Move It! ...continued...

Even though we intellectually grasp the idea that caged birds rarely show signs of progressive disease before they completely decompensate, we have been hopelessly slow to acknowledge the risks of premature death due to the common practices of keeping parrots. Most experienced clinical veterinarians recognize that a parrot presenting with signs of cardiovascular decompensation likely has a condition so advanced that response to treatment and favorable long-term prognosis is unlikely.6 In fact, experience dictates that even routine handling of older, sedentary birds (especially if they are overweight, underweight, or showing signs referable to liver disease or respiratory disease) can result in a physiologic crisis that will be hard for the owner to understand.

Missing from our literature, in a very glaring way, is what can be done to prevent or decrease the risk of advanced degenerative disease of the cardiovascular system in our aging pet birds. In this regard we are not alone. Until fairly recently, human cardiology functioned in the same manner; heart disease was typically addressed when the patient began to show serious decompensation or had a heart attack. For the last 15 years, heart disease was addressed as a “plumbing issue” and advanced techniques were developed to clear or bypass clogged arteries. These days, diet, exercise, and specific medications are the preferred treatment, and surgical intervention is used more to treat emergency presentations or intractable conditions.9

Sophisticated human medicine now focuses more on acquiring health and fitness than on responding to heart disease. The predisposing risks of improper diet and sedentary lifestyles have been clearly defined. It is now recognized that indulging in “feel good” food and tasty trans fats, consuming more calories than are burned, and living a sedentary life style can kill you!

So the challenge before us is to translate what we know into action. In the face of the numerous challenges of providing health care for birds, we can take some lessons from the best human physicians and provide guidance about acquiring health instead of just treating disease.

Process of Atherosclerosis

Although the clinical diagnosis of atherosclerosis is rarely effectively applied in birds, we do have a basic understanding of the pathology of the process and how function is affected. Thanks to our experienced avian pathologists, we know that grossly visible lesions in the large vessels reflect an end-stage process that began long before with damage to the endothelium of affected vessels.1,3,10 Current human literature pins the initial insult on deposits of low density lipoproteins. These deposits incite a serious and very chronic inflammatory process that, over time, thickens the walls of vital arteries. Myocardial infarctions in humans are thought to be triggered by a rupture of the inflammatory lesion. The rupture results in bleeding and clot formation. It appears that it is really the clot that can suddenly restrict or occlude blood flow with devastating results. Humans are at increased risk for heart attacks because the blood supply to the heart is limited to the coronary arteries. True heart attacks in birds are not considered common because of the very redundant blood supply to their heart muscle.8 This is one of the many unique adaptations for the physiologic demands of flight. Instead our sedentary psittacine population is more at risk for signs attributable to decreased blood flow in the aorta, brachiocephalic trunk, or carotid arteries.1–8,10 Signs may not occur until the animal experiences complete occlusion or unaccustomed exertion.1,4,6,7 Even when signs are present, they may not be easily interpreted as cardiovascular in origin.1,8

Exercise Physiology

The study of the physiologic processes of exercise has recently been fueled by both athletes and forward-thinking physicians and health care providers. Athletes are seeking to maximize performance in their quest to achieve goals previously considered unrealistic. The medical community is looking to support and maximize rehabilitation and disease prevention as an integral part of cardiac care.

In general terms, exercise physiology studies the ways chemical energy is transformed into mechanical energy by the machinery of the body. Maximum athletic performance reflects the finely tuned working of energy metabolism at a cellular level to fuel the muscles performing the work. Ramping up performance demands that all systems perform to capacity while maintaining homeostasis.11 At a cellular level, ATP is generated from several energy sources. Nonoxidative (anaerobic) pathways are involved in ATP production, but maximum power and capacity is achieved mainly by oxidation of glycogen. Muscles are supported by the actions of the respiratory and cardiovascular system. Maintaining homeostasis in the face of energy transduction is the primary job of the sympathetic nervous system and endocrine hormonal system. Training, or using the system, is the only way to prevent decline or improve function.11 Without some degree of earned “fitness,” exertion elicits endocrine responses that make sustained exertion impossible. When unused, the body demonstrates a limited capacity to respond to stressors whether that is demonstrated as muscle performance or maintenance of normal physiologic function.

