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Avian Medicine: General Avian Medicine:
Raptor Anatomy: Ten Key Facts

What is a raptor?

Raptors or birds of prey consist of members of order Falconiformes and order Strigiformes.

  • Owls belong to order Strigiformes. Barn owls (10 species) belong to family Tytonidae, while other owls (124 species) belong to family Strigidae (Fig 1).
  • BarredGHOsaw whetbarn owl

    Figure 1. Barred owl (Strix varia)(upper left); Saw-whet owl (Aegolius acadicus)(lower left); Great horned owl (Bubo virginianus) (upper right, image provided by Dr. Ed Ramsay); Barn owl (Tyto alba) (lower right, image by Stephen Barnett)

  • The order Falconiformes consists of family Cathartidae, Pandionidae, Falconidae, and Accipitridae (Fig 2). Cathartids are the New World vultures, while the family Pandionidae consists of one species, the osprey (Pandion haliaetus). Falcons and caracaras belong to family Falconidae. Accipitridae consists of Old World vultures, kites, harriers, hawks, and eagles.
  • ABEPatagonian falconRTHcondor

    Figure 2. American bald eagle (Haliaeetus leucocephalus, image by Linda Cronin)(upper left); Red-tailed hawk (Buteo jamaicensis)(lower left); Patagonian peregrine falcon (Falco peregrinus cassini) (upper right, image provided by Dr. Miguel Saggese); California condor (Gymnogyps californianus) (lower right, image by Dr. Ed Ramsay)


    Ten clinically significant facts

    If you are comfortable with psittacine anatomy and physiology, then you are well on your way to understanding raptors, however there are countless features that make this taxonomic group unique. LafeberVet has focused on ten amazing and clinically significant facts about bird of prey anatomy.

    1. Raptor claws or talons are needle-sharp (Fig 3) and the sharp, curved bill plays a key role in prehension. Although sometimes used for the killing blow, the beak does not always kill prey but instead can be used to damage the central nervous system thereby preventing escape attempts.
    2. owl talons

      Figure 3. Raptor claws or talons (arrow) are very sharp. Image by ‘minicooper93402′.

      With two viscious weapons at their disposal, restraint of raptors takes practice and training and should not be attempted without observation and supervision by others skilled in handling techniques.

    3. Many raptors are sexually dimorphic. The male is approximately 30% smaller than the female in many species, particularly diurnal birds of prey or falconiforms.
    4. Signalment is such a basic part of the clinical picture that many mammal veterinarians take for granted. It can be tremendously helpful to know—or at least suspect—that your patient is female.

    5. Two functional ovaries are often present in Falconiformes, and may be seen on survey radiographs.
    6. Although many falconiforms have a well-developed crop or ingluvies (Fig 4), owls merely possess a fusiform enlargement or widening of the esophagus instead of a true crop.
    7. falcon full crop

      Figure 4. Full crop (arrow) in a Peregrine/Saker falcon hybrid. Image by Dunleavy.

      Diseases associated with crop stasis such as Candida albicans overgrowth or “thrush” are sometimes seen in juvenile hawks, but much less commonly in owls.

    8. Since carnivores feed on relatively large, soft food items, the raptor stomach is adapted more for storage and is thin-walled, sac-like, and muscular. In fact the junction between the proventriculus and ventriculus is often difficult to identify externally.
    9. Of course the structure and shape of the raptor stomach informs normal radiographic anatomy as well as endoscopic results.

    10. In the final phase of gastric digestion in raptors, indigestible material, such as fur, feathers, grains, bones, teeth, and claws, are compacted into a pellet shape (Fig 5). Pellet formation occurs within the ventriculus. Muscular contractions then push the pellet up into the lower esophagus. From there antiperistaltic waves move the pellet toward the oropharynx where it is expelled. This oral expulsion of the pellet is called egestion or “casting”.
    11. pellets

      Figure 5. A collection of pellets collected from a Northern harrier (Circus cyaneus). Image by Jason Sturner.

      The contents of pellets provide valuable clues to biologists investigating the feeding habits of birds of prey (Fig 6). Egestion also provides an important clue to the health of the gastrointestinal tract for clinicians. Pellets are only egested once gastric digestion of a meal is complete. The average interval from feeding to egestion in owls ranges from 10 to 13 hours. The interval averages from 19.5-23.5 hours in hawks. Also, owls normally produce a pellet with each meal while hawks can eat more than one meal before casting. Most casts are egested before midmorning or after killing prey, but before the first feeding of the day.

      dissecting pellet

      Figure 6. Dissecting owl pellets. Image by Travel Manitoba.

    12. Gram-negative coliforms and clostridial organisms are part of normal GI flora. Normal fecal samples in carnivorous birds harbor a large variety of enteric bacteria. For instance, Clostridium spp. are a part of normal flora in birds of prey. Aerobic bacteria isolated from the cloaca of red-tailed hawks and Cooper’s hawks (Accipiter cooperii) includes coagulase-negative Staphylococcus, coagulase-positive Staphylococcus, Micrococcus sp., Streptococcus sp., Escherichia sp., and Salmonella spp. Raptors have also been implicated as potential reservoirs of Campylobacter spp.
    13. Another clue to your patient’s signalment can be derived from their eye since iris color can help to determine the age of some raptors. The iris of the juvenile red-tailed hawk (Buteo jamaicensis) is yellow and the iris darkens with age becoming chocolate brown in the mature RTH. The iris of the juvenile accipiter is yellow, while the iris of the 5-year old accipiter is ruby red (Fig 7).
    14. Coopers

      Figure 7. The accipiter iris turns ruby red (arrow) at 5-years of age. Click image to enlarge.

    15. In many birds, the eye is the most important sensory organ, and even partial impairment of vision has far-reaching consequences. Unfortunately, ocular lesions are a common problem in birds of prey (Fig 8).
    16. GHO hyphema

      Figure 8. Ocular lesions are a common physical examination finding in birds of prey. Image provided by Dr. Ed Ramsay.

      Although many raptors have one fovea, owls and some diurnal raptors are bifoveate. Owls and falconiforms have a well-developed temporal fovea that is dorsal and participates in binocular vision. A central fovea is located medially and participates in monocular vision.

    17. Although the dominant feature during fundic examination of all birds is the pecten, the appearance of the choroid can also be unique in some birds of prey.
    18. Choroidal vasculature is clearly visible in most young owls (Fig 9), which have a paucity of pigment in their retinal epithelium. Choroidal vessels are harder to appreciate in most diurnal raptors and in many cases are totally obscured by the overlying pigmented retinal epithelium. In these birds, the fundus is typically gray or brown to reddish-brown due to the pigment in the underlying retina and choroid.

      Choroidal vessels

      Figure 9. Choroidal vessels in an Eastern screech owl (Otus asio). Image provided by Dr. Christopher Murphy.



      References

      Gussekloo SWS. Feeding structures in birds. In: Bels V (ed). Feeding in Domestic Vertebrates: From Structure to Behavior. CAB International, Cambridge; 2006. Pp. 14-32.

      Klaphake E, Clancy J. Raptor gastroenterology. Vet Clin North Amer Exot Anim Pract 8:307-327, 2005.

      Korbel RT. Avian ophthalmology—A clinically-oriented approach. Proc Annu Conf Assoc Avian Veterinarians 2000. Pp 335-350.

      Murphy CJ. Raptor ophthalmology. Compendium Small Animal 9(3): 241-260, 1987.


      Written by Christal Pollock, DVM, Dipl. ABVP-Avian; Lafeber Company veterinary consultant on July 3, 2012.