Veterinary

RAPTOR NUTRITION: WHAT WE FEED THEM, WHAT GOES WRONG, HOW WE DEAL WITH IT.

N. A. Forbes BVetMed RFP DipECAMS FRCVS
Great Western Exotic Vets, Great Western Referrals,
Swindon, SN1 2NR. UK . http://www.gwreferrals.com

KEYWORDS

Raptor — Nutrition — Hatchery waste — Day old chicks — Rodents as food — Pigeons as food — Adenovirus

ABSTRACT

The aim of this paper is to review the available scientific and practical falconry text on raptor nutrition in order that vets can advise falconers on feeding regimes, as far as possible based on proven scientific research, assisted by practical information.

INTRODUCTION

The argument, that in the absence of detailed nutritional data the dietary needs of any individual species are most likely to be met by feeding a diet closely approximating to that which would be taken in the wild under ideal conditions (Kirkwood 1981), can be contested. Firstly, without detailed nutritional data, how can ‘ideal’ conditions be identified? Even a relatively accurate analysis of 90% of a wild birds intake may not be truly reflective of the nutrient profile of the diet (Brue 1994). In the wild most raptors are opportunistic eaters i.e. they eat anything which is available e.g. feathered and furred quarry also insects, reptiles and carrion. Whilst some species have adapted over many thousands of years to a certain food intake, in many others the environment in which they live and hence the food availability will have altered, often at a rate faster than the birds’ metabolism has been able to adapt (Brue 1994). A totally natural diet is impossible to replicate in captivity regimes (Dierenfeld et al. 1994), particularly because a wild bird has the option of choice (even if availability determines this), (indeed choice may vary with season and breeding activity), whilst a captive bird does not. In addition, captive birds may have different inherent nutritional requirements on account of their unnatural life style (Brue 1994). Wild birds often live short lives and death due to malnutrition is the most common cause of mortality in wild populations (Keymer et al. 1980; Hirons et al. 1979; Brue 1994). In essence, the modern falconer needs to develop feeding regimes based on the requirements of captive bred, raised and maintained birds as opposed to trying to replicate the, less than perfect, feeding patterns of wild raptors. Falconers bemoan the lack of scientific research into raptor nutrition for domesticated raptors. The primary reason to study nutrition, for the falconer, should be to improve the wellbeing of the raptors in our care. There are many factors that can influence both the quantity of food required by a raptor and its’ requirements for specific vitamins. Life style, husbandry, geographical area, different stages of the life cycle, for example the stage of development, growth rate, health status and production level of our birds can all affect their nutritional requirements. Our aim should be to achieve / maintain optimal health: greater longevity (achieving the full potential [flight and breeding] life span of your raptor) may be possible by optimising the diet as some dietary components may have protective effects, for example, antioxidants are known to help reduce cholesterol levels. Promote disease avoidance: nutritionally related disease can occur, which with knowledge can usually be avoided, for example:

DIRECT, because of inappropriate diet content or quantity:

  • Starvation;
  • Malnutrition / sub optimal nutrition;
  • Metabolic Bone Disease (Ca:P:D3 in balance) (i.e. rickets);
  • Obesity (leading most commonly to cardiovascular or liver disease);
  • Toxicities (e.g. excessive fat soluble vitamin supplementation, or mineral poisoning);
  • Competition for food between birds in the same aviary.

INDIRECT, as a consequence of altered requirements due to other conditions:

  • Management techniques and housing;
  • Rapid levels of neonatal growth;
  • Fledging;
  • Moulting;
  • Reduced or ineffective plumage leading to increased heat loss;
  • Breeding, egg laying and rearing;
  • Old age;
  • Increased or decreased exercise;
  • Following medical treatment e.g. antibiotics altering the gut flora;
  • During recovery after illness or treatment;
  • Altered ambient temperatures;
  • During periods of high stress e.g.:
  • Adverse weather reaction;
  • Weight reduction prior to entering;
  • Injury, change of aviary / husbandry or other conditions leading to sudden increases in metabolic rate.

