Abstract

This masterclass will review recommended nutrient requirements for psittacine birds and how these requirements were established. The class will also cover what the critical nutrients in companion parrots’ diets are and the health implications of their lacking these critical nutrients. Common feeding practices of parrot owners in the U.S. will be discussed, as documented by the surprising findings of two recent large feeding surveys of U.S. parrot owners. Finally, the lecture will cover significant nutrition-related diseases (such as atherosclerosis and gout) associated with these common feeding practices. We will also review the clinical relevance of these practices and what veterinarians and veterinary technicians can do to help bird owners feed their pets more complete nutrition so that companion birds live longer, healthier lives.

Introduction

Despite improvements in feeding practices by pet bird owners during the past two decades, companion parrots still suffer from many nutrition-related diseases that are recognized by avian veterinarians worldwide. The exact nutrient requirements for psittacine birds are not known, and dietary recommendations for these species have been extrapolated from studies done with poultry. To better educate bird owners, it is essential that veterinary professionals are familiar with which nutrients are essential to the health of pet birds, how pet birds’ nutritional needs differ from those of their wild counterparts, and what happens when these nutritional needs are not met. With this knowledge, veterinary staff can help educate bird owners about feeding their pets to prevent nutrition-related illnesses.

Feeding Strategies of Psittacine Birds

Psittacine birds are herbivores (consume plants) and are further subdivided based on general food consumption:¹

• Granivores (budgerigars, cockatiels, hyacinth macaws)
• Frugivores (some macaws, Amazon parrots)
• Nectivores (lorikeets, lories)
• Omnivores (sulphur-crested cockatoos, African gray parrots)
• Mixed (Scarlet macaw is a frugivorous granivore)

In all subdivisions, food consumed may vary based on nutrient availability, age, and sex.

Nutrients

Definition

Dietary components that provide energy and precursors for the synthesis of the body’s structural and functional macromolecules

• Macronutrients (lipids, protein, carbohydrates, water)
• Micronutrients (vitamins and minerals)

Essential Nutrients

• Required for optimal health
• Needed for metabolism
• Not synthesized adequately by the body
• Generally known for domestic Galliformes and extrapolated to other species

Nutrient Requirements of Birds

• Few nutrient requirements have been determined scientifically
• The minimal level that optimizes a bird’s health is considered the requirement
• Nutrient requirements are determined for three physiologic states:
• Basal: Those needed to maintain basic life functions
• Maintenance: Those needed for basal functions plus maintaining body temperature, interacting with other animals, finding food
• Total: All those needed for life at various stages (growth, reproduction, molting)
• Few studies have been done to establish nutrient requirements of psittacine and passerine species
• Nutrient requirements of companion birds have been extrapolated from those of Galliformes
• Some nutritional information has been extrapolated from studies of budgerigars and cockatiels
• These extrapolations do not account for several variables:
• Differences in efficiency of nutrient absorption between species
• Variable nutrient requirements for different species
• Variable requirements for different life stages and activities:
• Breeding/reproduction
• Exercise/flight/migration
• Growth
• Molting
• Aging

The Importance of Vitamins

Vitamin A

• Ingested as beta carotene and other carotenoids
• Degraded enzymatically in intestinal epithelium to retinal which is converted to retinol and retinoic acid
• Absorbed as retinol and retinol esters from GI tract
• Stored in liver
• Commonly deficient in diets of both captive and wild birds
• Exact requirement in psittacine species is unknown
• Feeding trials in female cockatiels showed that birds thrived at 2000–10,000 IU/kg diet
• Requirements in other species less dependent on grain in diet may be higher
• Deficiency leads to:
• Keratinization of squamous epithelium of respiratory and GI tract mucous membranes
• Metaplasia of salivary glands
• Blunted choanal papillae
• Widened choanal slit
• Anorexia
• Increased susceptibility to infection
• Impaired ocular rod function/night blindness
• Keratinization of conjunctiva/dry cornea/corneal abrasions
• Possibly increased vocalizations (seen in cockatiels)
• Possible change in feather pigmentation in some species

