammal eyes get their shape.  Types of eyeballs include:

• Flat eyeball — diurnal birds (active during the daytime) with narrow heads; a short bulbar axis results in a small visual image on the retina and lessened visual acuity.
• Globular eye — diurnal birds with wider heads, including passeriforms, most parrots and birds of prey; a cone-shaped intermediate region results in greater visual acuity
• Tubular eye — nocturnal birds of prey; the intermediate region is relatively elongated.

The shape of the avian retina is relatively flat, meaning that its surface lies near the point of focus for all directions of incident light. The wall of the eyeball consists of:

• Outer fibrous tunic — cornea and sclera
• Middle layer — vascular layer
• Inner layer — nervous (retinal) layer

Fibrous Tunic

The outermost layer, or fibrous tunic, maintains the shape of the eye. The cornea is relatively small, particularly in underwater swimmers. It is strongly curved in tubular and globular eyes. The refractive index between the air and the cornea is relatively larger compared with that between the water and cornea, and is nearly the same underwater.

The sclera in birds is reinforced with a continuous layer of hyaline cartilage, except at the scleral ossicles. These ossicles form a continuous ring of overlapping bones that support and form a base of attachment for the ciliary muscles. Mammals maintain their eye pressure with the fluid that is made internally. Birds also make fluid in their eyes, and this fluid is drained out by the scleral venous sinus, or canal of Schlemm. This sinus is at the sclerocorneal junction or limbus of they eye, and it needs to stay open to drain fluid or it will cause an increase in pressure like in mammals that get glaucoma.

Vascular Tunic

The middle layer of the eye consists of a continuous layer — the vascular tunic. It is composed of the choroid, ciliary body and iris. The choroid tends to be thick, highly vascular and darkly pigmented, and it provides a significant portion of the nutrition to the eye. The tapetum lucidum, a highly reflective surface in many species, has been observed in only a few species of birds — the goatsuckers, which are nocturnal. The choroid continues as the ciliary body and then the iris.

The ciliary body suspends the lens with its processes, the ciliary processes and its fibers, the zonular fibers that encircle the lens. The ciliary processes are pressed firmly against the lens with its muscles that provide accommodation. These muscles are striated in birds, and they are directly attached to the lens capsule. This unique feature allows birds to rapidly adjust their vision as they fly. These muscles are most highly developed in hawks because they require rapid accommodation as they descend in a dive to hit their prey target with great accuracy.

Accommodation is much different in birds compared with mammals. In mammals, the ciliary muscles result in a passive change of the thickness of the lens. Birds, however, use a variety of active mechanisms for accommodation of the lens. These mechanisms include:

• Using the posterior sclerocorneal muscle to force the ciliary body actively against the lens to change its shape
• Using the anterior sclerocorneal muscles to distort the center of the cornea (hawks, owls)
• Having a softer lens with powerful sclerocorneal muscles to force the lens to bulge through the pupil (diving birds). Water reduces vision because there is no longer corneal refraction, which accounts for 20 diopters.

The iris is often dark but may be highly colored. Male cockatoos often have a black iris, while females have a brown one.

The pupil is commonly rounded; pupils with an irregular margin may result from avian leukosis. Pupillary size is regulated by striated, rather than smooth muscles — the sphincter and dilator muscles of the pupil. Movement and size are rapidly changed in birds, but because of willful movement, a bird’s pupils often not respond to light in a pupillary exam because of stress. Birds also have the ability to regulate the quantity of light reaching the retina by migration of pigment in special cells embedded into the retina. With light adaptation, the pigment migrates to shield the receptor cells.

Lens

As indicated, the lens of birds is softer than that of mammals to provide rapid accommodation. Part of this softness results from the lens vesicle, which is fluid-filled. It lies between the annular pad (Ringwulst) and the body of the lens. The annular pad encircles the equator of the lens and is most pronounced in diurnal predators. In primates, the lens filters light below 400 nm, making it impossible to detect ultraviolet light. On the other hand, birds are able to visualize wavelengths down to 350 nm, allowing them to visualize many things we cannot. They are able to discern males from females in what we think are sexually monomorphic species. They can detect ripeness of food items because of this quality and hawks can visualize urine trails of mice. Now that is something very different than mammals and provides them with unique abilities to see things that we cannot see.

