RAPTOR NUTRITION: WHAT WE FEED THEM, WHAT GOES WRONG, HOW
WE DEAL WITH IT.
NEIL A FORBES BVETMED RFP DIPECAMS FRCVS
Great Western Exotic Vets
Unit 10 Berkshire House, County Park,
Shrivenham Road, Swindon, SN1 2NR
www.gwexotics.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
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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
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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.
HOW ARE NUTRIENT REQUIREMENTS QUANTIFIED?
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
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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
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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 biosecurity
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
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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.
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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 fatsoluble
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
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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 deyolking
of day-old chicks will also dramatically reduce their vitamin A content
and is not recommended except in specific situations, for example when
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feeding merlins, when yolk once a week is the maximum recommended frequency
(Forbes and Cooper 1993).
7. 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
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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
1. 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.
2. 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
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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.
3. 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.
4. 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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