DISEASE, leading to:

  • Reduction in appetite;
  • Reduction in availability of food (e.g. parasitism);
  • Diarrhoea — decreased absorption of nutrients and electrolytes in view of increased transit rates;
  • Reduced ability to store or mobilise nutrients, especially in liver disease.

GENERAL ILL-HEALTH, for example:

  • Metabolic disorders, e.g. liver disease, thyroid disorders, diabetes;
  • Neoplasia (i.e. cancers);
  • Senility

FOOD QUALITY, for example:

  • Excessive storage times reducing nutritional value;
  • Excessive storage times reducing water content;
  • Restricted food source / type, leading to limiting factors e.g. essential amino acids;
  • Poor hygiene precautions resulting in bacterial contamination;
  • Reduced quality food e.g. rancidity (excessive storage) which reduces vitamin E levels;
  • Usage of incorrectly balanced food supplements;
  • Excessive or inappropriate usage of food supplements.

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In establishing dietary requirements the goal is to determine what amount of food or particular nutrient is sufficient, if ingested routinely, to prevent impairment of health even if intake becomes inadequate for a short period, for the life stage and life style intended.

1. Maximum growth in the young
This is a common criterion used for commercial animals. However: whilst maximum growth is advantageous in birds destined for meat production, very rapid growth rates are often contra indicated in raptors (Forbes and Rees Davies 2000)

2. Maximum breeding production (to fledging)
This is also a common yardstick, although excessive production of you can harm the parents and result in poor quality off spring.

3. Prevention/cure of deficiency diseases
This depends on the observational endpoint chosen. (E.g., 5-10 mg of vitamin A per day prevents growth defects, but skin tissue becomes discoloured at this intake level). Seemingly this criterion could on occasion, therefore, be considered inappropriate in the light of the current concern for levels that promote optimal health as opposed to disease prevention.

4. Saturation of tissue
Determines the amount that will not cause any further increases in concentration of the nutrient in the tissues. Problem: some nutrients (e.g., fat-soluble vitamins) dissolve in adipose tissue, and will accumulate to toxic levels, leading to potentially life threatening diseases.

5. Balance studies
Method — measure input and output; when they are equal, assume the body is saturated. Assumes that the size of the body pool of the nutrient is appropriate and is not changed by the experiment. Assumes that higher levels of intake would do no good (clearly not true of water — hardly anyone would recommend just enough water to maintain balance). Such results are only relevant to the bird in that controlled environment, at that life stage.

6. Changes in a secondary variable
Changes in some secondary variable in response to the nutrient may be measured, e.g., changes in copulation frequency in tiercels in response to Vitamin E supplementation.

7. Amounts in typical diets
Sometimes it is difficult or impossible to determine the amount of a nutrient that is required. In such cases the amounts that seemingly healthy raptors in a wild population take in may be accepted as the norm. These levels, however, may be limited by population levels, prey availability, seasonal factors, lifestyle or geography (raptors in the wild may not need vitamin D in their diets, however, those kept in poorly designed, dark aviaries may).

WHAT IS AN ESSENTIAL NUTRIENT?

The classical definitions are:

Essential nutrient: substance that must be obtained from the diet because an animal cannot make it in sufficient quantities to meet its needs. Biotin is necessary in metabolism, but raptors normally produce sufficient quantities within their bodies. In contrast, pantothenic acid is equally necessary, but it is not produced internally. Hence, pantothenic acid is an essential nutrient.

  1. Macronutrient: nutrient needed in large amounts (many grams daily).
  2. Micronutrient: nutrient needed in small amounts (typically milligrams daily).

Conditional requirements: some substances are not generally considered essential to life, but might become so under specific circumstances (that is, conditional deficiencies are possible). The existence of conditional deficiency states may give rise to exaggerated claims of the importance of certain substances in normal diets, leading to the recommendation of unnecessary routine supplementation. For example the supplementation of a raptors diet with thiamine may be recommended for fish eating birds. These may improve in condition and cease fitting if the supplement is given. The additional thiamine, however, is only required, because of the naturally occurring ‘thiaminase’ (an enzyme which digests thiamine) in the fish, which is destroying the normally available levels of thiamine.