Retina

The retina of birds is relatively thickened and does not contain blood vessels, as occurs in mammals. This allows the entire space to pack in more “pixels” to see with. The retina consists of a non-nervous pigmented epithelium and a nervous layer composed of rods, cones, bipolar cells and ganglion cells. The ganglion cells collect to form the optic nerve at the optic or blind spot.

The rods and cones are the receptors in the retina. Rods are sensitive to the intensity of light, so nocturnal birds have mostly rods. In order to increase sensitivity to low amounts of light, several rods synapse with a single bipolar cell and several bipolar cells synapse with a single ganglion cell. However, the sensitivity of the owl’s eye may be due to its ability to gather more light (2½ times brighter than humans). Their ability to hunt in near darkness may result from their sense of hearing.

Cones are responsible for visual acuity and color vision. In diurnal passerines and predators, one cone synapses with a single bipolar cell, which synapses with a single ganglion cell. This one-to-one projection to the brain greatly enhances visual acuity, or the sharpness of detail.

Factors that affect visual acuity include:

• Relatively large eye
• Accuracy of focus on the regions of the retina because of the shape of the eye (ie, tubular)
• Magnifying capacity of the fovea
• Absence of blood vessels
• The visual acuity or one-to-one projection of receptor cells to ganglion cells
• Amount of contrast between an object and its background

The central area of the retina is the place of maximal optical resolution and may have a fovea. This fovea is commonly deeper than that of primates, thereby increasing visual acuity. The fovea may have one of three arrangements:

• A single, round central area in each eye, close to the optic axis. A horizontal central area is present in water birds and those that live in open plains, which allows the eye to fix the horizon at its reference point.
• Two foveate areas — one in the central area and a laterally situated temporal one. This is common in fast-moving birds, which require accurate perception of distance at relatively high speeds.
• A single foveate area temporally placed, as in owls.

Color Vision

There are three visual pigments in birds, with a possible fourth, that are sensitive to near ultraviolet wavelengths. In birds, cones additionally have an oil droplet within them with five different absorbency spectra. The droplets’ function is not entirely known, but they may directly produce color vision or may enhance contrast by acting as intraocular filters. For example, yellow droplets could remove much of the blue from the background, increasing contrast between an object and the blue sky.

In my previous column,  I discussed how we unintentionally stimulate our birds sexually and then we are stuck with a number of unwanted behaviors. It is also important for bird owners to understand the female bird’s reproductive system. Here is a detailed account of exactly what goes on during the egg-laying process, which will be especially helpful for those who wish to raise birds.

A hen’s ovary develops to its functional state based on the life span of her particular species. Many of the larger psittacines have development at about 4 to 7 years of age, while in smaller parrot species it is at 1 to 3 years of age. All birds have ovarian development during the breeding season; the ovary then shrinks during the non-breeding season.

As the ovary grows and matures, there is an outer definable cortex where the follicles develop with an inner medulla. The medulla becomes an irregularly shaped vascular zone that is centrally located. The vascular zone has nerves and blood vessels that supply the ovary along with smooth muscle and interstitial supporting cells. The vessels to the ovary come off of the abdominal aorta and the vena cava as short stalks, making it very difficult to remove the ovary from birds without significant bleeding from these large blood vessels. When the ovary is stimulated hormonally in the lay period, it bulges into the coelom or abdomen.

The ovary is located just caudal (or distal) to the lung and slightly caudolateral to the adrenal gland. This landmark is used for endoscopy when sexing a bird to determine its gender, called surgical sexing (a blood test is another way to determine a bird’s gender).

Three Phases of Development

In birds with a seasonal laying period, there are approximately three phases of development of the reproductive system: a prenuptial acceleration phase, a culmination phase, and a refractory period. During the onset of lay, or the prenuptial acceleration phase, the follicles begin to develop into a hierarchical pattern so that they don’t reach maturity simultaneously. As egg laying commences during the culmination phase, the ovary resembles a bunch of grapes because of its follicular development. During the resting phase, or the refractory period, the ovary diminishes in size as each follicle is reduced significantly and the ovary becomes quiescent (inactive).

During the acceleration phase, the oviduct increases in size to reach its full development in the culmination phase. This second phase is when the oviduct is capable of turning the ovum into an egg! In the resting phase, the oviduct shrinks to a very small thread-like structure.  While these structural changes are occurring, there are hormonal and behavioral changes that are happening as well. I discussed some of those in the last column.