Described as the “cascading risks” of the sedentary life, the risk of not using the machinery of the body is losing it. Less activity leads to weaker muscles, which leads to less activity. From the Mayo Clinic comes advice universally applicable to all animals: Use it or lose it! Research has shown that there can be huge benefits from small changes. Even small amounts of exercise will strengthen the cardiovascular system, strengthen bone and muscle, manage weight, prevent metabolic diseases, enhance the release of neurotransmitters that decrease depression, enhance gastrointestinal transit time, and restore diurnal rhythms. Even the Centers for Disease Control state that physical activity is essential for health and well-being and is an important adjunct in management of most chronic diseases including cancer, arthritis, osteoporosis, depression, and anxiety. It should not require a huge leap of imagination to apply these same recommendations for better health and fitness to our companion birds. Focusing on the research that demonstrates huge benefits from small changes, it should be relatively easy to introduce fun and simple ways to promote exercise.

Review of Adaptations for Flight: Birds are Meant to Fly

To review the adaptations for flight is to review everything that makes a bird a bird. From the function of the nervous system to the structure of the flight feather, psittacine birds are a fantastic example of an animal evolving to thrive in nature. Exploiting a wide variety of environments, parrots in the wild display a unique adaptability to live in most types of tropical and subtropical (environments). Some are able to withstand severe seasonal droughts and others migrate seasonally from swamplands to high elevations. At the root of the bird’s vigorous and adaptable physiology is an amazing cardiovascular system fueled by extraordinarily efficient respiratory function.

Cardiovascular and respiratory systems

Generally speaking, the heart of birds is larger and stronger than that of mammals of similar size. As the oxygen and nutrient demands of flight are high as compared to a mammal, the cardiovascular system of birds demonstrates certain unique attributes. The left ventricular muscle is 2–3 times thicker than the right, thereby creating greater systolic pressures. Myocardial tissue extends into the outflow valves and is connected to the Purkenje system, allowing for control over flow dynamics. The heart muscle itself is generously supplied via the right and left aortic sinuses. Avian hearts generate greater stroke volumes despite relatively lower heart rates and sustain higher mean arterial pressure. Vascular resistance is controlled by vessel size. Vessel size is under complex control of the nervous system. The innervation of arteries, arterioles, veins, and veinules is achieved by numerous nerve endings in the vessel walls. Circulating metabolites and circulating vasoactive factors result in a system fantastically responsive to the rigorous and changing demands of flight.12,13

As remarkable as the cardiovascular adaptations are, they pale in comparison to those for respiratory function. Powering flight is all about available oxygen. Avian lungs are exquisitely engineered to facilitate oxygen and CO2 diffusion. A complex system of air sacs serves to further decrease weight and provides a reservoir of air. Complete tracheal rings reduce resistance to airflow. The lungs are rigid and alternately filled and emptied of air by the action of intercostal muscles. Further enhancing respiratory function is the avian system of cross-current exchange between air capillaries and blood capillaries that allows constant uptake of O2 and removal of CO2.14

Just as the finely tuned mechanisms of the cardiovascular and respiratory tracts provide the power plant for flight, other adaptations equally miraculous in their economy complete the avian animal.

Adaptations of the musculoskeletal system and integument

All avian species have light pneumatized bones and skulls with less bone mass than a typical mammal. The lightweight beak replaces heavy jaws and teeth. Like most birds, psittacine species have a rigid backbone that provides structural support without heavy dorsal muscles. Lacking dorsal muscles to lift the wing, the antagonistic flight muscles that lift and lower the wing are located over the sternum and provide for lift via the supracoracoideus tendon attached to the top of the humerus. Most parrots have strong legs and sharp claws for landing and perching. The streamlined skin of most flighted birds, including psittacine birds, is remarkably thin. The skin supports and is covered by the feathers which offer protection, insulate, and create the air foil that permits flight.

Adaptations of the brain and behavior

The psittacine brain is characterized by a cerebrum proportionally larger than most birds’. Psittacine birds are recognized as intelligent animals and their larger brains probably allow for food recognition, memory, and the processing of information from the special senses. The thalamus contains nuclei that relay auditory, olfactory, gustatory, visual, and vestibular information to the cortex. The hypothalamus, located just ventral to the thalamus, integrates autonomic nervous system info and endocrine function. The well-developed cerebellum is responsible for equilibrium and coordinated muscle function.15 These adaptations are among those that allow psittacine birds to successfully exploit such a wide array of habitats around the globe. Flight above the forest canopy, facilitated by extraordinary cardiovascular function, allows access to vast areas for safe foraging and breeding.