OUTLINING THE BASICS OF A FEEDING REGIME

As a basic principle, it is important to remember that each raptor species has evolved over millennia to fill a very specific ecological niche (Brue 1994). The consumption of a prey animal by a raptor involves the bird eating casting (fur & feather), muscle, bone, viscera and the prey’s gut content. In supplying food to captive birds, all these elements should be considered. Any alteration to the birds diet, even from one prey species to another, in either captive or free living individuals can result in a change in the relative proportions of these materials consumed. It has been established that a raptors food requirement varies with body size. Buzzards, kites and eagles require approximately <10% wet weight, in food, of their body-mass per day, large falcons and Accipiter species 10-15%, whilst small falcons and accipiters 20-25% (Kirkwood 1980 & 1985). Total food requirement, therefore, can be seen as a correlation between an individual birds digestive efficiency and its metabolic rate.

COMMONLY USED RAPTOR FOOD

Day-old chicks: are often, mistakenly, considered to have the equivalent nutritional value of a single hen’s egg. This is not the case. The formation of an embryo within an egg and the development and subsequent hatching of a chick dramatically changes the chemical and nutritional value of yolk and albumen (Table 3). Day-olds are used as the basis of a staple diet for the majority of species of birds of prey. Offering a high protein, low fat diet with good levels of vitamins and calcium. In a recent study, the body composition of young American kestrels (Falco sparverius) fed on a diet of either day-old cockerels or mice were compared. This comprehensive study (Lavigne et al. 1994a & 1994b) provides ample evidence as to the nutritional adequacy of day-old cockerels as a food source for American kestrels. It should of course always be remembered that not all chicks, mice etc are equal, the nutrient value will in turn be governed by what they were fed on. The calcium levels, which are required by growing birds of prey, would be met by any of the whole prey outlined in Table 3 (Dierenfeld et al. 1994). Calcium levels, however, also need to be evaluated in relation to both dietary phosphorus (P) and vitamin D3. Ca:P ratios of 1:1 — 2:1 have been reported for indeterminate egg layers (poultry) with determinate egg layers i.e. those birds which lay eggs during a specific breeding season e.g. raptors, requiring lower levels (Bird & Ho 1976; Dierenfeld et al. 1994). Day-old chicks have the correct Ca: P ratio (the most important single factor) as well as good overall levels of calcium. The conclusion, is that day-old chicks are the ideal staple diet for most species of birds of prey, being nutritionally sound, with high ME/GE ratios, as well as being economically priced, readily available and convenient to use. As previously discussed, however, it would be most unwise to feed exclusively one type of food, therefore, a varied diet is always indicated.

Quail: At 6-wks old there appear to be no nutritional differences between male and female quail, however, at 16-wks of age marked differences appear: the nutritional quality of males remains unchanged yet the fat levels in female quail have almost doubled (Clum et al. 1997).

Age and sex differences in quail leads us to classify the main types that are available as follows:

5 week old male culls, 6 — 8 week old prime birds, 8 month old ex-layer birds, Vitamin E enhanced quail. Quail become sexually mature at 6 weeks of age, therefore, the most readily available quail are surplus males that are culled at 5 weeks old, i.e. those birds not required for breeding programmes. 6 — 8 week old birds are generally considered to be the best quail readily available and are suitable for most raptors. 8 month old layer birds are the by-product of egg production, frequently yolk and fat filled and often carrying significant levels of pathogens and disease. These birds can represent a bio-security risk to captive raptors. Vitamin E enhancement of quail fed to falcons, at the Peregrine Fund facility Boise Idaho has seen:

  • Improved libido effects in adults (increased copulation frequency);
  • Increased hatchability of eggs (59% to 83%);
  • Increased activity in chicks with, for example, food begging occurring between 4 & 10 hours earlier than in previous years (although one accepts this was not a controlled trial). It should be remembered that in the same way as our birds are as good as what we feed them, so in turn the food we feed our birds is only as good as what they, in turn, were fed.

Rats: notwithstanding the above comments regarding vitamin E enhanced quail, rats are naturally high in vitamin E, therefore, a strong argument exists for using both rat and quail as part of a feeding regime. Rats appear to be almost opposite to the quail in that the younger the rat the higher the vitamin content (Dierenfeld 1994).

