Julia B. Ponder, Michelle M. Willette The order Strigiformes comprises 220 to 225 extant species of owls divided into two families: Tytonidae (barn owls) and Strigidae (true owls). The two genera of barn owls, Tyto and Phodilus, represent less than 20 species. Most of the species living today are classified as Strigidae, which includes approximately 25 genera.26 Although the question has not been completely resolved at this time, recent systematics have aligned owls more closely with nightjars than diurnal birds of prey. Using the Sibley-Ahlquist taxonomy, the most recent addendum to the American Ornithologists’ Union combines Caprimulgiformes with Strigiformes (although they are discussed in separate chapters for the purposes of this book).50 With lineages extending back 70 to 80 million years, owls are one of the oldest groups of land birds.26 Modern day extinctions of owls such as the laughing owl (Sceloglaux albifacies) of New Zealand and the Mauritius owl (Mascarenotus sauzieri) are thought to be the result of habitat alteration and persecution.54 Habitat destruction is the greatest concern for many at-risk owl populations, including the Blakiston’s fish owl (Bubo blakistoni), the northern spotted owl (Strix occidentalis caurina), and many tropical owl species. A new species, the Rinjani scops owl (Otus jolandae) in Indonesia, has recently been discovered.46 Owls are found worldwide with the exception of Antarctica and some very remote islands. Most owls are nocturnal, with some species demonstrating crepuscular behavior and a few species hunting during the day. Owls possess several unique anatomic and physiologic adaptations relative to other birds or even other raptors. The skull design optimizes two critical senses for owls—hearing and vision. In up to one third of all owl species worldwide, large ear openings are placed asymmetrically on each side of the head to facilitate vertical location of sound. The right opening points upward and the left downward. The asymmetrical placement is critical for species that are nocturnal hunters, those that reside north of 35 degrees latitude where heavy snow cover often prevents visualization of prey, or both.31 Horizontal location of sound is assisted by a wide skull. Another cranial adaption in owls is found in the large, forward-facing eyes, which provide 60 to 70 degrees of binocular vision and a high level of stereoscopic vision for judging distances. The eyes are tubular in shape and have relatively large corneas for gathering light. The retina is specialized for dim-light vision, possessing more rods than cones (up to 56,000 per millimeter square [mm2] in the tawny owl, Strix alluco), and the rods contain high levels of rhodopsin, a light-absorbing pigment.31 In many species, the retina also has a tapetum lucidum, a reflective layer that increases the amount of light each rod receives. Unlike some other bird species, owls cannot detect ultraviolet (UV) light. Owls are far sighted and use the tactile bristle feathers around their beaks to feel objects up close. Owls have several unique anatomic differences in their gastrointestinal (GI) tracts relative to diurnal raptors. Unlike hawks, they do not possess a crop (dilation of the esophagus that stores food). Ingested food passes directly into the proventriculus, or glandular stomach. The pH of the ventriculus in owls averages 2.2 to 2.5 and does not provide sufficient acidity to break down fur, feathers, or bones. Through muscular contractions, the ventriculus forms a pellet, a compact bundle of indigestible foodstuffs, which is then cast at a meal-to-pellet interval of 10 to 13 hours.11 Owls do possess ceca, paired secretory organs at the juncture of the ileum and the colon. Fermentation (especially of cellulose), water and calcium resorption, and microbial action of both beneficial and disease-causing organisms occur in the ceca.33 Because of the blind-ended nature of these organs, food stuffs remain longer than in the rest of the GI tract, resulting in a product that is brown, homogeneous, and odiferous when excreted. Owls may eliminate their cecal contents in response to stress. The foot of an owl