Ovulation

Birds have a variety of mechanisms to reduce weight, which serve to compensate for part of the expenditure of energy for flight. One way to reduce weight is to only have the reproductive organs large during the culmination phase so that the bird does not have to carry all of that weight around other times of the year — particularly during migration! Another way for birds to carry around less weight is the laying of an egg that is incubated externally. Interestingly, no avian group is viviparous, that is, bears live young like people and other mammals. However, from their reptilian ancestry birds inherited an elaborate development process to form a heavily yolked, or cleidoic egg.

In order for the hen to have multiple offspring, each fertilized ovum must be fertilized sequentially and travel down the oviduct to form a shelled egg for external incubation. During the laying period or culmination phase, each follicle enlarges in a hierarchical arrangement so that they don’t mature simultaneously.

Each large follicle is suspended by a stalk that contains smooth muscle and a blood and nerve supply. The follicle is composed of a primary oocyte surrounded by six layers of tissue. The nerve supply consists of both adrengeric and cholinergic fibers, suggesting a role in ovulation. These tissues have an endocrine role, providing communication between the ovary and oviduct with the passage of each ovum to form an egg. Many birds have a stigma on all large follicles. This area is the point where the oocyte breaks through the follicular wall during ovulation.

Ovulation occurs under the influence of multiple factors, including a preceding LH spike (acute rise of luteinizing hormone). Ovulation, in the chicken, and presumably in other species, occurs at a relatively fixed time after oviposition or the time when the egg is laid (one-half hour in chickens). Ovulation with subsequent ovum capture by the infundibulum must involve at least some neural control.

Determinate Egg Layers

Some birds, such as crows and budgerigars (aka “parakeets”) are determinate layers. These birds lay a fixed number of eggs in their clutch. Indeterminate layers, such as cockatiels, are those bird species that can replace a lost egg by laying another. Double clutching is a technique to increase the number of eggs normally laid in a clutch by removing an egg, thereby stimulating the hen to replace that “lost” egg. This is why you should not remove the eggs in the nest of your companion bird as she lays them —  it will cause her to continue laying eggs, much like chickens do!

Internal Laying

The infundibulum is the proximal opening of the oviduct. This capture is helped by the anatomic arrangement of the left abdominal air sac. This air sac surrounds the ovary in its ovarian pocket except caudally, thereby acting as a conduit for the ovulated ovum to “fall” into the opening of the infundibulum. However, not all ovulated ovum are captured successfully. When the ovum does not reach the oviduct, but is laid into the coelomic cavity, it is termed internal laying. This process occurs more frequently at the initiation and termination of the culmination phase. Interestingly, this ovum can be absorbed without problems by the hen or can lead to egg peritonitis (the presence of yolk material in the coelomic cavity). 

A different process occurs with egg binding, when the egg  gets “stuck” in the shell gland or uterus. This is common when birds do not get enough calcium or protein in their diets, but there are other reasons as well. Seed-only diets are deficient in calcium and the amino acids that form the building blocks of protein, which is why a nutritionally balanced diet is especially important.

Egg Fertilization

Once the ovum reaches the infundibulum, spermatozoa must penetrate it rather quickly before albumen is laid down. This requires a well-timed wave of sperm to be in the appropriate location at the time of ovulation. In chickens, there is only a 15-minute window of time for penetration of the sperm for fertilization to occur. Fertilization is presumed to occur in this very proximal portion of the infundibulum.

After ovulation, the follicle where the ovum was expelled from undergoes rapid regression and absorption. During this regression, the cells in this follicle are replaced with hypertrophied granulosa cells laden with lipid. This lipid is converted to progesterone and other sex hormones to prime the oviduct for the development of the egg and to make the hen nesty and broody, which compels her to sit on her eggs for the required incubation time.

When the correct number of eggs are laid, the hen becomes involved in the incubation and caring of the chicks. It is during this time that the remaining follicles stop developing, and the ovary begins to regress until the next breeding season. As discussed in my previous column, an unsuspecting owner might unintentionally stimulate their bird to go back into egg-laying production or at least keep them in the stage of reproduction right before the lay period.

How The Egg Is Formed

The left oviduct in most birds is responsible for the formation of the shelled egg. It consists of five basic components that are only distinguishable histologically or by the stage of development of the egg in that portion of the oviduct. These components include, from proximal (in relation to the ovary) to distal: infundibulum, magnum, isthmus, uterus or shell gland and vagina.