Hamsters: nutritionally equivalent to rats, hamsters may be a good substitute for those falconers who do not wish to prepare rats. The thin skin and fur combined with their smaller size, means that hamsters do not require evisceration and can be fed whole.

Guinea pigs: are herbivores and so have long digestive tracts and require evisceration prior to feeding. Guinea pigs have very loose fur, which can quickly fill a falcon’s crop and should be totally skinned before feeding.

Mice: are typically the most expensive food available to smaller hawks and owls in terms of their cost to weight ratio. Clum et al. 1997 expressed concern over their particularly high levels of vitamin A. Additionally, their high fat content and low protein levels (Lavigne et al. 1994a & 1994b) suggests they are less suited to feeding to birds of prey than appreciated.

Wild prey species: any wild source of food (e.g. pigeon, game, road traffic kills) must be considered potentially contaminated. That animal failed the ‘fitness for life test’ and we do not know why. Such birds may be carrying pathogens, parasites or toxins. Many falconers’ feed ferreted, rifled or shotgun shot foods (especially rabbit and pigeon). Shotgun killed quarry should never be fed. Rifle bullets frequently fragment on impact, so even head rifle shot food should be discarded. Ferreted or hawk caught rabbits may contain lead pellets from a previous non-fatal shooting incident. Lead ingestion from the consumption of fallen shooters quarry is a major cause of mortality especially in free living eagles (Saito et al., 2000). Keepers should be aware of the clinical signs of lead poisoning (weakness of legs and wings, inability to stand, often grasping the feet each in the other, inco-ordination, poor appetite, green faeces, and weight loss). It only takes one lead pellet to kill a raptor; any suggestive signs should result in immediate presentation to an avian vet for examination and appropriate life saving therapy.

Other foods: the feeding of muscle (e.g. shin of beef) as a major part of the diet is unsatisfactory without supplementation. Birds flying on public display, are often fed beef as the public may object to seeing fluffy chicks or mice fed. This can lead to calcium deficiency even in adult birds presenting with central nervous signs or muscle cramps. Dietary composition is more critical in neonates than that of adults. The diet for chicks and growing eyasses must comprise whole carcasses, and not simply muscle (i.e. meat). When considering eyass diet it is important to study the food that is being consumed by the chick, rather than the food which is being offered to the parents, the two may be very different.

In conclusion, no one raptor diet can be ideal for all species. Day old chicks may make up the mainstay of raptor diets, but should be supplemented with variety of other wholesome foods, this is the case for both hunting and breeding birds. Falconers should not neglect the vitamin and other trace element requirements of their birds when limiting food intake in order to control weight for flight training.

PROBLEM AREAS TO BE AVOIDED IN FEEDING

1. Ignoring differences between species

There may be a temptation to feed the same feeding regime for all birds of prey. The nutritional requirements of hawks, falcons,eagles, owls, secretary birds or ospreys, vary between genera, with age, reproductive cycle and whether the bird is being flown, moulted out or free lofted. Wide variances exist between species, for example, European Kestrels (Falco tinnunculus) can breed successfully for several generations on an exclusive day old chick diet (Forbes & Cooper 1993). In contrast merlins (Falco columbarius) fed on the same diet will not thrive. Free living merlins consume a predominantly insect-based diet and a high fat diet may be a contributory factor in Fatty Liver Kidney Syndrome of Merlins (Forbes & Cooper 1993). The diet of free living Secretary birds (Sagittarius serpentarius) is predominantly snakes, which are lower in energy and higher in Ca:P ratio than most commercial raptor diets. Young fast growing Secretary birds fed on standard raptor diets may suffer a Ca:P:D3 in balance with resultant metabolic bone disease (rickets).

2. Unnecessary or excessive vitamin supplementation

Vitamin supplementation is not a good substitute for good basic nutrition (Sandfort et al. 1991, Forbes & Rees Davies 2000). Furthermore, if raptors are being fed a good diet, supplements will only be required at times of additional stress (e.g. training, moulting, breeding), if at all (Forbes & Rees Davies 2000).