is zygodactylous. When perched, digits 2 and 3 face anteriorly and digits 1 and 4 face posteriorly. Digit 4, however, is opposable and may assist in the restraint of prey by being placed in the forward position. The distal tibiotarsus is more rounded in owls compared with hawks, relating to the zygodactylous positioning of the digits. The tendons associated with the muscles of the tibiotarsus are calcified, providing increased strength to leg muscles, which are exposed to high stress forces.58 Determination of the age (aging) of owls on the basis of the molt pattern of flight feathers has been studied in a variety of North American owl species. The identification of multiple generations of feathers may be aided by using UV light to fluoresce porphyrin pigments. Distinct molting patterns may assist in the aging of many owl species up to age 3 or 4.59 Reverse dimorphism exists in many owl species. For example, size may often be used to sex snowy owls (Bubo scandiaca), northern saw-whet owls (Aegolius acadicus), boreal owls (Aegolius funercus), and great gray owls (Strix nebulosa), since less overlap exists in weight ranges between the sexes. In the northern saw-whet owl, wing chord measurements may also be used.37 In the snowy owl, distinct plumage differences, such as the number of bars on the tail and the amount of spotting on the back of the head may also be used to determine sex.47 It is critical to have a working knowledge of each owl species’ natural history to understand their captive housing and management needs. The choice of caging material and design should ensure that feathering is not damaged as the bird moves around the enclosure. Wood and some plastics may be good choices, whereas metal caging (chain-link, metal mesh, etc.) may be extremely damaging to the feathers, feet, and ceres of raptors. Consideration should be given to the flooring substrate if the owl will spend any amount of time on the ground. Small gravel (average 5 mm diameter) is the preferred choice for substrate that comes in direct contact with the bird. Most enclosures work best with two to three solid sides, multiple, strategically placed perches, access to water for bathing and drinking, and at least one area in which the bird may hide from the elements or from being viewed by the public. Shelter boxes are recommended for cavity nesters.2 Multiple owls may be maintained in one enclosure, although it is safest to not mix species in one display. Within a species, multiple-bird housing may work very well, but if the enclosure is not large enough to allow for personal space, aggression may occur. Aggression may also be a problem with new introductions into an established exhibit; adequate monitoring should be ensured. As many owls kept in exhibits have disabilities, their additional needs should also be considered when housing multiple birds together. Owl diets2,3 are diverse and vary by species in relationship to size, habitat, and feeding behavior. Small rodents comprise the bulk of most diets, but owls are opportunistic and feed on insects, invertebrates, fish, amphibians, reptiles, birds, small mammals, and bats. Captive diets include mice, rats, day-old chicks, quail, fish, chicken, guinea pig, and rabbit. Wild or domestically raised pigeons should not be fed to owls because of the risk of trichomoniasis and a host of viral diseases. Feeding hunter-killed prey sources carries the risk of lead poisoning from spent lead ammunition. Dead wild rodents and birds should also not be fed to owls, as these prey items may be a source of poisoning or diseases such as West Nile virus (WNV) infection. The food should be presented on a raised feeding area, which is easily accessed and protected from the elements and contamination from vermin. Most owl species should be fed once a day; smaller species may require twice-a-day feeding. Feeding is usually done late in the day. Exceptions include freezing temperatures and accommodating species that are more active during the day. A wide variety of whole-prey items should be offered. Food should be wholesome, freshly killed, or properly frozen and thawed to prevent nutrient loss and to limit