STRIGIDAE Owls FAUNA PARAGUAY

STRIGIDAE - OWLS
Fifteen Paraguayan species in three subfamilies, two of which can be further split into tribes. The subfamilies are based on skull characteristics and supported by genetic evidence. Their characteristics are discussed below. Owls are instantly-recognisable, generally nocturnal, predatory birds. The oldest known fossils date back 58 million years to the Paleocene.
Owls have a strong, hooked bill, cere at the bill base, sharp, hooked talons and cryptic plumage. The head is large with a rounded facial disk encircling large, forward-facing eyes and the tail is short. All owls are carnivorous or insectivorous. Skeletally the skull is rounded, the furculum is not fused to the sternum and the sternum has four notches. The bill is short, stout, slightly depressed and hooked at the tip. The cutting edges are sharp and the cere is covered with thick bristles. The feet are zygodactyl with two toes facing forwards and two backwards, an adaptation for perching but undoubtedly also improving the grip on prey. The third toe is longer than the second and the uropygial gland is naked. They are usually encountered alone or in pairs (few species are social) and females are usually larger than males.
The facial disk is composed of feathers with open vanes and bordered by a facial ruff that channel air towards the ear-openings. The ruff is less-developed in species that hunt visually rather than by hearing (as is the case with most nocturnal species). The inner ear is larger than in other bird groups and the auditory region of the brain is loaded with nerve cells. Muscles associated with the ears enable the owl to change the shape of the ruff and better "direct" their hearing. Owl hearing is sensitive across a wide-range of frequencies and they are capable of detecting much lower frequencies than other bird groups. The large ear-openings are usually crescent-shaped and are surrounded by folds of skin. Some genera have asymmetrical external ears, particularly exaggerated in Aegolius, and this allows the owl to pinpoint the exact location of its prey by sound alone by comparing the time it takes for a sound to arrive in each ear and the differences in pitch. This method is most effective with complex sounds.
Despite popular thought owls do not have acute night-time vision and see well eyesight during the day. The eyeball is long and tubular with a spherical lens maximising retinal image size. The eyes are well-spaced to improve depth perception which can be enhanced by bobbing the head. The eyes are supported by a sclerotic ring and protrude slightly from the skull. The ring renders the eyes immobile and the bird must turn its head to increase its field of vision. Owls are able to rotate the head through 270 degrees, but contrary to popular belief this is not an unusual ability amongst months. Owls have a duplex retina, but unlike in other birds, the number of rods greatly outnumbers the number of cones. This gives improved sensibility in low light conditions, but is at the expense of some colour vision.
The plumage is soft, owls lack an aftershaft but have a downy feather base. The flight feathers have a soft covering which assists in silent flight and in most species the leading edge of the outer primaries has a comb-like fringe. It is absent in day-flying species such as Glaucidium, and it has been hypothesised that day-flying owls have less requirement for quiet flight than nocturnal species. The wings are large relative to body-size and provide low wing-loading. Species that hunt from a perch tend to have shorter, more rounded wings and species that hunt in flight have longer, more pointed wings. The moult is extended and year-round, with the retrices often being shed simultaneously leaving the bird tail-less for a period. Well-developed "ear tufts" are found only in nocturnal species but their function is uncertain, it having been suggested that they aid in breaking up the outline of the head at the day roost, or alternatively that they play a role in signalling.
Owls lack a true crop, food is often swallowed whole and passes from the oesophagus, into the proventriculus (glandular stomach) and on to the gizzard (muscular stomach). It remains in the gizzard for a long period, during which time the digestible liquid remains move on to the duodenum and the indigestible solid remains are compacted together into a pellet. Reverse peristalsis then operates to eject the pellet via the bill, often accompanied with violent head motions. Owls are incapable of digesting bone and investigating the contents of pellets enables the diet of owls to be easily studied.
The basal metabolic rate of owls is lower than in other bird groups, the plumage is denser and they show less variation in daily body temperature. They frequently suffer from heat stress and respond by reducing activity, compressing the contour feathers, spreading the wings and gular fluttering (rapid vibration of the hyoid bone in the throat).
The majority of owl species are nocturnal and spend the day roosting in a tree. Even the Burrowing Owl Athene cunicularia, a species frequently seen during the day, hunts mainly at night. Roosting owls rely on cryptic colouration for protection and may be approached closely. Many species are polymorphic. Initially at least they may sleek the plumage in an attempt to be less conspicuous. Owls are frequent targets for mobbing and studies suggest that they choose roost sites where they are least likely to be mobbed. Owls are often aggressive in the area of the nest and may adopt a threat posture with wings spread and feathers raised to increase apparent size. Vocalisations are innate, not learned, species specific and have been used as a means to determine species limits in the family. Most species respond well to playback of their own call and many respond to playback of the call of other species.
Typical owls are monogamous and many form long-term pair bonds. They do not build nests, using old stick nests of other species or cavities. Eggs are pure white. Those of cavity-nesting species tend to be more rounded as there is no danger of them rolling out of the nest. Incubation is by the female, the male spending the time hunting and feeding his partner as she sits. Females develop a large brood patch.