Vitamin D

• Cholecalciferol (vitamin D3) and ergosterol (vitamin D2)
• Birds synthesize cholecalciferol from cholesterol in skin exposed to ultraviolet (UV) light
• Most pet birds aren’t exposed to enough UV light, so they a need dietary vitamin D source
• If birds are exposed to adequate UV light, they don’t have a dietary requirement for vitamin D
• Without adequate UV light, requirement in chickens and turkeys is 200–1100 mg/kg diet
• As calcium (Ca) levels in diet decrease, phosphorus (P) levels increase, Ca:P ratio is affected and dietary requirement for vitamin D3 increases
• Birds cannot convert vitamin D2 into D3 as cats and dogs can
• Vitamin D3 acts as hormone to regulate Ca and P metabolism
• Essential in bone mineralization and eggshell formation
• Deficiency leads to:
• Thin eggshells in hens
• Osteomalacia
• Pathologic bone fractures
• Excess can lead to toxicity with over-supplementation:
• Ca is mobilized into blood and mineralizes soft tissues (joints, kidneys, heart, blood vessels, pancreas)
• Documented with over-supplementation of hand-feeding formula and pelleted diets
• Susceptibility varies by species, with macaws possibly more susceptible

Vitamin E

• D-tocopherol found in plant leaves
• Transported by plasma lipoproteins and incorporated into lipid of cell’s plasma membranes
• Stabilizes membranes by reducing free radical peroxidation
• Works with superoxide dismutase, glutathione peroxidase, catalase to protect cell membranes
• Affects conversion of arachidonic acid into prostaglandins and thromboxanes, thereby altering immune response
• Dietary requirement varies depending on dietary levels of polyunsaturated fatty acids, vitamin A, betacarotene, and rancid fats, plus presence of other dietary antioxidants (i.e., selenium)
• Deficiency leads to:
• Cell membrane dysfunction
• Encephalomalacia (“crazy chick disease” = torticollis, vocalization, wing-flapping)
• Muscular dystrophy
• Ventricular myopathy
• Exudative diathesis
• Red blood cell fragility
• Excess can lead to deficiency of other fat-soluble vitamins

Vitamin K

• Has anti-hemorrhagic activity
• Ingested in plant leaves
• Cofactor of hepatic microsomal carboxylase that catalyzes carboxylation of glutamate in the function of osteocalcin, clotting factors VII/IX/X, and prothrombin
• Rapid turnover
• Not stored long in body
• Low requirement
• Increases with infection, liver disease, vitamin K antagonists (antibiotics that deplete intestinal microflora producing vitamin K)

Omega Fatty Acids

Omega-6 and -3 polyunsaturated fatty acids (FA):

• Affect immune, cardiovascular, renal, and nervous system function
• Reduce inflammation and arthritis
• Omega-3 FAs produce fewer inflammatory cytokines than omega-6 FAs (arachidonic acid) that produce inflammatory prostaglandins, leukotrienes
• Most birds eating seeds and grains are not consuming a balance of polyunsaturated fatty acids but are getting more omega-6 FAs than omega-3 FAs
• Lack of omega FAs results in increased inflammation (itchy skin, unkempt feathers, poor feather quality)

Calcium

• Most important dietary mineral
• Needed for bone mineralization, eggshell calcification, nerve conduction, muscle contraction
• Requirement for psittacine bird species isn’t known
• Chickens require 0.1% of total diet for maintenance
• Egg-laying chickens require 3.3% of total diet
• Egg-laying budgerigars and cockatiels have been shown to require 0.85% and 0.35%, respectively, for normal egg calcification
• African gray parrots may require more Ca than other psittacine bird species, as they more commonly develop hypocalcemic seizures
• Requirement may be reduced when birds ingest larger amounts of pellets containing vitamin D3
• Requirements may change with different life stages (i.e., increased with growth)
• Deficiency:
• Occurs from:
• Too little dietary Ca
• Excess dietary P
• Too little dietary vitamin D3
• Is common in psittacine birds on all-seed/nut diets containing little Ca and vitamin D3
• Leads to egg-binding in laying birds
• Excess:
• One anecdotal report (D. Phalen 2005) of renal mineralization in budgerigars fed pellets exclusively
• No evidence of renal mineralization noted in budgerigars fed all-pelleted diets in my 26-year career as an avian veterinarian recommending pellets to all psittacine bird species

Essential Amino Acids

• In chickens: arginine, leucine, isoleucine, lysine, methionine, phenylalanine, valine, tryptophan, threonine, and possibly glycine, histidine, proline because of insufficient synthesis relative to use
• Budgerigars also require glycine because they cannot synthesize enough
• Birds eating high-fat seed diets commonly cannot ingest enough seeds to meet their amino acid needs and eat until they are fat