Birds demonstrate a high degree of hardwired behaviors that typically are triggered by environmental stimuli. Genetically driven hardwired behaviors are another example of the economy of the animal that allows flight. These hardwired behaviors are fascinating and unique and include sexual behaviors. As weight-saving measures, gonads are typically involuted and inactive until triggered by environmental stimuli. Female birds have only one ovary. Species-specific environmental signals stimulate the hormonal cascades that stimulate the transition from tiny inactive gonads to functioning organs that can become hundreds of times larger.16

Risk Factors

Forward-thinking human cardiologists are pursuing lifestyle changes centered around diet and exercise to prevent the development of debilitating heart disease. The role of different types of dietary fats in the development or prevention of disease has advanced nutrition to the forefront in today’s battle for heart health. Trans fats, once thought to be a healthier alternative to saturated fats like palm and coconut oils, turned out to be extraordinarily damaging. Over time, it became apparent that elevated triglycerides, a known marker of heart disease risk, was largely the result of excess sugar and simple carbohydrates in the diet. Diabetes and obesity, often the result of overindulgence in these same items, have been linked to serious heart disease in humans. Dubbed the “metabolic syndrome,” specific disease risks relating to obesity, prediabetes, diabetes, and hypertension have been solidly tied to diet and a lack of exercise.

Although we have little hard data on optimal nutrition for psittacine species, we have overwhelming circumstantial evidence that diets based in polyunsaturated fats (seed-based diets) will predispose to obesity, atherosclerosis, cardiac disease, hepatic lipidosis, and multiple vitamin and mineral deficiencies. We have minimal data on measurable physiologic parameters like triglycerides, low density lipoprotein, high density lipoprotein, or cholesterol levels in our companion bird population, particularly as they relate to cardiac disease.17 Avian nutritionists, veterinarians, and pathologists have long considered the role of omega-3 fatty acid deficiencies as a factor in long-term problems associated with omega-6 rich polyunsaturated seed diets.1,6 The addition of omega-3 fatty acids, a fragile but essential nutrient (often obtained from flax seed, evening primrose, or borage oils) has been considered to have therapeutic and preventative value in cardiac and vascular disease in humans and companion birds.1,6,9

Sadly, it has been avian pathologists who have presented us with the circumstantial evidence that indicts the typical “lifestyle” of companion psittacine birds. The real evidence that speaks to a vital need for exercise and optimal nutrition is seen in healthy free-living psittacine populations. From a physiologic perspective, caged parrots are functioning at a small fraction of their capacity. Physiologists investigating the metabolic rate of birds in flight demonstrated an astounding 6–8% increase over the same birds at rest.17 This is in contrast to humans that increase their metabolic rate a mere 2–3% under the most strenuous athletic conditions. Given their capacity for vigorous flight, a lack of exercise and its consequent effects on physical condition may well correlate with some of the incidence of cardiovascular disease in captive birds. Avian pathologists have noted an alarming incidence of silent but serious cardiovascular disease, including atherosclerosis,1,6 and human risk factors for atherosclerosis7 appear to have transferability to psittacine species.1 The sedentary life of most companion and aviary birds seems likely to predispose to numerous degenerative conditions, including artherosclerosis and heart disease.

Continuing to intervene only once disease is recognized will not serve our profession or our avian patients. We need to follow the lead of the best human physicians and provide bird owners with guidance about acquiring health for their birds, instead of just treating disease. Just as humans have benefited from walking or other exercise regimens designed for their bodies, psittacine birds may benefit from another aerobic activity, flight or flapping flight, that is designed for their bodies. Veterinarians and biologists participating in efforts to recondition sedentary psittacine birds for eventual release or large enclosure flight witness in effect a rebirth. Depending on the age, health, and starting fitness level of the birds, these birds’ return to flight requires a gradually earned vigor and skill. Avian veterinarians and falconers involved with conditioning or rehabilitation of birds of prey approach the process with careful attention to diet and training. Introducing sedentary birds to exercise through natural flapping or even flight has the potential to improve their cardiac health and overall condition.

Move It!