The wall of the oviduct is similar in its basic structure to other tubed organs of the body. It consists of an epithelium with an underlying submucosa, external layers of smooth muscle, and covered by peritoneum. The epithelium of the oviduct has two predominant cell types; a ciliated and a glandular cell that is similar to a goblet cell.

The number of each cell type varies in each portion of the oviduct based on its function. For example, the magnum has a much larger proportion of unicellular glands, involved in the production of albumen. The ciliated cells have cilia that beat toward the cloaca in most areas.

There are multicellular tubular glands that are found opening into mucosal folds in each region of the oviduct except for the vagina. The mucosal folds spiral slightly along the oviduct to help rotate the eggs as it moves distally. The outer layers of smooth muscle are thought to propel the developing egg down the oviduct and transport the sperm by reverse peristalsis to the infundibulum.

Infundibulum

The infundibulum consists of a funnel for ovum capture and a tubular region. The funnel opening is toward the ovarian pocket of the abdominal airsac. Sperm have been observed to be stored in the glandular grooves and tubular glands of the infundibulum. The release of the ovum causes the release of the sperm into the area for fertilization. It must be fertilized at this point before traveling to the next region.

The tubular region is also described as the chalaziferous zone. The tubular portion secretes a thin layer of dense albumen, the chaliziferous layer of albumen. These chalazae twist around the yolk, and thereby tighten as they turn when traveling down the oviduct. In chickens, the ovum passes through the infundibulum in approximately 15 minutes.

Magnum

The next region of the oviduct is the magnum, the site where the majority of the albumen is added to the egg. The albumen is secreted primarily by the numerous branched tubular glands. The ovum remains in the magnum approximately three hours in chickens.

Isthmus

The isthmus is the site for the formation of the two shell membranes (inner and outer), the addition of a small amount of protein to the albumen and the initiation of calcification of the shell. Only the glands of the isthmus produce sulfur-containing amino acids, important for shell membrane production. Passage through this short segment of the oviduct takes approximately 75 minutes in chickens.

Uterus or Shell Gland

There are two anatomically distinct portions of the uterus or shell gland in fowl. The cranial part is short. The distal portion is pouchlike and where the egg remains for approximately 20 hours in fowl. In addition to longitudinal folds, there are transverse folds that result in lamellae with a leaf-like appearance. These lamellae flatten against the developing shell. The uterus initially is responsible for plumping of the egg, whereby water and electrolytes are added to the albumen. This occurs first and then calcification proceeds.

The uterus has a remarkable ability to extract large amounts of calcium from the blood stream, thereby requiring the release of calcium stored in the long bones. The shell consists of shell membranes, testa and cuticle. The testa is composed of an organic matrix with calcite — a crystalline form of calcium carbonate. The outermost organic layer is the cuticle. The cuticle is water repellent, reduces evaporative loss, and it forms a barrier for microorganisms.

The vagina is a conduit for the egg to pass from the oviduct through the cloaca and out the vent. The process of expelling the egg is termed oviposition. The time required for oviposition varies among species. The vaginal sphincter separates the uterus and vagina. This sphincter relaxes at the onset of oviposition. It is the smooth muscles of the uterus that propel the egg through the vaginal sphincter and into the vagina. The presence of the egg produces neuronal stimulation. This causes a bearing down reflex of the cloaca, passing the egg out through the vent.

Additionally, there are folds in the region of the sphincter, the spermatic fossulae or the sperm host glands. These glands represent the primary site for sperm storage, an important adaptation to allow for rapid fertilization of the next ovum immediately after its ovulation. Ovulation often occurs shortly after oviposition of the egg that is fully formed.

The Egg Inside & Out

The fully formed egg is composed of a germinal disc, yolk, yolk membranes, albumen, and a shell. The germinal disc is normally a circular, opaque, whitish spot. In chickens, it is approximately 3 to 4 millimeters in diameter. It represents the remnant of the nucleus. If the ovum is fertilized, it is termed the blastoderm and represents the cells of the developing chick embryo.

The yolk consists of mostly lipo- and phosphoproteins. The yolk serves as the main source of nutrition for the developing embryo. There are four yolk membranes; two from the follicle and two provided by the infundibulum. These membranes from a barrier between the yolk and the albumen, but allow for the movement of electrolytes and water.

