General Biology

The species in the order Passeriformes are often referred to as “passerines” or “perching birds.” This is the most diverse order of birds, comprising more than half the known bird species, more than half the bird genera, and more than half the bird families. Passerines have a worldwide distribution and inhabit all but the circumpolar habitats. Passerines are often the most abundant bird taxa in any given habitat. Passerines are relatively small birds, the largest species being the common raven, which weighs greater than 1.5 kilograms (kg), and the smallest being the short-tailed pygmy-tyrant with an average weight of 4.2 grams (g).

The order Passeriformes is often subdivided into suboscines and oscines, the latter often being referred to as “songbirds.” Suboscines include the suborders Eurylaimi (broadbills, asities, pittas), Furnarii (ovenbirds, woodcreepers, antbirds), and Tyranni (cotingas, manakins, tyrant-flycatchers). Suborders Acanthisittae (New Zealand wrens) and Menurae (scrub-birds, lyrebirds) are often considered intermediate groups. All other passerines are in the subdivision of oscines.

Unique Anatomy

Passeriformes species are thought to be monophyletic, with the following anatomic features consistent across all species: (1) an aegithognathous palate, (2) unique syringeal anatomy, (3) an incumbent hallux (digit 1 is at the same level as the other digits of the foot) creating an anisodactyl foot, (4) unique arrangements of plantar tendons, (5) the tensor propatagialis brevis, which attaches to the forearm as well as the humerus, (6) bundled spermatozoa with a coiled head and large acrosome, and (7) distinctive foot anatomy, which allows independent action of the hallux.^10,34

Passerines have a crop, but the cecum is vestigial or absent in most species. The right and left nasal sinuses do not communicate as they do in psittacines. The cranial thoracic air sacs and the clavicular air sacs are fused in passerines, resulting in seven total air sacs rather than the usual nine seen in other birds. Most passerines have 10 primary remiges, although in some oscines the tenth primary is reduced or vestigial.¹⁰ Oscines have complex muscle morphology of the syrinx that allows for complex vocalizations, which are often learned. These traits make some passerines excellent mimics (e.g., mockingbird, mynah, etc.). The syrinx of suboscines is more basic and therefore produces less complex vocalizations. Several species of pitohui are known to have batrachotoxins in their skin and feathers, which serve as a toxic chemical defense.⁹ Much as in dart frogs, this toxin is believed to originate from insects in the diet.

Special Housing Requirements

Although small in size, passerines require ample amounts of enclosure space. In addition to enclosure size, elevated perching opportunities (above any perceived threats) and ample visual barriers (which may be achieved with live or artificial plants) may help to reduce stress in these birds. Chronic stress caused by inadequate enclosures or inappropriate husbandry is a major problem in passerines. Many of the common diseases in passerines (e.g., candidiasis, aspergillosis, mycobacteriosis, atoxoplasmosis) are considered to be caused by opportunistic pathogens that are often associated with immunosuppression resulting from chronic stress. Species-specific temperature requirements, social dynamics, lighting or photoperiod, and diets should also be carefully researched to reduce stress. Nest-building materials given to passerines should be carefully chosen, as some materials (especially fine synthetic fibers) are known to cause leg entanglement and constriction.²³ Small enclosures with wire mesh are known to cause feather damage (particularly rectrices and remiges) when the birds hang on the wire, possibly hindering flight. If a small enclosure is needed for transport or hospitalization, those with smooth, solid walls are preferred over those with wire mesh sides.

Feeding

Passerines have a metabolic rate that is approximately 60% higher than in other bird taxa. When calculating the basal metabolic rate (BMR) in kilocalories, the formula for passerines is BMR = 129 (W^0.75), where W is the weight in kilograms. This difference in metabolism may have an effect on drug pharmacokinetics, requiring higher dosages, more frequent dosing intervals, or both in passerines compared with other birds. Daily water requirements for passerines are also higher than for other birds and may be as high as 250 to 300 milliliters per kilogram (mL/kg) daily.