Incorporating exercise into the life of companion birds will first require a philosophical shift. It will require avian veterinarians to expand their approach beyond the diagnosis of clinical disease and demand that we become well versed in the biology of the species we treat. As our patients may be cockatiels or macaws, our approach will have to be knowledgeable and applicable for the client and their pet bird. To be effective, we must be able to identify the species kept as companion birds, understand their biology, and weed through the complexities of the owner’s perceptions.

Although statistics reflect that most birds never have the benefit of veterinary care, many motivated owners visit avian veterinarians for yearly check ups. Others seek veterinary care for routine wing and nail trims. These visits offer a unique opportunity to educate owners proactively about the common health risks of companion psittacine species. New clients can be made aware of the philosophy of the practice to include a wealth of information about the natural history of the species, behavior traits, and ways to promote health. As traditions of acquiring health have become popular and widely publicized, owners are more likely to relate and welcome information.

History and management information as vital part of exam

The exam will have to include a fairly comprehensive history of management that will reveal to the inquiring mind the role of the bird in the individual’s home. Carefully designed questions of the client can declare loudly whether they ascribe to “commercial management” including the traditional seed diet and indoor caged existence or whether they have incorporated newer dietary recommendations, recognizing the need for enrichment and a healthy lifestyle. Most clients will obviously fall between these 2 extremes. Our job is also to ferret out the real situation when our clients have learned the correct answer to our questions but either intentionally or unintentionally cater to the whims of their companion bird. Many owners will declare that their bird eats only a formulated diet and vegetables but regularly share their own 3 generous meals. To really help our patients and clients, we have to build into our schedule adequate time to accurately assess the situation. It is not uncommon for avian veterinarians to accept the owner’s assessment of a healthy, balanced diet and an ideal lifestyle even when all evidence is to the contrary. An individual bird’s eating habits will often reflect the owner’s diet. It is very common that the bird’s fondness for foods like ham or ice cream will only be revealed with ongoing conversation.

Using routine care to educate and promote health

The most common diagnostic test offered to avian patients is the physical exam. The physical exam opens the door to other routine and non-routine diagnostic tests. As most companion psittacine species will mask any signs of early illness, it becomes the avian veterinarian’s job to note and interpret details. Focusing an owner’s attention to weight gain, muscle loss, feather abnormalities, old injuries, plantar erosions, or intolerance of handling can open the mind of the owner to the predictable but often invisible signs of loss of condition. These often subtle signs inevitably precede debilitating disease. Cardiovascular disease, in most cases, will develop very slowly over time. Most pet birds are sheltered from physical exertion. This lack of exertion can allow for development of disease and may prevent symptoms from being recognized.

A health exam by a qualified avian veterinarian is a critical first step in any wellness plan, especially if unaccustomed exercise or exertion is planned. The nature of the avian physical exam, because it usually involves some kind of restraint, is in effect a kind of “stress test.” Careful attention to physiologic responses can be invaluable in evaluating the bird’s fitness. Respiratory rate, heart rate, and the presence of arrhythmias or murmurs must be carefully noted. Abdominal or subcutaneous fat is abnormal in healthy psittacine birds and reflect metabolic changes that predispose to heart disease. Attention to feet and wings may reveal clues about how often and how well the bird moves around. A thorough physical exam can reveal old injuries that might limit stability or flight. The eyes should be evaluated for cataracts or other conditions that may impair sight. Feet and nails must be healthy to facilitate stable perching or landing. Severe wing clips and tails damaged by repetitive falls may need time to grow out.

Just as the physical exam supports a thorough anamnesis, lab work will support the findings of the physical exam. Even budget-minded owners usually accept simple diagnostics like a PCV/TP that can speak volumes about the general health of the bird. Anemia, hypoproteinemia, or gross lipemia can reflect serious underlying conditions. When accepted by the owner, chemistry panels and serologic tests can help refine the clinical picture and, in many cases, support the need for lifestyle changes.

Among avian veterinarians, there is general agreement about the alarming incidence of cardiovascular degenerative conditions. There is consensus that early clinical signs are uncommon if not rare. There is also agreement that the incidence of this disease is seriously underreported. Diagnostics are usually regarded as stressful. There is consensus that several species are especially at risk, including Amazons (Amazona species), African grey parrots (Psittacus erithacus), and cockatoos (Cacatua species).