The albumen is less viscous than the yolk. Its solid component consists of ovomucin, which is mostly protein. The albumen in an egg is divided into dense and thin albumen. The dense albumen has a higher content of ovomucin than the thin albumen. The chalaziferous layer surrounding the yolk membranes is dense albumen. The chalaza represent twisted strands of ovomucin fibers. The egg has an inner and outer layer of thin albumen with a middle layer of dense albumen. The albumen suspends the embryo in a relatively aqueous environment. Its protein component acts as a source of nutrition for the embryo. Albumen also is considered to have antibacterial properties.

The shell, as indicated above, consists of the two shell membranes, the testa, and the cuticle. Generally, the size of the egg is related to the size of the parent. However, its relative weight diminishes as the body weight increases. The eggs of precocial species (offspring are born covered with down, having open eyes and capable of leaving the nest within a few days of hatching) are often larger than those layed by altricial species (helpless, naked and blind when hatched. The shape is also species-dependent and appears to be related to the shape of the hen’s bony pelvis. Spherical eggs are associated with a deep pelvis oriented in the dorsoventral direction, while elongated eggs occur with dorsoventral narrowing.

The owner should examine the eggs and note the sequence of eggs when they are laid. When the shell thins or the eggs become misshapen, there is great cause for concern. This represents a hen that is not able to provide enough calcium and/or protein in the diet. There may be additional concerns, so it is important to get the bird to an avian veterinarian as soon as possible.

Coloration of the egg shell is very diverse. These ranges of color result largely from variations of two pigments; the red-brown porphyrins and the blue-green biliverdins. These pigments are deposited as crystals throughout the calcified testa. The surface texture of the shell also varies. Most commonly, the shell is smooth with a slight sheen. It can also range from the highly glossy egg of woodpeckers to the pitted eggs of the ostrich.

Birds are remarkable, and egg laying is a unique biologic event. It was designed to reduce weight with flight. We often use the egg as a symbol of birth and hope. With spring come eggs and hope for a new year!

Why put off to tomorrow what you should do today … especially if it can be a lifesaver for your pet bird? Here are five choices to make now to better your bird’s health.

1. Choose an avian veterinarian and schedule an appointment.

Establish contact with an avian veterinarian before you find yourself in dire need of one. Unfortunately, some pet owners are forced to frantically search the Internet or phone book for a veterinarian because their pet became sick or injured. The danger with this is multi-fold; in the case of an emergency, precious time can be wasted not only in trying to find the phone number for a veterinarian, but searching for the physical address of a clinic you’ve never been to. In this harried approach, you also run the risk of bringing your bird to a veterinarian who little to no experience treating birds.

Many of us take the time to find a doctor we trust and like; it should be no different when it comes to our pet birds. Save yourself a headache — and possible heartbreak — by researching a few veterinarians in your area and choosing the one who best meets your needs (ex. location, hours, avian experience, bedside manners, etc.). The next step is to take your bird to your chosen vet for a well-bird exam.

2. Choose to keep your bird up-to-date on diagnostic tests.

If you are traveling at all in the year ahead and plan on having your bird stay at a pet boarding facility while you are away, you don’t want your reservation to be denied because you neglected to keep your bird up-to-date on its annual diagnostic tests. Most pet boarding facilities that accept avian clients require a health certificate from a veterinarian stating that your bird has been tested for, and is free of avian diseases such as Proventricular Dilitation Disease (PDD), psittacosis and Beak and Feather Disease (PBFD).

If you plan on taking your bird along with you on your travels, especially if you are going out of state or taking your bird on a airplane flying, you might need a certificate of health from your veterinarian within 30 days or less prior to your departure date.

3. Choose to pay attention to your bird’s poop — no joke!

Kudos to you if you make an effort to change your bird’s cage liner every day, but before you “roll and throw,” pause to see what your bird’s gastrointestinal tract has been up to. You don’t need a crime-scene-style investigative kit to do this, you just need a general idea of what constitutes a normal dropping for your bird.

The next time you bring your bird to the vet, take along the dirty liner from your bird’s cage. (Don’t tightly roll up the liner, which can smear the droppings. Instead, slide it loosely into a grocery store bag for easy transport) and ask your vet to show you what makes up the droppings. Interestingly, some foods can influence the dropping’s color. Birds that eat mostly seeds usually have green-colored droppings, while those on a mostly pelleted diet have brown-colored droppings. Certain fruits can also influence the color of your bird’s feces … a few fresh blueberries, for example, can color the feces purple!