Protein

• Feathers comprise the largest protein-containing portion of a bird’s body
• Requirements increase with growth, reproduction, molting
• Required amino acids limit rate at which protein can be turned over and may require increased food ingestion to meet protein needs
• Theories about increased protein ingestion leading to gout in birds are not substantiated

Formulated Diets

• Nutritionally balanced products to be fed to companion birds include:
• Pellets
• Extruded diets
• Pelleted material packed between intact (not ground up) grains and seed
• These diets are considered nutritionally balanced according to recommendations known for Galliformes and extrapolated according to AAV panel recommendations
• Few studies have been done determining nutritional needs of varied groups of psittacine bird species originating from diverse backgrounds
• Nutritional balance will change at different life stages (growth, reproduction, etc.)
• Pellets:
• Mixture of ground grains with vitamins and minerals
• Liquids are commonly added to mixture and then heated with dry or steam heat and forced through holes in a die with a roller
• Pellets produced are of constant shape and size
• Extruded diets:
• Commonly referred to as “pellets”
• Mixture of ground grains with vitamins and minerals
• Mixture is forced through an extruder with high heat and under pressure
• Moisture is added as steam, which evaporates as mixture passes through extruder
• Different-shaped holes in extruder plate determine the size and shape of food pieces created
• High heat kills infectious organisms

Feeding Practices of Companion Parrots in the U.S.

General Practices

• Companion birds are not typically fed balanced diets
• Bird owners typically provide seed, produce, and a variety of other table foods, hoping birds will self-select nutritional balance from diverse options
• Typically, birds select out high-fat options and become obese
• To combat this trend, for the past 20+ years, avian veterinarians have recommended feeding formulated pelleted or extruded diets that are nutritionally balanced as the base diet

Problems with Feeding Formulated Diets

Bird owners often resist feeding these diets because:

• Birds don’t readily transition to them
• Owners think of them as boring, lacking variety, and limiting opportunities for foraging
• Owners add produce and other table foods to formulated diets to increase variety and foraging

Pet Birds ≠ Wild Birds

Pet birds’ nutritional requirements differ from those of wild birds:²

• Wild birds need calories to fly and to forage for food, mates, nest sites
• Wild birds graze all day without becoming obese, because they constantly burn calories
• Pet birds become obese because they sit in cages all day
• Pet birds are provided with food, so they don’t have to hunt
• Pet birds bond to owners as “mates” and don’t seek out other birds
• Food availability changes seasonally in the wild, so birds get more variety and balance
• Pet birds typically are fed the same foods every day, often in excess

Problems with Produce-Feeding

• Fruits and vegetables provide some micronutrients but are not the solution
• Produce is the basis of a healthy diet for relatively slow-growing, slowly reproducing humans
• Cannot provide adequate, balanced nutrition to meet the needs of rapidly growing, quickly reproducing pet birds
• Domestically grown produce has higher energy and water content and lower essential nutrient levels than wild counterparts
• Excessive produce-feeding may cause dietary nutrient dilution

Recommendations for Feeding Produce³

• Produce may be fed with a formulated diet to provide micronutrients, variety, and enrichment through foraging
• High water content of produce makes nutrient dilution minimal when fed as a complement to formulated diet
• Nutrient dilution is more common if formulated products make up <50% of total diet
• Table foods (including produce) should be no more than 25–30% of total diet so as not to upset nutritional balance

Seed/Nut Diets—A Persistent Problem

• Most common diet still fed to companion birds
• Are high in fat
• Lack fat-soluble vitamins (A, D, E, K) and critical minerals (calcium, phosphorus, sodium, zinc, iron, iodine, selenium, manganese)
• Are deficient in key amino acids (lysine, methionine, riboflavin, pantothenic acid, niacin, choline)
• Are overall nutritionally imbalanced
• Will be preferably consumed by pet birds if mixed with healthier food choices (selective eating)
• Veterinarians agree that pet birds’ seed overconsumption causes many health problems:
• Seed contains excess fat (cholesterol and triglycerides)
• Seed lacks essential vitamins and minerals
• Seed overconsumption leads to nutrient deficiencies
• Lack of vitamin A → gout, poor skin/feather quality, respiratory problems
• Lack of calcium, vitamin D → egg binding, soft-shelled/shell-less eggs
• Seed overconsumption leads to nutrient excesses
• Excess fat → obesity, arthritis, high cholesterol, stroke, atherosclerosis, fatty liver/liver failure