The problem is two-fold:

a. Incorrectly balanced supplements, for raptors i.e. a vitamin/mineral supplement based on the nutritional requirements of one species is unlikely to be suitable for another (Angel & Plasse 1997, Forbes & Rees Davies 2000). All fat-soluble vitamins compete with each other for absorption. Hence if any one of the fat-soluble vitamins is available in excess there can be competitive exclusion in the fat micelle. This leads to an antagonistic interaction among the vitamins. A vitamin supplement formulated for one species may well be incorrect for another. Any supplement used should be one prepared professionally specifically for raptors.

b. Inaccurate supplementation, either in an attempt to ‘do good’ i.e. in the mistaken idea that if one pinch is good, two pinches are better, or simply through lack of accurate manufacturers guidelines. In a study undertaken at Houston Zoo (Angel & Plasse 1997), wide variations were found amongst individual keepers’ interpretation of the quantities of supplements that should be added to avian diets. “A pinch” was found to weigh between 0.1 and 1.9 g. Vitamin supplementation added directly to the food has also not shown any detectable differences in health although food supplementation when provided in the food to prey species, has shown benefits to the secondary consumer (Dierenfeld et al. 1989).

In conclusion, varied, whole animal diets are desirable as they require little or no supplementation (Carpenter et al. 1987, Burnham et al. 1987, Dierenfeld et al. 1994, Bruning et al. 1980, Lavigne et al. 1994a & 1994b, Forbes and Rees Davies 2000).

3. Monotypic diets — (being provided with only of one kind of food)

Despite the adequacy of day-old cockerels as a staple food for many species of raptors, monotypic diets are unlikely to be advisable. Manganese deficiency, for example, has been documented in captive raptors fed a diet containing exclusively rat (Clum et al. 1997).

4. Monophagism — (habitual eating of only one kind of food)

Comparative work on digestive efficiency of birds of prey has shown that the Common Buzzard (Buteo buteo), a generalist species, has a greater digestive efficiency on a wider range of prey than the Peregrine Falcon (Falco peregrinus), a specialist species (Barton & Houston 1993). Such variation in the ability of different species to extract nutrients from their food requires the falconer to consider the dietary suitability for his own species and to ensure that the birds of prey in his care do not become locked into eating a narrow selection of foods. Raptors have no innate nutritional knowledge. Like children who would eat burgers and sweets daily if allowed, raptors may be selective. Only enough food of a single type per day should be fed, with diet variation taking place over a period of time, in order to ensure that large enough portions of each food type are eaten thereby maximising the nutritional advantages of each food consumed.

6. Excessive food provision

Birds eat to satisfy energy demands, so on a diet high in energy e.g. a high fat diet; they will eat less and therefore may not obtain the required micronutrients or trace elements from the food they consume. Although the dietary requirements of a captive raptor are less than that of a wild bird, their micro nutrient and trace element requirements will be the same, i.e. proportionately they require more trace elements. Whilst food energy content control is strict in flying birds (for weight control), it is less certain in aviary birds, such that obesity can arise. Excessive feeding leads to selectivity, potentially deficiencies, obesity and the potential for food decay, ingestion of spoiled food and the attraction of vermin.

5. Incomplete diets

Whole diets comprising flesh, bone, skin and casting materials are preferable to partial diets comprising just lean meat. Bones, for example, found in pellets cast by the gyrfalcon, (Falco rusticolus), were heavily modified by digestion, with traces of digestion observed on more than 80% of articular ends, nearly 100% of broken surfaces and on some shafts. It would appear, therefore, that the digestive tract of falcons are adapted to cope with bone structure and that the high levels of digestion found suggest that bones form an important part of the diet of birds of prey.

6. Over enthusiastic evisceration

The liver of an animal stores over 90% of the vitamin A content of a carcass as well as many other vitamins (Annex B). The evisceration of animals, therefore, beyond the removal of the intestines (where necessary) should be avoided. The routine de-yolking of day-old chicks will also dramatically reduce their vitamin A content and is not recommended except in specific situations, for example when feeding merlins, when yolk once a week is the maximum recommended frequency (Forbes and Cooper 1993).