The feeding strategies of passerines are as diverse as the species. The order includes species that exhibit carnivory, frugivory, nectivory, granivory, insectivory, and various combinations of omnivory. Diets for captive birds should replicate the natural history of the species and should be as varied as possible to reduce nutritional deficiencies. Some of the passerine species within the superfamilies Corvoidea, Muscicapoidea, Sylvioidea, and Passeroidea lack the L-gulonolactone oxidase enzyme necessary to synthesize vitamin C and therefore require ascorbic acid in their diet.⁷ Some species with red-colored plumage (e.g., some Carduelis spp. and some Euplectes spp. ) require carotenoids in the diet to maintain the normal intensity of the red pigment in the feathers.

Some species of passerines (e.g., tanagers, birds of paradise, starlings, mynahs, and manakins) are sensitive to excess iron in the diet, which leads to iron accumulation (hemosiderosis) and damage (hemochromatosis) in the liver (see Noninfectious Diseases below). Diets containing between 25 and 50 mg/kg iron on a dry-matter basis have been shown to prevent hemochromatosis in iron-sensitive species.¹⁴ However, higher dietary iron content may be needed for breeding birds and growing chicks.²⁰ Reducing substances that may enhance iron absorption (e.g., citric acid, ascorbic acid), adding substances that may bind to iron in the diet to prevent absorption (e.g., tannins, phytates), or both are additional methods employed to prevent hemochromatosis in iron-sensitive species.²⁰

Feeding stations for flocks of passerines should be designed to allow sufficient space for all birds to feed. Multiple feeding stations are recommended to reduce starvation caused by conspecific and interspecific aggression. Perches should be positioned such that feces do not drop below to contaminate the feeding areas. In wild house finches (Carpodacus mexicanus), increased risk of mycoplasmal conjunctivitis caused by Mycoplasma gallisepticum has been associated with tube-style feeders, and the risk was lowered with platform-style feeding stations.¹²

Restraint and Handling

All passerines may be manually restrained safely for short, nonpainful procedures. Longer or painful procedures should be performed with the birds under anesthesia. Initial capture may be difficult and stressful. In large aviaries, smaller capture cages containing feeding stations to attract the birds as well as doors that may be closed remotely are helpful. Birds may either be lured into these capture cages with food, or they may be trained by using operant conditioning to regularly use the cages. Once in a smaller enclosure, hand nets are most often used for capture of passerines. Dimming the room lights may facilitate a quicker capture.

Passerines may be manually restrained by using one of two primary methods, depending on the species and size. The first technique may be used on most species (Figure 31-1). The bird’s neck is extended by using the thumb and the forefinger or by using the index and middle fingers, taking care not to apply excessive pressure to the structures within the neck. The body is then supported using the same hand. Pressure should not be exerted on the body cavity, as it may impede movement of the keel and result in hypoventilation, loss of consciousness, and rapid death. For larger passerines, the other hand may need to be used to restrain and extend the feet and legs. The second method of manual restraint is often employed by bird banders and should only be used on the smaller passerine species (Figure 31-2). The tibiotarsi of both limbs are held between the index and middle fingers. The hocks of both limbs are allowed to partially flex, and the tarsometatarsi of both limbs are then held between the index finger and the thumb. The body and the neck are left unrestrained. To avoid excessive strain on the limbs and to aid in normal respirations, the bird should be held in a normal sitting posture, with the majority of the weight resting on the top of the handler’s fingers.

Most therapeutic drugs used in passerines are the same as those used in other avian species. The higher BMR of passerines should be considered during selection of therapeutic dosages and dosing frequencies, as absorption, time to maximum concentration, and half-life may be altered significantly. Metabolic scaling may be of assistance when estimating dosages or dosing frequencies for some drugs used for the first time in passerines. Commonly used antimicrobial and antiparasitic drugs are presented in Tables 31-1 and 31-2.¹³ Because of the wide variations in species and the lack of pharmacokinetic data for most passerines, the use of any therapeutic drug warrants caution and careful monitoring of signs of overdose or lack of efficacy.