4. Choose to stash some cash for your bird.

Create a healthcare savings account for your pet bird, and  contribute to it each month for your bird’s healthcare needs. Adding $10 or $20 a month can help you budget for your bird’s routine exams and potentially ease the sting of an unexpected medical treatment. Also, consider buying pet health insurance for your pet bird. Shop around for the coverage that best meets your needs; ask what procedures are covered on the plan and what the deductible will be for your pet bird, as the price can vary depending on your bird’s age and species.

5. Choose to weigh your bird regularly.

Instead of guessing your bird’s weight by how much heavier or lighter your bird seems when perched in your hand, it’s time to taking guessing out of the equation. Purchase a scale that weighs in grams (not ounces) and do a weekly weigh-in of your pet bird. Weight ranges vary depending on the bird species, so ask your avian veterinarian for your bird’s ideal weight range. Report any fluctuations outside of your bird’s established weight range to your avian veterinarian, as this can be a sign of ill health.

A gram scale can also be helpful if you are converting your bird to a healthier diet. Some birds can take a while before trying a new food, which is why many aviculturists recommend continuing to offer the old diet along with the new offering until the bird consistently eats the new diet; essentially you are weaning your bird off its former diet and onto a new one. Regular weigh-ins can help you monitor your bird’s weight to make sure it is not losing weight during this transition. 

In my last column, I discussed some of the unique features that help birds to fly — from hearts that pump five times their volume per second, to their respiratory system, which allows them to aerate more quickly with a larger blood volume compared to that of mammals. Their shape reduces turbulence, and their organs are lighter in weight. Yet one organ that is relatively heavy is their intestines — their guts. So let’s look at the structure and function of this complex organ, and at adaptations birds use to make the most of it.

From Beak to Crop & Into The Esophagus

The gut, or GI tract, starts anatomically at the bird’s beak and ends at its vent. The proximal portion of the GI tract consists of the beak, oropharynx, cervical esophagus, crop and thoracic esophagus. The esophagus of birds is distensible and sits on the right side of the neck — just the opposite of what occurs in mammals. Some birds, but not all, have a distensible portion of the esophagus, which is the crop or ingluvies.

The crop is an esophageal diverticulum and varies in size depending on the species and the age. Young chicks have a much larger crop than adult birds of the same species. In addition, the size of the crop decreases in size with sick birds that are not eating normal volumes of food. This is important when determining the volume to be fed when gavage-feeding critically ill birds. In psittacine birds (parrots), the crop normally extends from the right side of the neck at the base of the thoracic inlet to the left before narrowing into the thoracic esophagus. From the thoracic esophagus, which lies on the left, the GI tract continues into the stomach. 

The Stomach — A Rotatory Grinder

The stomach of birds consists of the proventriculus, isthmus and ventriculus or the gizzard. It lies on the cranial left side of the abdominal cavity. Since birds do not have a diaphragm, it is termed the abdominal coelom. The proventriculus is the proximal portion and is the glandular portion that secretes the digestive enzymes. There is a narrow isthmus or intermediate portion followed by a saclike to highly muscular gizzard.

The gizzard acts to grind food, making it more readily available for the enzymes to digest. In psittacine species, there is a rotatory motion of the food in the stomach with food moving from the proventriculus to the ventriculus for grinding and then back into the proventriculus. This normal pattern can be disturbed by a variety of disease processes, with the most noted being proventricular dilitation disease or PDD.

The Small Intestine — Reduced Length & A Clever Strategy

The anatomy of the small intestine includes the duodenum, jejunum and ilium. The majority of the pancreas lies between the 2 limbs of the duodenum. The splenic portion is located near the back of the bird and is not as closely associated with the GI tract. This portion contains more of the endocrine portion of the pancreas. The bile duct and pancreatic ducts open near the terminal portion of the ascending duodenum.

In most species of birds, the jejunum and ileum are composed of a number of U-shaped loops along the edge of the dorsal mesentery, which is the membrane that takes origin from the back and hangs into and supports the intestines. From our perspective for understanding flight, birds have a limited length to their small intestines and have a unique peristalsis-retroperistalsis pattern to digest and absorb nutrients.

What that means is that they reduce the length and hence the weight of the intestines — a brilliant move! Then they run the digesting foods back (retroperistalsis) and forth (peristalsis) in this shorter segment until digestion and absorption has been completed—so they don’t need to move a bunch of guts around like mammals!

