Biology

The order Caprimulgiformes (nightjars and allies) comprises five families and 120 species of large-eyed, wide-mouthed, superbly camouflaged birds. Family Podargidae (frogmouths), Family Aegothelidae (owlet-nightjars), and Family Caprimulgidae (nightjars and nighthawks) are predominantly from Australasia. The European nightjar (Caprimulgus europaeus) is migratory between Europe and Africa. Family Steatornithidae (oilbirds) and Family Nyctibiidae (potoos) are from South America. Despite their superficially similar external appearances, taxonomists argue that Caprimulgiformes birds differ distinctly in many anatomic features. Strong evidence suggests sister taxa status between Aegothelidae (owlet nightjars) and the diurnal Apodiformes (swifts and hummingbirds) and that perhaps all six families belong to a clade with a shared common ancestor.²⁰ Caprimulgiformes species also share morphologic affinities with Strigiformes (owls).⁹

Most birds in this order are nocturnal and insectivorous and live in bonded pairs during the breeding season, but the oilbirds set themselves apart by living in colonies in caves by day and feeding on fruit by night.

The tawny frogmouth (Podargus strigoides) will be the main focus of this chapter because among the members of this order, it is the most commonly maintained one in captivity, with 273 specimens in 92 institutions worldwide (International Species Information System [ISIS], 2012).

A distinctive feature of all Caprimulgiformes species is their excellent camouflage. Species that roost and nest in the open by day rely on their cryptic coloring and cryptic postures for protection (Figure 24-1). Nightjars roost and nest on the ground, and their colors match their local substrate. When danger approaches, birds of this order flatten their plumage, extend their neck, close their eyes to mere slits and remain motionless to blend into the background. The plumage of all Caprilmulgiformes species is not only intricately shaded but also soft, loose, and fluffy, facilitating both camouflage and silent flight.⁵

Sensory rictal bristles on the face are another feature of the order, although they are absent in potoos. These bristles also assist with camouflage by obscuring the outline of the beak.

A naked vestigial uropygial gland is present in most species but absent in frogmouths and potoos, which maintain their plumage with the assistance of large femoral powder down patches. Powder down is absent in the other families.^5,20

Respiratory System

Most Caprimulgiformes species produce their vocalization via a tracheobronchial syrinx. Oilbirds have an asymmetrical bronchial syrinx with which they produce echolocating sonar clicks, which enable them to navigate in the absolute darkness of roosting caves (Figure 24-2).^5,13,25

Gastrointestinal System

All Caprimulgiformes species have a vestigial, flaplike tongue, which contributes little to the swallowing process (Figure 24-3). Caprimulgiformes species have no crop, and large ceca (5 centimeters [cm] in tawny frogmouths) are present in all species except owlet-nightjars.^5,20

Caprilmulgiformes species have low basal metabolic rates (BMRs) compared with other birds, with the Podargidae having the lowest avian metabolic rate (40% to 70% of the BMR for an equivalent-sized nonpasserine). This low BMR is reflected in unusually low physiologic values of body temperature in the order of 37° C to 38.5° C, heart rate of 125 to 150, and respiratory rate of 10 to 20.^9,16 Tawny frogmouths and potoos are heat tolerant but will pant when the ambient temperature exceeds 40° C.^16,17

Special Housing Requirements

Aviaries should be large enough to allow flight when the birds are active at night, with a recommended minimum measurement of 3 meters (m) width, 6 m length, and 3 m height. Large-gauge wire mesh may be used to allow nocturnal insects to enter the aviary. Vegetation should simulate a eucalypt woodland, and natural perching should have a range of diameters and heights to allow choice and avoid pododermatitis. An undercover area, approximately one third of the aviary, should contain high perching for day time roosting in an area visible to the public. Frogmouths often choose to roost in sites exposed to heavy rain. Hollow logs placed vertically and forked perching with thick textured bark in a colour that blends with the birds’ plumage makes an attractive display (see Figure 24-1). A natural aviary substrate provides extra prey and behavioral enrichment, and a sand area under roosting perches facilitates cleaning. The diurnal roosting behavior and temperament of tawny frogmouths allows a unique display opportunity in that they may be placed on a perch outside an aviary or in a classroom, and they will usually sit tight for hours if provided with some browse for cover and minimal supervision.

Owlet-nightjars roost in hollows during the day and so are not suited to outdoor displays unless perspex viewing ports or spy cameras are used. Owlet-nightjars require at least two horizontally placed roosting logs or nest boxes per bird, fixed high on walls in a sheltered area of the aviary.³ Nocturnal house displays have been tried with varying success.