Venipuncture may be performed with the same technique used in most other avian species. For passerines, the right jugular vein is the preferred site. The ulnar or medial metatarsal veins may also be used but may be limiting because of the small size. Clinicians should be very careful about the volume of blood collected. For healthy, hydrated passerines, blood up to 1% of the bird’s body weight may be obtained (i.e., 1 mL for every 100 g of body weight). This volume should be reduced in debilitated birds or those with a history of blood loss. Lithium heparin may be used as an anticoagulant for passerines, although it may affect the complete blood cell count (CBC) through clumping of white blood cells (WBCs), artifactual changes during the staining process, or both. EDTA (ethylenediaminetetraacetic acid) may be used in some species, but it is known to cause lysis of erythrocytes in some passerines (e.g., corvids). For passerines, the lymphocyte is the predominant WBC, and lymphocytosis may be the primary response to stress. The packed cell volume (PCV) of most passerines falls between 40% and 55%, a PCV of less than 35% indicates anemia. Smears made from the buffy coat of a hematocrit tube increase the likelihood of identifying intracellular parasites such as Atoxoplasma spp. within mononuclear cells.²⁹ The reference intervals for the complete blood count and serum chemistry values for some of the passerines commonly kept in zoos is included in Table 31-3.

TABLE 31-3

Reference Intervals for the Complete Blood Count and Serum Chemistry Values for Some of the Passerines Commonly Kept in Zoos*

