Strigiformes

(Owls)

Class Aves

Order Strigiformes

Number of families 2

Number of genera, species 27 genera; 206–215 species

Evolution and systematics

Barn owls (Tytonidae) and typical owls (Strigidae) constitute a distinctive order (Strigiformes) of nocturnal predators. With the possible exception of owlet-nightjars (Aegothelidae), they are unlikely to be confused with any other group of birds. Like diurnal birds of prey (Falconiformes), they have a strongly hooked beak and sharp talons that they use to capture live animals. This convergence in morphology and behavior led early taxonomists to classify hawks and owls in the same order, the Raptores. However, morphological and genetic data clearly indicate that nightjars (Caprimulgiformes), rather than hawks, are the closest living relatives of owls.

The oldest known fossil owl, Ogygoptynx wetmorei, was found in 58-million-year-old Paleocene deposits from Colorado. Fossil owls from the Tytonidae and three extinct families are known from roughly 50-million-year-old Eocene deposits in North America and France. Bubo poirrieri and Strix brevis from the Lower Miocene of France and the United States, respectively, at 22–24 million years old, are the oldest fossils attributed to the Strigidae. Thus, tytonids likely arose before strigids in the evolutionary history of owls.

Determining the number of living species of owls has proven to be difficult because many congeneric species are similar in appearance. The discovery that vocalizations provide important clues to owl taxonomy has radically altered the view of how many species exist. Studies based on vocalizations, often supplemented with DNA evidence, have resulted in a much larger species list than was envisioned in the 1990s. In their 1991 book, Owls of the World, Rob Hume and Trevor Boyer recognized 151 species. By 2000, however, the leading references treated more than 200 species. Depending on which authority is followed, the grand tally rests between 207 and 215 species: 16–18 in the Tytonidae (two genera) and 189–197 in the Strigidae (25 genera).

Physical characteristics

Owls vary in size from the diminutive elf owl (Micrathene whitneyi) at 1.41 oz (40 g) to the massive Eurasian eagle-owl (Bubo bubo) at 9.25 lb (4.2 kg). Few other avian orders exhibit such a large range in body size. Most species have a large head, a short neck, a facial disc that surrounds forward-pointing eyes, and cryptic plumage. Strigids differ from tytonids in having a rounded rather than a heart-shaped facial disc, four rather than two notches in the sternum, a rounder skull with relatively larger eye openings, and a naked uropygial (oil) gland. In addition, the talon on the third toe has a smooth edge in strigids versus a serrated edge in tytonids.

Plumage colors are dominated by browns and grays distributed in complex patterns that help provide camouflage for roosting owls. Some tytonids have nearly solid white underparts, although their upperparts are cryptically marked. All tytonids have dark brown eyes. In contrast, many strigids have bright yellow irides (plural of iris), and the two species of white-faced owls (Ptilopsis) have striking orange-red irides. The sexes are virtually identical in appearance in most species; when differences exist, females tend to be darker and more heavily marked than males, especially on the underparts.

About half of the strigids have a distinctive plumage feature that is somewhat of a misnomer, the so-called ear tufts. Prominent ear tufts are restricted to nocturnal forest species and may function to provide camouflage during daytime by breaking up the outline of an owl that is perched in vegetation. Ear tufts have nothing to do with hearing. They are absent in tytonids except for the oriental bay owl (Phodilusbadius),

which has two short tufts formed by extensions of the upper edge of its facial disc.

Owls rely heavily on their keen sense of hearing to locate and capture prey at night. They can perceive a wide range of sound frequencies, and they use differences in arrival time and intensity of sounds at each ear to obtain precise information on the horizontal and vertical location of a sound source. The ability of owls to localize a sound source in this manner is enhanced by extremely large ear openings that are positioned asymmetrically on the sides of their head.

Owls have large eyes with large pupils, and unlike most birds, their retinas contain a preponderance of rod cells that are sensitive to low light. Despite their night-adapted vision, owls do not see well when it is extremely dark. On average, the minimum amount of light needed to see an object is two times lower in tawny owls (Strix aluco) than in humans, but variation in performance is such that the light-gathering ability of the most sensitive human eyes is similar to that of the least sensitive owl eyes. An owl's vision is sufficient for it to avoid obstacles in low light but does not allow it to see all objects below the forest canopy on the darkest nights. Thus, forest owls probably rely on spatial memory to avoid obstructions, or else they restrict their activity to the crepuscular hours when there is more light.

Distribution

Owls inhabit every continent but Antarctica. About 80% of the species occur in the tropics: 25% in Central and South America, 25% in Asia, 20% in Africa, and 10% in Australasia. The barn owl (Tyto alba) is nearly cosmopolitan, occurring throughout the tropics and in much of the United States and Europe. Six owl species are Holarctic, one of which, the short-eared owl (Asio flammeus), also has colonized the remote islands of Hawaii and the Galápagos. At the other extreme, 15 species of Otus have minuscule ranges on tropical islands in the Pacific and Indian oceans. Some mainland species also have restricted ranges. The Itombwe owl (Tyto prigoginei) is known from only two locations in the Congo, and the long-whiskered owlet (Xenoglaux loweryi) is confined to a small area in the Peruvian Andes.

Habitat

Owls occupy all major terrestrial habitats, from moist tropical jungles and temperate coniferous forests to grasslands and deserts, but approximately 95% of all species live in some sort of forested habitat. They also use human-altered landscapes such as farmlands, pastures, and suburban woodlots. Some Otus, Bubo, Glaucidium, and Ninox coexist with humans in wooded neighborhoods in many parts of the world. Fish-owls (Ketupa) of Asia and fishing-owls (Scotopelia) of Africa specialize on aquatic prey and are closely tied to streamside forests and mangroves, but no owls are strictly aquatic. In general, owls are absent from extremely high elevations and from the harshest deserts.