Poor preparation, storage and handling

The manner and duration of storage can dramatically affect food quality and nutrient levels. Blast feeding of day-old chicks, for example, produces a significantly higher nutritional quality end product when compared to slow freezing in a domestic chest freezer. If meat products remain at room or body temperature for any period during the euthanasia, freezing, storing, transport, storage, thawing, feeding process, bacterial levels which are bound to be present will be permitted to multiple — rapidly creating a dangerously contaminated diet. Food kept for protracted periods (>3m) in domestic and commercial freezers deteriorates in nutritional quality, particularly in terms of water-soluble vitamins and vitamin E. Freezing is a drying process and long-term storage (unless sealed) can reduce the water content of food. As birds of prey obtain the majority of their water intake from their food, moisture depletion caused by long-term storage can cause potential problems during warm weather. Food should always be sourced from reputable suppliers with modern large-scale freezing plant and with sufficient turnover of stock to ensure that the food supplied has been frozen immediately after culling and is supplied as soon afterwards as possible. The temptation of bulk buying to obtain quantity discounts, with subsequent long-term storage in domestic freezers should be avoided. The method of killing should be ascertained and it should be certain that no toxic or noxious substances could be in the food. Barbiturate poisoning has occurred in both wild and captive raptors after birds have been fed the carcasses of animals euthenased with pentobarbitone. Other possible toxic contaminants include alphachloralose, mercury, mevinphos and other pesticides. Animals or birds fed to raptors must not have been on any form of medication, or medicated withdrawn food prior to their death. The feeding of day old poults hatched from antibiotic treated turkey eggs has led to infertility (Forbes & Rees Davies 2000). The potential risks of zoonotic (diseases transferable to man from animals) infections should always be considered when handling raptors or their food.

VETERINARY ASPECT OF RAPTOR NUTRITION

Common deficiencies and excesses

Although this is already covered, since this subject is so important the practical aspects of Ca:P:vitamin D3 are also considered, in greater depth, here. Ca:P:D3 in balance, metabolic bone disease (MBD), also commonly known as rickets is the most important nutritional deficiency of raptors. Birds may present with signs ranging from slight bowing of the legs, longitudinal rotation of the tibio tarsae to major multiple folding fractures of the skeleton and even fits. MBD is most likely to occur in fast growing larger species. Breeders should be advised not to feed such species ad libitum, but rather to restrain the potential growth rate. ‘Angel wing’ or ‘slipped wing’ (an outward rotation of the section of the wing from which the primary feathers originate) has been experienced in several fast growing larger raptors, in particular when being imprinted. This is readily controlled if diagnosed early by bandaging the primaries against the body, together with Ca, vitamin D3 supplementation and restriction of the growth rate. The diet must comprise of whole carcasses, i.e. not simply muscle (i.e. meat). The author has investigated calcium deficiencies in free living Golden eagle (Aquila chrysaetos) and European buzzard (Buteo buteo). In the former case the young were parent reared in an area with limited ground game (rabbit or hare). The birds were feeding predominantly on fallen sheep and deer carcasses. However, the young were only consuming meat from the carcasses (as sheep and deer bones were too large for young to ingest). The buzzards were rearing young in an area with a significant rabbit die off due to myxomatosis. Food was plentiful and rabbit bones were too large for young buzzard chicks, moreover in view of excessive food availability selectivity of ingestion was encouraged. A similar situation can arise when a breeder feeds a whole carcass diet of rabbit and pigeon for the parent rearing say, young Harris’ hawks (Parabuteo unicintus). Either the young are unable to consume the larger bones or the parents feed what is easiest. The result is severe MBD. It is always a question of what food is consumed by the birds rather than what is provided. Calcium deficiency may also be encountered in neonates produced by a hen with significant renal pathology, or from one which has laid an excessive number of eggs (due to egg pulling or multiple clutching). Any hen likely to ‘multiple clutch’ should be supplemented with Ca, D3 as soon as the first clutch is completed. Calcium deficiency due to inadequate D3 levels is less common in raptors in comparison with psittacines as most captive raptors have access to day light, this could change in the event of enforced housing due to avian influenza risk.

