PASSERIFORMES Classification

asseriformes (perching and songbirds) is the largest order of birds. It contains nearly CHAPTER 60 percent of all bird species ranging in size P from the tiny Weebill (80 mm in length) to the Superb Lyrebird (130 cm long, including a 72 cm tail). Canaries, finches, starlings and mynahs are examples of passerine birds that are common in cap- tivity. Passerines are widely distributed throughout the world, and all passerines share a common aniso- dactyl foot structure with three unwebbed toes 43 pointed cranially and one caudally. The altricial young are usually naked when hatched and are reared in a nest. The order is believed to have originated in Gond- wana, the ancient southern continent. Convergent evolution makes differentiating species based on morphologic grounds difficult, and in the past, the anatomy of the syrinx has been a primary means of PASSERIFORMES classification. Recent DNA hybridization and protein electrophoresis studies are modifying traditional views. One group of passerines is now believed to have evolved and radiated in eastern Gondwana, proto-Australasia (these include such species as bow- erbirds, lyrebirds, birds of paradise, honeyeaters, Australian grass finches and fairy wrens). Another group, the suboscines or primitive songbirds are be- Patricia Macwhirter lieved to be of western Gondwanan origin (current- day South America). These include such primitive South American species as contingas, manikins, ant- birds and gnateaters. More advanced songbirds like Fringillidae finches, sparrows, warblers, starlings, thrushes and sunbirds may also have evolved via western Gondwana and migrated from this area to become widely distributed.10 While Passeriformes are widespread in the northern hemisphere, there are very few endemic families and many of the species are migratory.56 1173 CHAPTER 43 PASSERIFORMES digestive system. For example, although Cochlosoma sp. may cause inapparent infections in adult Ben- Anatomy and Physiology galese (Society) Finches, significant mortality may occur in juvenile Gouldian Finches being fostered by Bengalese parents.15 Passeriformes have become adapted to a wide variety Nestling Estrilid finches (eg, Gouldian and Zebra of ecological niches. The anatomic and physiologic Finches) normally have characteristic luminous differences expressed within the order reflect these mouth markings. Mucosal patterns are species-spe- specific evolutionary patterns (Table 43.1). cific and help to guide parents to their own chicks within the recesses of dark nests. Digestive System The spleen in most passerines is oblong, not spheri- cal, as it is in Psittaciformes. The basal metabolic rate (BMR) of passerine birds is generally about 65% greater than that of non-passerines, and their body temperature is about two de- Respiratory System grees Centigrade higher (around 42°C). While some In most Passeriformes, unlike in Psittaciformes, the desert passerines such as the Zebra Finch have been right and left nasal sinuses do not communicate. In known to survive months without drinking water, passerine birds, separate samples for rhinal disease most small passerine birds drink from 250 to 300 (bacterial, viral, chlamydial) cytology examination ml/kg body weight of water each day and may eat up should be taken from each sinus if a bilateral nasal to 30% of their body weight daily. These figures are discharge occurs (Figure 43.1).9 higher than those for most non-passerines, which tend to be larger birds. Singing ability is highly developed in many passerine species and is related to the complexity of the Most passerine species have a narrow, triangular syringeal anatomy. Some species have the capacity to tongue compared with the thick blunt tongue of par- sing duets with themselves by alternately using op- rots. The tongue is rarely involved in clinical problems. The tongue of passerines may become hyperkeratotic at the tip and extend rostrally through the beak. The syndrome appears to cause few clinical problems, but the hyperkeratotic tissue can be slowly trimmed back with a pair of strabismus scissors, taking care not to cut healthy mucosa. Other parts of the digestive tract differ depending on the species’ feeding patterns. A ventriculus is present in gra- nivorous and insectivorous species such as finches, but not in species such as honeyeaters that consume nectar and soft foods. If present, cecae are generally small and vestigial. Inexperienced Passeriformes breeders may present chicks for evaluation of a “sore” or “swelling” on the neck. This clinical sign is usually determined to be the crop distended with seeds and visible beneath thin, featherless skin. There is no production of crop milk in passerines as there is in pigeons, but some finches will regurgitate crop contents to feed their young. Pathogens can be FIG 43.1 Passeriformes, like this canary, have right and left transmitted from parent to offspring during this paranasal sinuses that do not communicate as they do in Psittaci- process, particularly with foster-raised chicks that formes. If bilateral oculonasal discharge suggestive of sinusitis is occurring, it is wise to collect samples from both the right and left probably did not receive yolk-derived antibodies sinuses for culture and cytologic evaluation. Conjunctivitis in this against microorganisms from their foster parents’ canary was responsive to tylosin therapy (courtesy of Michael Murray). 