From Ceca to Vent

The next physiologic principle involves the last part of the GI tract. This final segment of the GI tract includes the large intestine or rectum and the cloaca. The large intestine begins at the level of the paired ceca. The large intestine consists of a short and straight segment and is probably homologous to the mammalian rectum.

The ceca — if the species of bird has them — arise at the junction of the end of the small intestine and the beginning of the large intestine or rectum. Parrots, however, don’t have ceca or they are rudimentary, and the ceca of passerines are small. Poultry and ostrich have very large ceca. The large intestine of birds functions similarly to mammals; its major task is to reabsorb water. That is an important factor for flight as well. To see how that works we need to explore the cloaca.

The cloaca of birds is divided into the coprodeum, urodeum and proctodeum. Its basic organization is similar between species, except for the possible phallus in waterfowl and ratites. The coprodeum is the cranial-most compartment and it stores fecal material. The coprodeum is continuous with the large intestine, but is separated from the urodeum by the coprourodeal fold.

The urine and urates are stored in the urodeum prior to evacuating the cloaca, as the ureters open into this receptacle. The oviducts or ductus deferens open into the urodeum. The more caudal fold is the uroproctodeal fold that separates the urodeum from the central and distal-most cavity; the proctodeum. The cloacal bursa opens into the proctodeum. The vent is the opening of the proctodeum to the outside.

The Role Of Retroperistalsis In Water Reabsorption

Interestingly, urine from the ureters flow into the urodeum. The urine of birds is not as concentrated as that of mammals so that means that there may be a larger volume and hence weight of the water. But birds have solved that problem because they can move that urine by retroperistalsis from the urodeum into the coprodeum to the large intestine.

Remember, the large intestine is designed for the reabsorption of water. So birds have again solved a problem using a physiologic function — retroperistalsis! This ability to run fluids and gut contents backwards help solve the weight problem for flight. The shorter GI tract works more efficiently while reducing weight. Retroperistalsis of the urine from the kidneys help to reduce carrying water around and allows water to be used more efficiently. Birds are their own miracle! How brilliant from a physiologic perspective!

Birds And The Miracle Of Flight

Humans have often marveled at the ability of birds to fly and sometimes suffered terribly in our initial efforts to join them through technology. Birds have solved the problem through aerodynamic design of their own, streamlined bodies, reduced weight, a powerful heart and an efficient respiratory system, among other adaptations.

Also important are their adaptations to their guts that allow rotatory digestion in the stomach, peristalsis and retroperistalsis in shortened small intestines, and water absorption that allows effective water management. While their GI tract is relatively short, it works efficiently to fuel the flight that thrills us. From starling to eagle, they’re all pretty impressive from a flight perspective.

We talk about miracles, particularly during the holiday season, but one that we as bird owners often marvel at is the miracle of flight! The adaptations necessary for birds to fly are considered among the most intriguing puzzles of all in biology. Bats are the only mammal that have the ability to fly on a dynamic wing, while insects have true flight but usually on a fixed wing.

Humans have yearned to fly, but it took the last 100 years for us to perfect machines for flight — we just can’t do it even with wings strapped to our arms! And, in these 100 years, we have finally gotten closer to the abilities that birds have been endowed with — from “flying” deep into the water to flying far into the outer reaches of the atmosphere. How is it that birds can do this so effortlessly? What allows them to make this miracle happen?

A Warm-Blooded Heart

A number of factors are involved for this to happen. Having evolved from reptiles that had developed a four-chambered heart, birds also developed the ability to maintain their own internal body temperature, to be “homeothermic.” They can be, on average, 9 degrees warmer than mammals, which can account for a number of other issues that we can explore in the future. Warm bloodedness allows animals to maintain a constant body temperate, and that is important for providing the energy necessary for flight.

Flight takes an incredible amount of energy; up to 20 times the energy of just sitting. The ability to maintain an internal body temperature is also accompanied by the development of a four-chambered heart. That type of heart is important for providing the energy for pushing the blood to oxygenate the tissues, and it is critical in generating the power for flight.

A Balancing Act

So for flight, form does indeed follow function to be successful. There are strict limitations for the shape and size of a flying animal or machine. Aerodynamic efficiency needs to have sufficient power combined with structural and muscular strength. Part of this equation also requires that weight be kept to a minimum or even more power and strength is needed—stretch the limits too far and flight just does not occur! This has resulted in a certain uniformity of design in all birds that fly. So a parrot has a similar overall structure to that of an eagle, heron or a duck. The anatomical parts of birds are perfectly suited for life in the air. All are designed to allow for the Miracle — from weight reduction to power and all of the aspects needed to fly!