Diagnostic evaluation of cardiovascular disease

In the case of any abnormal findings on the physical exam, a workup to rule out other debilitating conditions is necessary. If clinical presentation suggests a cardiac problem, radiographs are indicated. This information can be augmented by an electrocardiogram and blood pressure measurement. Ultrasonography is considered the ultimate diagnostic tool in the evaluation of cardiac function and has the advantage of requiring minimal restraint; however, relatively few clinicians have the proper equipment or expertise to obtain and interpret images.18

Respect for the fragile condition of the patient often necessitates clinical response without the benefit of diagnostics. Some clinicians have started looking at blood levels of triglycerides, low density lipoproteins, and high density lipoproteins hoping to find predictable indicators of early disease.19

Medical intervention in heart disease

Medical treatment of cardiac disease in companion birds is usually instituted late in the process and usually fails. As many patients are presented as critical emergencies, stabilization of the patient, stress reduction, supportive care, and evaluation and treatment for other medical problems is the first priority.

There are only a few drugs commonly used in companion birds to treat primary heart disease. The goal of therapy is to restore circulatory function, reduce congestion, and improve cardiac output. Cardiac glycosides (digoxin), diuretics (furosemide), and angiotensin converting enzymes (ACE-) inhibitors (enalapril) are commonly used.6

Acquiring fitness with lifestyle modifications

Many birds and owners will warm to the idea of major lifestyle change when it is framed within the context of their bird’s biology. We are still battling huge ignorance about where different species come from. Too many consider Hawaii an original habitat for parrots. Rarely are our clients aware of whether their bird is from the neotropics, Africa, Indonesia, or Australia. The concept of different species being evolved for radically different habitats is very foreign. We are also a geographically impaired society. Many are surprised to learn that huge glaciers grace the Andes just above the cloud forests where the many rivers that feed into the Amazon pass through the native habitat of many psittacine species. Although psittacine birds are generally limited to tropical and subtropical latitudes, temperatures can drop far below those considered tropical, especially at higher elevations. Whereas biologists regard psittacine species as inordinately adaptable, most companion parrot owners are compulsive about protecting their birds from elements outside the living room. We have sorely neglected the need to approach serious health care for birds within the context of their ecology and biology. Realistic and effective approaches to sustainable health in our psittacine birds will need to incorporate an understanding of their unique biologic needs and native behavior.

To Fly or Not to Fly

Assuming that full flight is the only way to effectively exercise companion birds is naïve and impractical. There are many ways to exercise companion birds other than flying. Flying is a huge leap of conditioning for the average caged bird. It may also be a huge leap for many owners. Given the very sedentary style of life for most pet birds, a gradual approach to restoring fitness is very reasonable. We are still at a point where fresh air and sunshine are more commonly regarded as risky. A breeze is considered as dangerous as a “draft”. The concept of parrots living in the wild, exposed to the elements of nature and living as they have for eons, is far removed from the typical notion of pet parrot ownership. Successfully incorporating life style changes into the lives our captive parrot population will happen only if it is presented in a reasonable and easy-to-implement fashion.

Overcoming owner and bird resistance

Many companion bird owners are quite comfortable with the status quo. Most parrots were sold as juveniles that looked like adults. Cute, compliant, and usually quiet, these babies soon grew up but their appearance and size changed minimally. Many slid from being loved as babies to falling in love with their owners. Critical normal avian developmental milestones of fledging and living independently were bypassed.20 Often, normal adult parrot behaviors are interpreted as threatening and vigorous birds can be uncomfortably loud. Many birds have easily trained their keepers to honor routines and food preferences. Owners risk screaming and biting if they do not demonstrate expected compliance. Turning the tables requires education, motivation, and a series of small successes.

Fresh air and sunshine and the chance to see the world

A first and vital step towards health is one easily achieved by most owners. A second cage outdoors can provide a horizon-expanding experience for companion parrots. Small birds can be relocated to a safe area on porches or decks where they can be easily observed in their own cages. Even 10–15 minutes outside can offer the benefits of fresh air and some ultraviolet light, and these small fories can provide a small success when owners observe their pets’ enjoyment. In cases where logistics or seasonally inclement weather limit access to the outside, an open window or patio door can offer the same advantages. Relocation to a novel site usually evokes interest and more activity and takes advantage of the instinctive behavior to roost and forage in different locations.