1174 SECTION SEVEN COMPARATIVE MEDICINE AND MANAGEMENT TABLE 43.1 Distinguishing Features of Selected Passerine Birds be traced to a single individual bird. The American mockingbird has simi- Approx. lar abilities and may mimic the General Family Number of Characteristics Composition sounds of companion Psittaciformes. Species Emberizidae New World 550 Cup-shaped open nest, female incubates, Like psittacine birds but unlike rat- finches, cardinals, diverse group, 9 primary feathers ites and penguins, passerine birds buntings, Cuban finch have a highly developed neopul- Estrildidae African, Asian 125 Palatal markings in young, monogamous, monic and paleopulmonic parabron- and Australian dome-shaped nest with side entrance, 10 chi. This allows for highly efficient finches, waxbills, primary feathers oxygen exchange. In most passer- nunias, parrot finches ines, the cranial thoracic air sacs are Fringillidae True finches, 125 Cup-shaped open nest, female incubates, 9 fused to the single median clavicular canaries, primary feathers, 12 long tail feathers sac, making a total of seven air sacs goldfinch, as opposed to the nine air sacs of chaffinch psittacine species. Passeridae Sparrows, finches 32 Bulky domed nest of dried grass, breed in colonies, seed-eating, finch-like birds Ploceidae Weavers, 145 Dome-shaped, covered, woven nest, do not Reproductive System whydahs, queleas sing, some species parasitic Sturnidae Starlings, mynahs 108 Dark, iridescent, or brightly colored, In general, only the left ovary and colonial, nest in holes, long straight bills, oviduct develop in normal female mimicry ability passerines. Occasionally a nonfunc- tional right ovary will develop in female sparrows. Both testicles develop in males and during the breeding season these may reach enor- mous proportions in relation to the size of the bird. These physiologically enlarged testicles should not be mis- taken for pathologic conditions. Role of Light in Reproduction Temperate-evolved species (including canaries) are usually dependent on daylight intervals for reproductive performance. Increased daylight hours trigger the release of luteiniz- ing hormone (LH) in responsive indi- viduals. The precise light interval varies among species but the physiologic control mechanism appears to be similar. Light stimulates photore- FIG 43.2 A hand-raised European starling can develop an extensive vocabulary and have a voice quality that is similar to its relative, the mynah bird (courtesy of Mark Spreyer). ceptors in the brain, probably in the hypothalamus, where there is a cir- cadian rhythm of photosensitivity. If posite bronchi. Roller canaries are specifically bred light coincides with the period of sensitivity, luteiniz- and trained for their singing ability. ing hormone releasing factor (LHRF) is released and The ability to mimic the human voice is well devel- gonadotrophin secretion is increased. If light coin- oped in some passerines, notably mynahs, starlings cides with the insensitive phase of the rhythm there and corvids (Figure 43.2). Among Australian passer- is no response. Gonadotrophin release in turn, trig- ines, lyrebirds are legendary in their ability to mimic gers the release of sex hormones. the calls of other bird species, and rain forest gullies There must be a rest period following a long light will re-echo with an apparent chorus of calls that can exposure to allow the photoreceptive system to reac- 1175 CHAPTER 43 PASSERIFORMES tivate and once again be responsive to increasing day and early molting. Molting causes an immediate ces- length. Stimulation of the reproductive cycle is best sation of breeding activity. accomplished by a progressive increase (four-week period) of exposure to light. In general, the maximum After a period of long daylight hours, birds become effect of increasing day length will occur when a refractory to photostimulation, and plasma concen- passerine individual is exposed to 10-14 hours of trations of both LH and FSH begin to fall. In male light. In males, the release of testosterone may occur White-crowned Sparrows, for example, this occurs in less than

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