Common Name	Common Shama Thrush	Raven	Golden-breasted Starling	Azure-winged Magpie	White-headed Buffalo Weaver	Fairy Bluebird	Red-billed Leiothrix	Bali Mynah	Red-capped Cardinal	Taveta Golden Weaver	Superb Starling	Blue-grey Tanager
Scientific Name	Copsychus malabaricus	Corvus corax	Cosmopsarus regius	Cyanopica cyana	Dinemellia dinemelli	Irena puella	Leiothrix lutea	Leucopsar rothschildi	Paroaria gularis	Ploceus castaneiceps	Spreo superbus	Thraupis episcopus
WBC (x103 cells/µL)	0.00–16.63 (7.87)	0.00–16.99 (8.61)	1.84–23.88 (8.83)	0.00–13.34 (6.66)	1.92–29.99 (9.72)	2.21–33.41 (11.94)	0.00–14.02 (6.41)	2.01–27.97 (10.00)	0.00–17.76 (7.56)	0.00–21.16 (9.40)	1.21–24.98 (7.97)	1.82–22.65 (7.68)
Hematocrit (%)	37.1–59.9 (48.5)	28.8–52.3 (42.9)	35.1–63.4 (49.1)	38.1–60.2 (49.1)	37.0–61.4 (51.4)	37.7–61.9 (50.5)	36.9–57.3 (46.4)	35.1–54.3 (44.2)	40.1–68.1 (54.1)	37.2–55.3 (46.3)	34.0–58.8 (47.6)	43.1–59.6 (51.1)
Heterophils (x103 cells/µL)	0.00–7.11 (2.61)	0.00–7.75 (3.92)	0.44–6.73 (2.32)	0.00–5.00 (2.24)	0.22–8.22 (2.80)	0.30–8.37 (2.83)	0.00–2.67 (1.09)	0.69–7.62 (2.92)	0.00–3.24 (1.45)	0.00–4.82 (1.98)	0.15–6.12 (1.82)	0.15–4.96 (1.40)
Lymphocytes (x103 cells/µL)	0.00–9.53 (4.08)	0.00–8.51 (3.61)	0.55–16.78 (5.47)	0.00–8.61 (3.58)	0.82–15.67 (5.11)	0.55–20.84 (6.31)	0.00–9.32 (3.91)	0.54–18.53 (5.54)	0.00–13.32 (5.12)	0.00–12.95 (5.53)	0.36–15.50 (4.47)	0.60–15.76 (4.70)
Monocytes (x103 cells/µL)	0.00–0.600 (0.184)	0.00–0.532 (0.178)	0.00–1.460 (0.298)	0.00–0.578 (0.166)	0.00–1.290 (0.270)	0.00–2.173 (0.506)	0.00–0.681 (0.219)	0.00–1.218 (0.284)	0.00–0.301 (0.087)	0.00–1.189 (0.155)	0.00–1.078 (0.224)	0.00–1.862 (0.363)
Eosinophils (x103 cells/µL)	0.00–0.893 (0.276)	0.00–0.885 (0.264)	0.00–1.053 (0.130)	0.00–0.377 (0.089)	0.00–1.789 (0.355)	0.00–1.697 (0.304)	0.00–1.298 (0.230)	0.00–1.371 (0.269)	0.00–0.245 (0.068)	0.00–1.004 (0.305)	0.00–1.656 (0.274)	0.00–0.567 (0.106)
Basophils (x103 cells/µL)	0.00–1.835 (0.632)	0.00–1.653 (0.363)	0.00–1.366 (0.244)	0.00–0.413 (0.092)	0.00–3.189 (0.521)	0.00–5.616 (1.185)	0.00–1.557 (0.511)	0.00–1.631 (0.328)	0.00–0.814 (0.166)	0.00–1.361 (0.454)	0.00–2.089 (0.352)	0.00–3.093 (0.651)
Glucose (mg/dL)	NA	302–464 (377)	218–474 (333)	205–542 (369)	214–507 (372)	188–412 (298)	NA	203–443 (324)	NA	NA	237–448 (345)	139–464 (297)
Uric acid (mg/dL)	NA	0.8–18.3 (6.1)	3.5–32.8 (13.5)	0.0–26.7 (13.5)	3.0–28.8 (11.5)	3.0–22.5 (10.2)	NA	2.3–20.7 (8.8)	0.0–24.6 (10.6)	0.0–23.4 (11.5)	3.3–27.9 (11.8)	1.4–18.8 (8.1)
Calcium (mg/dL)	NA	6.9–9.8 (8.4)	6.4–10.3 (8.2)	7.2–10.3 (8.9)	5.3–10.3 (8.4)	5.9–11.0 (8.5)	NA	6.7–10.3 (8.6)	NA	NA	6.7–10.3 (8.3)	5.4–11.9 (8.6)
Phosphorus (mg/dL)	NA	0.6–5.8 (2.3)	1.3–10.0 (4.3)	0.0–7.2 (3.4)	0.0–7.0 (3.6)	0.9–14.3 (5.0)	NA	0.8–8.5 (3.4)	NA	NA	0.4–8.4 (3.2)	0.0–7.3 (3.4)
Sodium (mEq/L)	NA	143–170 (156)	132–179 (157)	143–177 (161)	142–176 (159)	142–181 (162)	NA	139–177 (158)	NA	NA	144–169 (157)	NA
Potassium (mEq/L)	NA	1.2–5.0 (3.3)	1.0–5.4 (3.4)	0.2–4.4 (2.5)	0.4–4.1 (2.3)	0.7–4.7 (2.8)	NA	1.5–5.6 (3.0)	NA	NA	1.2–4.9 (3.2)	NA
Chloride (mEq/L)	NA	109–131 (120)	91–142 (117)	NA	106–132 (118)	105–134 (119)	NA	99–130 (119)	NA	NA	106–131 (118)	NA
Total protein (g/dL)	NA	2.6–5.3 (3.8)	2.0–5.9 (3.4)	2.1–4.4 (3.3)	1.6–4.8 (3.3)	2.3–6.2 (3.6)	NA	2.5–5.6 (3.8)	NA	NA	2.0–5.2 (3.4)	1.6–5.2 (3.5)
Albumin (g/dL)	NA	0.8–3.6 (2.0)	0.5–2.7 (1.4)	0.5–3.0 (1.8)	0.2–2.3 (1.3)	0.3–2.8 (1.7)	NA	0.9–3.3 (1.8)	NA	NA	0.5–3.1 (1.5)	0.0–3.7 (1.8)
Globulin (g/dL)	NA	0.4–3.5 (1.8)	0.5–3.3 (1.9)	0.2–3.0 (1.6)	0.2–3.7 (1.9)	0.4–3.5 (1.9)	NA	0.4–3.6 (2.0)	NA	NA	0.2–3.7 (1.9)	0.0–4.4 (1.9)
Alkaline phosphatase (IU/L)	NA	0–120 (64)	0–484 (226)	NA	0–345 (164)	0–170 (78)	NA	68–459 (223)	NA	NA	0–479 (246)	NA
Aspartate aminotransferase (IU/L)	NA	132–478 (275)	148–583 (316)	0–884 (458)	64–362 (229)	185–759 (359)	NA	145–507 (282)	0–424 (232)	NA	136–737 (324)	110–846 (355)
Creatine kinase (IU/L)	NA	0–318 (157)	75–1333 (448)	0–1087 (530)	0–907 (490)	340–2253 (893)	NA	154–1407 (535)	NA	NA	78–905 (343)	0–2013 (1030)
Cholesterol (mg/dL)	NA	93–271 (183)	101–278 (194)	NA	73–345 (216)	63–253 (164)	NA	103–279 (185)	NA	NA	66–204 (137)	66–243 (154)

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