Efficient Respiratory System

In a previous column, I talked about the unique respiratory systems of birds.  That uniqueness allows for one-way movement of air with the blood in a cross current multiplier to efficiently allow for the removal of carbon dioxide with the capture of oxygen in their very effective air capillaries. These air capillaries are smaller but have a thinner lining to whisk the oxygen into the red blood cells on the other side of the mucus membrane while dumping carbon dioxide to be exhaled.

Additionally, the ability to use the air sacs to move blood with a fixed lung allows for packing a huge number of these oxygen-capturing air capillaries into the lung surface, compared with the alveoli of mammalian lungs. So in effect, us humans just couldn’t get enough oxygen for lift off or to maintain flight even if we could get airborne! On the other hand, birds’ respiratory systems allow them to take in oxygen efficiently and to get rid of carbon dioxide, and to maintain flight.

Streamlined Shape For Reduced Drag

The body shape of birds allows for smooth, sweeping lines that reduce turbulence and minimize air resistance. Their shape is perfectly suited for flight – they have pointed front ends with smooth contours and a compact, but light, body. Even their legs disappear during flight to reduce drag – their landing gear comes up with takeoff and flight but, when landing, birds bring their legs down and spread their toes to increase drag needed for slowing the body for landing. You have probably seen ducks with their tails and legs down with toes out during landing!

So birds are like airplanes that put down their wheels for landing. Insects, by contrast, have a very different outward appearance. They are often blunted with a number of appendages that do not retract well so that they are sticking out in all directions. Insects are slow fliers at 10 to 15 miles per hour, while there are some birds that can fly upward of 100 to 200 miles per hour. Interestingly, there is 100 times greater drag when flying at 100 miles per hour than there is when flying at 10 miles per hour. A bird’s significantly greater efficiency of streamlining allows for those high airspeeds with a minimum of energy.

Weight Reduction with Strength and Flexible Rudder

Weight reduction is one of the important adaptations necessary for flight. The skeleton of birds has been fused in many locations to reduce weight while adding strength. In the skull, a large number of the bones have been fused and the infraorbital sinus has been added to fill it full of air to reduce weight and add buoyancy. In contrast, I am sure you have noticed how heavy your head becomes when you try to do a number of sit-ups properly!

Weight reduction with centralized strength is important for flight. The vertebral column in birds has been fused from the distal thoracic region with the lumbar vertebrae to just before the vertebrae of the tail, the coccygeal vertebrae.  These fused vertebrae then make a rigid platform to work off of to thrust the body forward during the downstroke of the wing beat.  This strong rod is fixed to the bird’s “landing gear” through a structure called the synsacrum.

The synsacrum represents the fused ilium, ischium and pubis, and it is then fused to the vertebrae, including the caudal thoracic, lumbar, sacral and proximal coccygeal vertebrae.  The distal coccygeal vertebrae are moveable except at the tip, where the tail feathers insert.  That allows the tail to move so that it can act as a rudder! Those of you whose birds have had broken tail feathers know that this can seriously compromise their ability to maneuver.

A Good Skeletal Architecture

As you would expect, the landing gear of the legs need to act to absorb shock. This is accomplished through the flexion of the joints, while the head of the femur is firmly placed in a confined hip joint that continues to transmit the landing forces through the fused synsacrum to the fused vertebral column. Pretty ingenious I would say!

The thoracic girdle also has fused elements where the wing is connected to the body. The thoracic girdle is composed of the clavicles that fuse to make the wishbone, the coracoids and the scapulae. The coracoid is the stout bone that connects the humerus of the wing to the sternum. The sternum, with its long keel, provides the broad surface for the insertion of the pectoral muscles for the down and upstroke of the wing. The ribs insert onto the sternum, and these ribs, in turn, attach to the vertebral column.

With each downstroke, the force moves from the wing across the sternum through the ribs to the vertebral column, which then, because of its fused parts, acts to propel the bird’s body forward. The clavicles act as a transverse spacer bar to not allow the chest cavity to collapse with the force exerted during the downstroke. The birds’ fused skeletal architecture is an elegant adaptation for flight, providing structure while managing the forces of flight effectively.

And More To Come …

In the next discussion, I will discuss how a bird reduces weight of some of the organs for flight, as well as some ingenious ways that anatomy has been modified to meet the physiologic demands during flight.