The Bird Health and Nutrition Link

Diet Impacts Health

• Veterinarians, bird breeders, and bird-owning communities recognize the association between diet and health:^4,5
• Veterinarians treat many common bird health issues:
• Reproductive disease/egg-laying problems
• Liver disease
• Kidney disease
• Feather-picking/skin problems
• Many health issues in birds may be due to poor nutrition:
• Atherosclerosis/stroke
• Obesity
• Fatty liver disease
• Kidney failure/gout
• Egg-binding
• Feather-picking/skin disease
• Poor feather quality
• Veterinarians link many bird health issues to nutrient excesses and deficiencies:
• Atherosclerosis/stroke—excess fat/cholesterol/triglycerides
• Obesity—excess fat
• Fatty liver disease—excess fat
• Kidney failure/gout—lack of vitamin A
• Egg-binding—lack of calcium, vitamin D
• Feather-picking/skin problems—lack of vitamin A
• Poor feather quality—lack of vitamin A, protein

Atherosclerosis^7-9

• Accumulation of foam cells, lipids, calcium, and cellular debris leading to large fibrous-fatty plaques narrowing arterial lumen
• Extremely common in older, female Amazon, African gray, and male Quaker parrots
• Amazon and African gray parrots may have increased incidence of atherosclerosis due to lack of exercise and consumption of high-fat diets
• Increases in total plasma cholesterol have been associated with increased incidence of atherosclerosis in psittacine species
• Budgies fed a 2% cholesterol/15% lard diet developed hypercholesterolemia and significant atherosclerosis in 6 months
• Quaker parrots fed a 1% cholesterol diet developed significant atherosclerosis in 2 months and advanced atherosclerosis in 4 months
• Lesion severity and arterial cholesterol content were correlated with plasma cholesterol concentration

Supplementing Seed Isn’t the Answer

• Many seed manufacturers mislead bird owners into thinking their products are nutritionally complete by coating seed hulls with supplements discarded by psittacine birds as they eat
• Well-intentioned owners offer a variety of healthy food choices with seeds and expect their pets to choose for proper balance
• When offered a varied diet, pet birds do not select for nutritional balance but select out what they like best (selective eating), which is typically the seeds
• Offering an abundance or a wide variety of food choices stimulates excessive reproductive activity, since food diversity signals breeding
• Homemade diets aren’t the answer
• Bird owners add foods to seed-based diets to try to provide variety and balance, but homemade combinations usually lack essential nutrients and contain excesses

Exotic Bird Ownership in the U.S.

Birds are Popular Pets

• American Pet Product Association 2019 survey of U.S. pets
• Surveyed 25,109 households as a representation of total U.S. households
• 5.7% of U.S. households owned pet birds = 6.9 million households
• Average of 2.98 birds/household
• 20.6 million pet birds in the U.S.
• ZuPreem 2014 survey of U.S. bird population
• Surveyed 27,202 households as a representation of total U.S. households
• 6.1% of U.S. households owned pet birds = 7.4 million households
• Average of 2.50 birds/household
• 18.6 million pet birds in the U.S.

ZuPreem 2014 Survey

• 1,680 bird caregivers with 4,200 birds of varying species
• Surveyed bird owners
• Divided birds into size categories
• Asked “What are bird owners looking for in a diet for their pets?”
• As expected, the response “provides complete and balanced nutrition” was the top food benefit sought by bird owners
• But the overwhelming response to “contains seed” was a surprise!
• Bird owners are not looking for sunflower seed-free foods
• Also not looking for “contains nuggets/pellets” at a high rate
• Owners who feed pellets/nuggets have concerns that birds won’t eat it
• A significant number also don’t think nuggets/pellets are nutritious enough!
• Conclusions:
• More birds eat seeds and nuts than any other foods
• Seeds and nuts are a greater percentage of U.S. birds’ diets annually than any other foods
• In terms of total food fed to birds in the U.S. in 2014, seed was, on average, 50% of their total consumption
• 56% of birds in the U.S. eat no pellets
• Of birds eating nuggets/pellets, this food made up only 31% of their total diets
• Seeds and nuts are consumed by U.S. birds more than other foods, regardless of bird size
• Seed makes up a larger part of small birds’ diets and a smaller part of large birds’ diets
• If a bird eats pellets, it’s only about 30% of any size bird’s diet
• Seeds make up a greater percentage of U.S. birds’ annual diet, regardless of bird size