When carefully orchestrated, companion birds can easily and safely be set up safely outside in inexpensive screen tents that provide protection from mosquitoes, wild birds, and other pets. Bonded birds accustomed to being indoors and within line of site of owners will need to be educated about the fun and benefits of the outdoors. Visual barriers often help the very bonded, insecure pet bird learn to enjoy some independence. Back packs made of stainless steel or secure mesh offer a chance for a walk/hike or bike ride in a secure environment. Some birds may tolerate or even learn to enjoy a harness.


Clipped birds, especially Amazons and macaws, can be offered a secure perch or loop of rope with good traction and encouraged to flap with a repeated up down motion. First attempts should be limited to 5–10 flaps and the exercise tolerance and recovery time of the bird noted. The time flapping can be slowly increased as owner and bird gain strength. This simple and quick maneuver works to develop strong feet, mimics the wing action of flight, and requires variable degrees of exertion. Birds tolerant of a hand-held dowel are good candidates for this exercise. Birds suspicious or afraid of a dowel can be “flapped” on the hand. A hand towel over the hand offers protection from sharp claws. Even birds with severe disabilities can be held in the hand and will flap while touring inside or outside. Wing trims can be tailored to an individual’s body type, level of fitness, and environment.

Safe enclosures

Outdoor enclosures are available in many sizes, made from expensive or inexpensive materials, and offer an array of security features. An outdoor enclosure can be as simple as a commercial pet bird cage. A standard bird cage will provide minimal security and will usually necessitate close supervision. Pet bird cages are not safe enclosures for birds at dawn, dusk, or overnight. Nocturnal predators like raccoons are crafty and lethal. Pet bird cages offer little protection from bothersome rodents or birds of prey. Birds left unattended outdoors, day or night, in pet bird cages should be within another secure enclosure. Screen tents are seasonally available and provide shade, some security, and protection from mosquitoes.

Aviary enclosures are available in many sizes and usually constructed of galvanized wire or stainless steel. Traditional aviary wire is ½-inch X 2-inch galvanized after welding wire. This wire offers protection from nocturnal predators, is inexpensive, and is nontoxic until ingested. Powder coated steel enclosures are also on the market. Stainless steel mesh, ½-inch X ½-inch (.063), is commercially available and safe for housing most psittacine species, but it is hard to work with. Zoo Mesh® is a stainless mesh suitable for small psittacine birds (Cascade Coil Drapery, Portland OR, USA). Phantom Mesh® (Cascade Coil Drapery, Portland, OR, USA) is a coiled wire mesh available in stainless steel. Aviaries were available as modular kits, but lack of sales caused them to be discontinued. The manufacturer would consider taking orders again if demand increased.

Considerable thought and planning should go into any aviary plan. Many companion bird owners have unreasonable expectations about populating and maintaining an aviary outside. Mixed species aviaries are often a recipe for disaster. Many pet birds can enjoy the freedom of exercise and the outdoors in an aviary and retain their gentle nature. Some species are predictably easy to flock and some are impossible. Some birds will shed their tameness like an old coat. Insights into species-specific innate behaviors and seasonal changes in behaviors are best obtained ahead of time.

Outdoor enclosures intended for gardens can be adapted for supervised outdoor time. Costco (Costco Wholesale Corporation, Kirkland, WA, USA) carports can be enclosed in inexpensive wire and fitted with ropes and swings. They offer enough space for large birds to fly in and gardens can be arranged to offer vegetables for foraging.


Introducing dietary and exercise modifications to our companion psittacine patients has the potential to improve their overall general health and to create happier birds. Specific benefits may include an improvement in cardiovascular health, especially for those species prone to artherosclerosis. There are reasonable ways to bring exercise and a healthier lifestyle to companion birds. Histories, health exams, and diagnostic tests can be used to prepare individualized plans appropriate for each bird.


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12. Sturkie PD. Heart and circulation: anatomy. In: Sturkie PD, ed. Avian Physiology. 4th ed. New York, NY: Springer-Verlag; 1986:130–166.

13. Orosz SE. The avian cardiovascular system: anatomy and physiology for the clinician. Avian Specialty Advanced Program. Proc Annu Conf Assoc Avian Vet. 2004;3–10.

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Due to space limitations, the reference list is limited to 20. Additional references are available from the author upon request.