Subfamily Striginae: "Typical Owls"

TRIBE OTINI "SCREECH OWLS" - Two Paraguayan species in the genus Megascops.
The largest and most diverse owl group. New World species with two different vocalisations, a long, trilled territorial song and a shorter aggressive song. Characterised by small size, short legs, short, rounded wings and prominent ear-tufts.
TRIBE STRIGINI "WOOD OWLS" - Six Paraguayan species in the genera Strix (4) Pulsatrix (2).
Medium to large forest owls with rounded heads lacking ear-tufts. Asymmetrical external ear-opening, round facial disk and densely-barred plumage in Strix. Pulsatrix large, uniform brown plumage, powerful talons and bold facial markings. Pulsatrix produce a strangely vibrating song, Strix gives horrific screeches and wails.
TRIBE BUBONINI "EAGLE OWLS" - One Paraguayan species in the genus Bubo.
Largest and most powerful owls. Feathered toes and legs. Well-developed facial disk and prominent, widely-spaced ears. Asymmetrical external ear-opening. Quiet bubbling song at low frequency.

Subfamily Surniinae: "Little Owls"

TRIBE AEGOLIINI "SAW-WHET OWLS" - One Paraguayan species in the genus Aegolius.
Dumpy, large-headed. large-eyed owls with short tails and no ear-tufts. Relatively long wings and tarsi feathered to base of toes. Woodland habitat. Greatly exaggerated asymmetry in shape of slit-like ear-openings, extending the full height of the skull and extending to the bony structure of the ears.
TRIBE SURNIINI "PYGMY OWLS" - Two Paraguayan species in the genera Athene and Glaucidium.
Small, compact owls, insectivorous or bird hunters. Lack well-developed facial disk. Athene unique for social behaviour and nest in abandoned mammal burrows. Short tail, rounded wings and long legs with bristled toes. Male larger than female. Active by day in area around the nest but hunts at night. Glaucidium are tiny owls and are frequently diurnal or crepuscular in behaviour. They like comb-like fringes on the outer primaries. Toes and talons long. "False eyes" on the back of the head presumably act to dissuade predators and mobbing. Frequently cocks or flicks the tail.

Subfamily Asioninae: "Eared Owls"
Three Paraguayan species in the genera Asio. Slender with long wings and low wing-loading. Typically hunt by coursing harrier-like over open ground. Prominent ears located centrally on the head. Tarsi feathered and toes covered with short plumes. Plumage boldly-streaked below. Asymmetrical external ear-opening.

REFERENCES
Campbell B & Lack E 1985 - A Dictionary of Birds - T & AD Poyser.
Konig C, Weick F, Becking JH 1999 - Owls: A Guide to the Owls of the World - Pica Press.
Marks SJ, Cannings RJ, Mikkola H 1999 - Strigidae Handbook of the Birds of the World Volume 5 - Lynx Ediciones.