Pet Bird Feeding Facts

• Despite improvements in pet bird nutrition in recent years, birds in the U.S. still consume predominantly seed
• Most pet birds never see a veterinarian, so veterinarians never get the chance to improve most birds’ diets
• Some nutritionally related conditions are less commonly seen in pet birds today because of better diets:
• Sterile nasal granulomas in African gray parrots
• Gout in all species
• Yet, many nutritionally related diseases persist in pet birds despite increased knowledge about pet birds’ nutritional needs
• Statistics revealed in the ZuPreem survey explain why veterinarians continue to see these nutritionally based problems

How to Solve the Pet Bird Nutrition Problem

Formulated Products

• Are the solution as the base diet
• Provide all the “required” nutrients (including vitamins and minerals) without excessive fat, to promote birds’ health
• Offer a manageable volume of food for a bird to consume daily that contains complete and balanced nutrition
• May be fed as base diet with smaller amounts of vegetables and fruit to provide micronutrients and enrichment through foraging
• May provide variety to birds’ diets through different shapes, sizes, +/- colors without the risk of “selective eating”

Need to Create Awareness About Formulated Diets

• Veterinarians and technicians can continue to stress to bird owners the importance of these products and the detriment of seeds
• Formulated food manufacturers can continue to invest in educating bird owners about the health benefits of their products
• Pet stores can educate employees about the value of formulated products and the harm of seed-based diets
• Bird owners whose birds eat formulated diets can share their experiences

Difficulties with Seed to Pellet Conversion

• Veterinarians see a very small fraction of the total pet bird population
• Bird owners who never see veterinarians may not be aware of the importance of pelleted diets at all
• Many bird owners get frustrated or too stressed to continue conversion process if birds don’t eat pellets immediately, and they just give up
• Veterinary staff can encourage bird owners to convert pets to pelleted diets, but owners need ongoing support during conversion process
• Bird owners who don’t see veterinarians and try converting their birds to pellets need even more support and encouragement

ZuPreem Diet Conversion Project, 2018–19

Overview

• Goal is to provide support to veterinary staff and to bird owners as they convert pet birds off of seed-based diets and on to formulated diets
• Aim is to transition birds to healthier formulated products, regardless of brand or variety
• Veterinary staff examine pet birds to ensure they’re healthy
• Veterinarians ask bird owners to follow one of three conversion methods or a variation of one
• ZuPreem aids veterinary staff in answering owners’ questions during conversion process
• Owners are then sent follow-up surveys to see how conversion process went, what problems they encountered, what conversion methods worked best, etc.
• More than 175 veterinary hospitals from across U.S. and Canada enrolled
• Each hospital sees a minimum of 20 birds per week
• All birds must receive a veterinary checkup and be deemed healthy before conversion starts
• Project has been taking place for >1 year now
• All hospitals receive material to promote diet conversion:
• Samples of pellets for large and small birds
• Literature and links to online videos describing and showing conversion methods
• With 175 hospitals x minimum 20 birds/week x 50 weeks = >180,000 birds whose diets could be improved with pellets

ZuPreem Diet Conversion Methods

• ZuPreem website has detailed description of three conversion methods:
• “Birdie’s choice”
• “Tough love”
• “Slow and steady”
• Each method is broken down into a series of simple steps
• Each step has several tips to help owners implement
• Each step is described in words plus easy-to-follow illustrations
• Each method is also described in a real-life video
• Owner surveys:
• Post-conversion surveys sent digitally or hard copy, depending on owners’ preference, through veterinary hospitals to owners to gather info about conversion process
• Owners fill out surveys and send back to hospitals
• ZuPreem gathers survey results from hospitals
• Statistics are run on conversion survey results
• Project results, presented at ExoticsCon 2019 by Dr. Alison Cummings,¹⁰ will be published to help aid veterinary staff and bird owners in understanding how to make future diet conversion easier
• Survey questions:
• What species is your bird?
• How old is your bird?
• Is your bird male or female, or is the gender unknown?
• Were you able to convert your bird to eating pellets?
• Which of the three methods did you use to convert your bird?
• If another method, please describe
• To what brand did your bird convert?
• Which variety of that brand?
• How long did it take for your bird to convert to eating pellets?
• What problems did you encounter when you started the conversion process?