Tawny frogmouths acquire most water from their food or from rain. Their legs are unsuited to walking to the edge of a pond to drink. An elevated, broad, shallow water supply should be available near the favored roost site, and a range of perching should be available so that the birds may sit in the rain if they choose to. Nightjars have been observed taking water on the wing, much like swallows, and skimming along the surface of a lake.

Most members of this order are adapted to a diet of nocturnal insects and small vertebrates, with the exception of the oilbird, which is a frugivore. The wild tawny frogmouths diet consists of 78% insects, 18% other invertebrates (worms, slugs, and snails), and 4% vertebrates (small mammals, amphibians, reptiles, and birds).⁹ The proportion of vertebrates in the diet increases in winter when insects are less abundant. The heavily ossified and muscled bills of Caprimulgiformes species form a stong snap trap, enabling them to eat larger prey, which they crush or vigorously beat on branches before swallowing.⁹

Most food is obtained by pouncing to the ground from a tree or other elevated perch. Flying insects are caught on the wing and swallowed whole. Ingested grit and stones help break down prey.^5,9,11

Oilbirds eat the fruits of a wide range of tree species, predominantly palms, laurels, and incense trees. They feed on the wing and swallow the fruits (up to 6 cm in diameter) whole. The seeds are regurgitated, and mounds of decaying seeds are left on the floor of their roosting caves.⁵

Restraint and Handling

Most Caprimulgiformes are docile birds in captivity and are assessed as low risk or innocuous to handlers. Usually, handlers do not need gloves or protective clothing to protect themselves, but a towel is useful to handle aggressive wild frogmouths. Most species adopt their stick posture when approached during the day and are then easy to capture by hand. Net capture may be required at times, and the occasional individual may be aggressive and fly at the face of keepers when approached. Such birds may generally be gradually conditioned with food rewards to remain perched. The beak of a tawny frogmouth may exert significant crushing force, so handlers should avoid bites by grabbing wild or aggressive birds from behind and controlling the head. The feet of Caprimulgiformes are weak and harmless. The smaller species drop their feathers to avoid predation, so they must be handled gently but firmly. Owlet-nightjars are nervous birds but are caught easily during the day from their roost logs or boxes.

Anesthesia and Surgery

Preanesthetic evaluation is recommended, as well as stabilization of dehydrated or debilitated birds with warmed subcutaneous lactated Ringer solution (up to 40 milliliters per kilogram [mL/kg]). Isoflurane administered via a T-piece and face mask and then via endotracheal tube is the anaesthetic of choice (typically 5% induction, 1% to 2.5 % maintenance to effect). The epiglottis is absent in avians, which increases their susceptibility to aspiration. For birds weighing 200 to 400 g, fasting for 2 to 4 hours, followed by intubation with an uncuffed tube, is advisable. The phalanges make a suitable attachment point for pulse oximetry. The birds should be placed in lateral or ventral recumbency as soon as possible after surgery to reduce inspiratory effort.

Traumatic injuries requiring surgery are common. Closed midshaft fractures of the radius or ulna where one bone is still intact have an excellent prognosis, with a “figure-of-eight” support bandage immobilizing the elbow and the carpus for 2 to 3 weeks, followed by early ambulation to avoid contracture of the patagium. Open fractures carry a worse prognosis, but surgical repair is certainly possible. Fractures within a centimeter of a joint have an unfavorable prognosis because of the possibility of arthrodesis caused by diffuse calcification common in avian fracture healing. Serious oral injuries, including beak fractures, pharyngeal lacerations, and tongue injuries, may occur from mouth-to-mouth fighting between incompatable individuals. Pharyngeal wounds may be so deep that they progress to osteomyelitis and septicemia.²¹ Beak fractures may be repaired successfully by wiring. The tongue also may be injured during force-feeding, as it may be pushed back and creased, later dropping off at the site of trauma.²¹ Injured tongues generally heal well. Tawny frogmouths have a high risk of trauma caused by motor vehicles because automobile lights illuminate prey, which attracts the birds to approach the roads for foraging. Owlet-nightjars and nightjars are at greater risk of predation becaue of their small size and ground dwelling habits.

Cataracts are common in captive and rescued tawny frogmouths, generally occurring secondary to trauma and may be removed via phaco-emulsification if preoperative electroretinography indicates retinal health and a likely return of sight.

Nociception in birds is similar to that in mammals.¹⁸ Meloxicam is the analgesic and anti-inflammatory agent of choice at a dose of 0.3 to 0.5 mg/kg intramuscularly (IM), intravenously (IV), or orally (PO), twice daily (BID).¹⁸ Other nonsteroidal anti-inflammatory drugs (NSAIDs; diclofenac, carprofen, flunixin, ibuprofen, and phenylbutazone) have been associated with nephrotoxicity, visceral gout, and mortality in Caprimulgiformes species, but with administration of meloxicam to over 700 birds from 60 species, no mortalities have been reported.⁷