microbial load. The intestines of previously frozen mammals and poultry (except day-old chicks) should be removed, as these items are a potential source of Clostridium. Intake should be monitored, and uneaten food should be promptly removed. Supplementation is not usually required if owls are fed good-quality whole food items. Exceptions are thiamine and vitamin E supplementation needed for diets high in fish content, breeding situations, and growing chicks. A source of water for drinking and bathing should always be made available, except during freezing temperatures and in medical housing. Hunting behavior may be used for behavioral enrichment in some species. Live crickets, mealworms, crayfish, frogs, and fish have been introduced into owl enclosures. A diet of live food may carry some risks, including parasites, injuries from the prey, and poor public reception. Perches should be placed strategically to help the bird feel comfortable in the enclosure and provide enriching views. Since owls perch in areas where they feel safe and not necessarily on perches that are the best for the health of their feet, it is critical to provide them with several suitable perches. For most owl species, rounded or beveled perches work best. These may consist of dowels, beveled 2 × 4 inch (or 5 × 10 cm) wooden boards cut at species-specific angles or natural branches (oak is recommended) of varying diameter.2 Generally, a rounded perch should not be so wide that the owl’s foot is flattened when the bird perches. Also, if natural branches are used, they must be replaced every few months or sooner when the bark wears off, leaving a smooth surface. If an owl develops bumblefoot, perch locations, sizes, and substrates should be evaluated, focusing on those the owl uses most frequently. The location of the lesions on the feet may further assist in identifying the problem. Feathers may be damaged by perches, enclosure walls, ceiling, and floors. Bent, tipped, or broken feathers are all signs of management problems and need to be addressed to stop further damage. For example, perches should be placed far enough from the wall so that when a bird turns around, it does not brush or rub its wing or tail feathers against the wall. Broken feathers may be repaired by a process called imping, in which a molted feather from the same species, sex, and feather position is used to replace the broken one. A short piece of whittled bamboo (or guitar string in small owls) is glued into the hollow shaft of the broken feather and used to secure the replacement feather.2 To prevent breakage, bent feathers may be straightened either with a feather straightener or a small moist rag heated for 30 seconds in the microwave oven. In captivity, the beaks and talons of owls need regular maintenance, as they grow throughout the year. In the wild, natural wearing and reshaping occur with exposure to varying weather conditions, larger bone sizes of prey, and a variety of uneven surfaces that owls rub (feak) their beaks on to clean and maintain the shape. The manual trimming and reshaping of beaks is called coping and is most often done with a rotary tool such as a Dremel rotary tool. When using the tool, care must be taken to ensure that the facial bristle feathers do not get caught by the rotating bit. If this happens, serious injury may result. Recommended preventive medical measures of owls include monitoring weight on a frequent basis; routine physical examinations; obtaining baseline hematology and chemistry values; baseline radiography; periodic fecal examinations; serology, as appropriate; plasma banking, as practical; vaccinations in species susceptible to WNV; and prophylactic medication in species susceptible to plasmodiasis and aspergillosis. Blood smears and the buffy coat should be evaluated for hemoparasites. As in all species, a thorough, systematic examination is the cornerstone diagnostic and should be conducted in a fashion similar to that in other birds. Appropriate restraint is required for handler and patient safety and to minimize patient stress. Traditional diagnostic tests such as hematology and blood chemistry, imaging, parasitology, bacteriology, cytology, and necropsy are all applicable to owls, although it may be difficult to find species’ normal values to compare results. Often, only a single case report or the result from a closely related species is available for comparison. Establishing baseline values for hematology, chemistry, and radiology during routine physical examinations may provide important information to offset these challenges. Select hematology and chemistry results are listed in Table 23-1. TABLE 23-1 Select Physiological Reference Intervals for Select Owl Species25