ZuPreem Diet Conversion Project Goals

• Identify which conversion methods work best for different species
• Determine average time for conversion of each species
• Determine which specific pellet brands and varieties worked best for particular species
• Identify most common problems owners encounter during conversion process and how to overcome them
• Set bird owners up for successful future pellet conversions

ZuPreem Diet Conversion Project Findings

• Conversion rate
• Only 3/83 birds were reported to not convert (conversion rate of 96%)
• Birds that did not convert followed the “slow and steady” method
• Adult lovebird and juvenile green-cheeked conure
• Both liked pellets, but owners were concerned they were not eating enough, so they continued to mix in seed
• Juvenile budgerigar
• Bird continued to eat around the pellets, so owner gave up
• Life stage vs. time to conversion
• Regardless of life stage, 64% birds converted within 7 days and 90% within first month
• Conversion within 3 days: adult 44%, geriatric 41%, juvenile 33%
• Conversion within 7 days: juvenile 76%, geriatric and adult 59%
• Geriatric group was the only group to have 100% conversion, with 94% converting in <1 month
• Time to conversion by method
• Fastest method: Tough love with 90% of birds converting within 1–3 days
• Most successful overall: Tough love with 100% of birds converting within the first month
• Slowest method: Slow and steady, with 56% converting within 7 days, and 87% within the first month
• Slow and steady was the only method that had birds not convert (5%)

ZuPreem Diet Conversion Project’s Future Goals

• Follow birds before and after conversion to pelleted diet:
• Body weight
• Body condition score
• Activity level
• Blood lipid (cholesterol, triglyceride) concentrations
• Overall health
• Hypothesize that birds on formulated diets will have more appropriate body weights/condition, improved activity level, more normal lipid concentrations, and overall better health than those on seed diets

Take-Home Messages

• Despite making great strides in pet bird nutrition over the past 20+ years, most bird owners are still ignorant about proper nutrition for their pets
• Pet birds are still predominantly eating high-fat, seed and nut diets and are not consuming balanced diets
• To get the proper balance of vitamins, minerals, amino acids, and other key nutrients, pet birds must consume a base diet of formulated pelleted diet
• The overwhelming majority of birds will convert to a pelleted diet from a seed-based diet
• For most birds, diet conversion takes less than 1 week regardless of life stage, conversion method, or bird family
• Conversion method was the only variable found to significantly affect conversion time when it comes to pet birds’ conversion to pelleted diets
• The veterinary community—in association with pet stores, breeders, and bird food manufacturers—faces the ongoing task of better educating the bird-owning public about proper nutrition for companion birds

References

1.  Orosz SE. Clinical avian nutrition. Vet Clin Exot Anim. 2014;17:397–413.

2.  Perron F, Grosset C. The diet of adult psittacids: veterinarian and ethological approaches. J Anim Physiol Anim Nutrition. 2014;98:403–416.

3.  Brightsmith DJ. Nutritional levels of diets fed to captive Amazon parrots: does mixing seed, produce, and pellets provide a healthy diet? J Avian Med Surg. 2012;26(3):149–160.

4.  Shoemaker NJ, Lumeij JT, Beynen AC. Nutrition-related problems in pet birds. Tijdschr Diergeneeskd. 1999;124(2):39–43.

5.  Koutsos E, Gelis S, Echols MS. Advancements in nutrition and nutritional therapy. In: Speer BL, ed. Current Therapy in Avian Medicine and Surgery. St. Louis, MO: Elsevier; 2016:142–176.

6.  Beaufrere H. Atherosclerosis: comparative pathogenesis, lipoprotein metabolism, and avian and exotic companion mammal models. J Exot Pet Med. 2013;22(4):320–335.

7.  Beaufrere H, Ammersbach M, Reavill D, et al. Prevalence of risk factors associated with atherosclerosis in psittacine birds. J Am Vet Med Assoc. 2013;242(12):1696–1704.

8.  Beaufrere H, Cray C, Ammersbach M, et al. Association of plasma lipid levels with atherosclerosis prevalence in Psittaciformes. J Avian Med Surg. 2014;28(3):225–231.

9.  Beaufrere H, Nevarez JG, Wakamatsu N, et al. Experimental diet-induced atherosclerosis in Quaker parrots (Myiopsitta monachus). Vet Pathol. 2013;50(6):1116–1126.

10.  Cummings A. The pellet puzzle: an evaluation of three proposed pellet conversion methods. In: ExoticsCon Proc. 2019:148–149.