Obstructions

Casting: is the indigestible parts of the carcass, normally consumed and then regurgitated as a pellet by raptor. This includes hair, feathers and in some cases (e.g. owls) skeletal elements. Casting should not be given to any chicks under 12 days of age, and for some species (e.g. Merlin) not until 20 days of age. This applies in particular to ‘hard’ casting such as rodent fur, whilst chick down is considerably easier to deal with. Young chicks are typically unable to cast such material; leading to a proventricular obstruction and death. Clinically a firm swelling may be palpable caudal to the edge of the sternum. Standard medical treatment using prokinetics, oral and parenteral fluid therapy, and oral liquid paraffin is typically ineffective. Surgery of such debilitated neonates typically results in the chicks death. If instead the chick is force fed for a few days, so it increases in size, it will then typically be able to pass the casting itself. Breeding females with developing ovarian follicles and a swollen active oviduct may have difficulties with excessive casting due to lack of coeliomic space. Casting should be reduced rather than increased in pre-egg laying females. A normal raptor will produce a casting 8 — 16 hours after a meal. Birds cannot be fed again until they have cast. If feeding occurs prior to casting, a small intestine obstruction can arise. If presented with a thin or a weak bird, where it is desirable to increase the birds condition (weight), then frequent, small, cast free meals of readily digestible food (e.g. skinned day old chicks), should be given. As soon as the crop is empty the bird may be fed again.

Inadvertent ingestion of indigestible matter: On occasions organic material may be consumed with food (e.g. peat or vegetable material from nest ledges, wood shavings, which the bird is unable to cast. In such cases an ingluviolith or proventricular impaction may occur. Harris’ hawks are considered the most intelligent of the common captive raptor species. They will at times ‘play’ with materials in their surroundings and can ingest various foreign bodies. One example is that they can learn to untie the knot tethering their leash to the perch. The leash can be pulled free of the swivel and the bird can then swallow the leash necessitating an ingluviotomy, although the bird will often cast it back itself. Large foreign bodies may be safely left 24 hours, in the expectation that the bird will naturally cast them. Owls, both in captivity and in the wild, occasionally eat very long twigs (on occasions 6 — 8 inches long). The bird may appear in appetent, uncomfortable and miserable. Sometimes the twig is ‘cast’, but on other occasions, it may perforate the crop or proventriculus with a grave prognosis. Endoscopic or surgical removal may be necessary. Another form of obstruction seen especially in the larger owls is the ingestion of pea gravel. The bird is presented with a history of having a good weight but marked loss of body condition. Gastric distension by the gravel reduces the bird’s appetite and little or no food is ingested. The condition is often advanced by the time of presentation.

Ingestion of over size food items: the feeding of rabbit or hare carcasses with intact femurs can cause problems. The bone may pass directly into the proventriculus and be digested. However, in larger raptors the bone may rotate into a transverse position in the crop or proventriculus. The bone may form an obstruction in the crop or perforate the gut leading to a terminal peritonitis. If the bone is broken (preferably without sharp ends) before feeding the problem does not arise. A similar situation can develop when pheasant or chicken necks are fed whole. The neck usually passes down straight, but occasionally will double over in the crop or distal oesophagus becoming. On occasions, birds will eat uncommon prey items. The most unusual obstruction encountered by the author was when a female red tailed hawk (Buteo jamaicensis) which had caught and eaten a hedgehog (Erinaceous europaeus). Initially the bird was fine, but after 18 hours with no casting, she was presented for examination. Barium contrast radiography confirmed the presence of multiple spines and fur lodged in the proventriculus. The obstruction was successfully removed via abdominal surgery.

Decreased motility: ’Sour Crop’ is a common and often rapidly fatal crop stasis. Ingested meat is held within the crop being maintained at 38 — 40oC, with no gastric acid or enzymes present to prevent bacterial multiplication. This occurs most commonly in thin or sick birds which are given an excessive crop of food. The most urgent action required is to empty the crop, which will generally require veterinary intervention. The most rapid and atraumatic method is, with the bird anaesthetised and entubated crop, ingluviotomy is performed, the crop lavaged with warm and closed immediately or a day or two later.

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