Strigiformes
Biology
Anatomy and Physiology
Management
Housing
Diet
Management of Feet, Feathers, Beaks, and Talons
Preventive Medicine
Diagnostics
Tests
Units
Short-eared Owl
(Asio flammeus)
Burrowing Owl
(Athene cunicularia)
Eurasian Eagle Owl
(Bubo bubo)
Verreaux’s Eagle Owl
(Bubo lacteus)
Snowy Owl
(Bubo scandiacus)
Great Horned Owl
(Bubo virginianus)
Mean
Reference Interval
Mean
Reference Interval
Mean
Reference Interval
Mean
Reference Interval
Mean
Reference Interval
Mean
Reference Interval
White Blood Cell Count
*103 cells/µL
8.62
0–17.16
7.44
2.20–16.49
12.77
3.76–30.69
14.00
0–26.53
9.78
3.06–26.11
13.08
4.14–27.71
Red Blood Cell Count
*106 cells/µL
2.44
*
2.39
1.33–3.46
2.28
1.39–3.16
Hemoglobin
g/dL
11.10
4.5–17.4
13.40
8.02–18.30
Hematocrit
%
43.40
33.00–53.1
44.70
29.40–55.00
39.60
29.10–47.80
36.50
26.10–46.70
43.00
28.10–54.10
41.30
32.60–51.20
MCV
fL
179.00
*
184.50
110.00–256.90
176.50
134.80–221.50
MCH
pg
42.50
10.40–68.40
58.80
37.30–81.30
MCHC
g/dL
25.00
10.80–38.08
32.20
22.90–41.30
Heterophils
*103 cells/µL
3.67
0–8.74
4.00
0.97–10.92
6.88
1.76–18.59
7.81
0–16.52
4.78
1.25–12.71
7.37
2.14–17.13
Lymphocytes
*103 cells/µL
3.55
0.00–7.55
2.53
0.50–6.82
4.68
0.87–14.50
4.72
0–11.84
3.74
0.74–12.05
4.18
0.88–11.01
Monocytes
cells/µL
378.00
0–1286
278.00
0–1199
394.00
0–1952
328.00
0–899
271.00
0–1192
537.00
0–2215
Eosinophils
cells/µL
663.00
0–2924
338.00
0–1704
595.00
0–3401
879.00
0–2857
226.00
0–1322
599.00
0–3174
Basophils
cells/µL
131.00
0.00–572
169.00
0.00–915
147.00
0.00–770
99.00
0–420
83.00
0–511
196.00
0–1157
Glucose
mg/dL
299.00
212–395
321.00
209–450
350.00
281–426
317.00
222–409
335.00
221–456
336.00
256–417
Blood Urea Nitrogen
mg/dL
7.00
1–12
6.00
0–11
Creatinine
mg/dL
0.30
0–0.70
0.50
*
Uric Acid
mg/dL
9.10
0–16.70
8.60
1.80–25.90
9.20
2.50–22.90
8.80
0–17.40
9.00
2.60–20.20
9.00
3.00–19.80
Calcium
mg/dL
9.20
7.30–10.80
9.40
7.30–12.00
9.80
8.00–13.00
10.00
8.00–11.70
9.50
7.40–11.60
9.40
7.70–11.60
Phosphorus
mg/dL
4.70
*
3.90
1.30–9.10
5.60
0.60–9.60
4.70
1.70–8.00
4.80
1.50–10.30
5.30
1.90–11.40
Ca/Phos ratio
3.00
1.00–6.70
1.90
0.70–3.10
2.20
0.60–3.60
2.40
1.00–5.00
2.20
0.90–4.80
Sodium
mEq/L
153.00
135–169
155.00
142–167
155.00
143–165
156.00
140–174
157.00
143–173
Potassium
mEq/L
2.50
0.30–4.50
3.10
0.90–5.00
3.20
1.80–4.50
3.00
1.50–6.10
3.00
1.20–5.00
Na/K ratio
67.10
7.50–116.80
55.10
16.40–87.90
50.80
27.30–72.30
58.00
27.50–103.40
57.80
25.10–120.20
Chloride
mEq/L
118.00
107–128
119.00
107–129
120.00
*
116.00
107–127
118.00
101–130
Total Protein
g/dL
3.30
1.90–4.40
3.50
2.50–4.80
3.70
2.50–5.20
4.40
3.10–5.70
4.00
2.40–6.50
3.80
2.60–5.60
Albumin
g/dL
1.60
0.50–2.50
1.60
0.80–3.30
1.80
0.10–3.20
1.60
0.80–2.40
1.50
0.90–2.50
1.60
0.80–3.10
Globulin
g/dL
1.80
0.60–2.80
1.90
0.30–3.10
1.60
0–3.50
2.80
1.70–3.90
2.30
0.30–4.60
2.30
0.40–4.40
Alkaline Phosphatase
IU/L
55.00
0–106
31.00
5–58
39.00
11–111
51.00
16–163
Lactate Dehydrogenase
IU/L
367.00
0–1071
274.00
0–628
662.00
0–1812
490.00
0–1134
Aspartate Aminotransferase
IU/L
250.00
0–447
164.00
68–322
164.00
55–331
142.00
36–230
272.00
108–570
188.00
86–347
Alanine Aminotransferase
IU/L
120.00
12–215
38.00
*
34.00
0–66
32.00
0–70
Creatine Kinase
IU/L
446.00
0–1012
428.00
94–1235
485.00
0–1080
298.00
0–596
584.00
140–1592
633.00
128–1688
Gamma-glutamyltransferase
IU/L
5.00
0–16
Amylase
IU/L
731.00
211–1296
679.00
*
270.00
97–435
385.00
0–830
Total Bilirubin
mg/dL
0.20
0–0.40
0.20
0–0.60
Cholesterol
mg/dL
250.00
99–378
191.00
105–280
218.00
89–330
237.00
143–364
184.00
112–298 
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Strigiformes
Chapter 23