Behavior

Owls are well known for being nocturnal, but many species are active in daylight, including snowy owls (Nyctea scandiaca), short-eared owls, and most species of Glaucidium. By day, nocturnal species perch quietly in a concealed site, using their cryptic plumage to help blend into their surroundings. Owls often are considered fearless because they allow close approach by humans before taking flight. However, this "tameness" is part of their behavioral repertoire to avoid detection by predators.

Given that many species are nocturnal, it should come as no surprise that vocal displays are important in the lives of owls. An owl's hoot is its song, which typically is used to attract mates or to repel conspecific intruders. In Ninox, Aegolius, and Asio, songs are emitted exclusively by males and function mostly to attract mates. Once a male becomes paired, he stops singing and generally restricts his vocalizing to brief calls during food deliveries. In many other owls, both pair members emit similar songs for territory advertisement and courtship, although males are more vocal than females.

Adult owls typically exhibit little movement outside of their territories. Fewer than 20 species are truly migratory, and in only half of these is migration undertaken by a large segment of the population. Species that feed on cyclic rodents like voles and lemmings tend to be nomadic, moving large distances to new areas when prey populations dwindle at their previous breeding sites.

Feeding ecology and diet

Owls consume a wide variety of prey types, and although a few species occasionally eat carrion, all owls catch and eat live animals. Many species eat invertebrates. Seventy-five percent of the 40 species of Otus for which diet information is available appear to specialize on insects. Amphibians are not major prey item of any owl species, but fish-owls and fishingowls often eat frogs. Some Otus and Glaucidium incorporate snakes and lizards into their diet.

Birds seldom account for a large number of prey items in owl diets, although some Glaucidium seem to specialize on them. Small mammals such as shrews and rodents are major prey items for many medium and large-sized owls, especially in northern latitudes, and the larger eagle-owls (Bubo) often feed on medium-sized mammals such as hares, rabbits, skunks, and even monkeys.

Reproductive biology

Most owls defend exclusive breeding territories and are non-social while nesting. Burrowing owls (Athene cunicularia) and long-eared owls (Asio otus) do not defend territories and may nest as close as 50 ft (15 m) from conspecifics. Mating systems tend toward monogamy, with one male and one female maintaining an exclusive pair bond while caring for their young. DNA fingerprinting has been used in studies of genetic parentage in four species of strigids, and in each case the birds proved to be genetically monogamous. Extra-pair copulations have been observed in burrowing owls and flammulated owls (Otus flammeolus). Several species of Asio regularly form communal roosts of 10–30 or more birds during the nonbreeding season.

Most owls breed only once per year. Barn owls are striking exceptions, regularly breeding twice per year in temperate latitudes and virtually year-round in the tropics. Mammal-eating species often nest in late winter or early spring, whereas insectivorous species generally nest in late spring or early summer. To attract a mate, males begin vocalizing about a month before nesting begins. Breeding displays often involve courtship feeding in which males bring prey items to females. Copulations are seldom observed because they take place at night.

Owls have a decided penchant against nest building. Many of the medium-sized and large species use old stick nests built by other birds, niches in broken-top snags, or cavities in cliffs. Still others nest on the ground, including snowy owls, short-eared owls, and marsh owls (Asio capensis). The burrowing owl is unique in nesting below ground in burrows constructed by badgers, prairie dogs, and ground squirrels or in natural or man-made holes. The smallest species nest in tree cavities excavated by woodpeckers.

Owls lay white eggs that are roundish in shape. Small insectivorous species lay small clutches (two to four eggs), as do large carnivorous and piscivorous species. On average, the small-rodent specialists lay larger clutches (five to eight eggs), especially when prey are abundant. Eggs are usually laid at two-day intervals, and incubation typically begins with the

first egg, resulting in nestlings of very different ages within a single nest.

Females perform all of the incubation and brood-rearing duties, whereas males provide most of the food for the female and young. Incubation periods range from 22 days in the smallest species to 32 days in the larger ones. Upon leaving the nest, young of the cavity-nesting species are fairly well developed and somewhat adept at flying, whereas young of the open-nesting species leave the nest two to three weeks before they can fly, hopping or walking along tree branches to distance themselves from the nest. These "branchers" have high survival, and their early departure from the nest probably reduces their vulnerability to predation.

Conservation status

According to BirdLife International's Threatened Birds of the World, 27 owls are at risk of extinction, and another 20 species are considered Near Threatened. Fourteen of the 27 high-risk species are endemic to small islands, which are especially vulnerable to human disturbance and invasion by exotic predators, and 12 occur in tropical areas that have been devastated by timber harvest. Only one, Blakiston's eagle-owl Bubo blakistoni, occurs outside the tropics. It, too, is threatened by habitat destruction as well as by indiscriminate shooting. Two species of owls receive protection under the Endangered Species Act of the United States government. The ferruginous pygmy-owl Glaucidium brasilianum is listed as Endangered, and the spotted owl Strix occidentalis is considered Threatened. Populations of both species have declined due to destruction of critical habitat in the United States.

Habitat loss is the biggest problem faced by owls. Forest fragmentation disrupts the functioning of communities, and because owls are at the top of the food chain, they are highly susceptible to the negative effects of these changes. Moreover, some owls do not tolerate disturbance, so human encroachment can make an area unsuitable even under moderate levels of habitat alteration. Other threats that may be important on a local scale include collisions with automobiles and fences, electrocution, illegal shooting, and pesticides.

Significance to humans

Regarded as symbols of wisdom by the ancient Greeks and serving as a source of delight and wonderment into the twenty-first century, humans have been captivated by owls for millennia. Owls have been depicted on coins, currency, and in numerous forms of art, and they also have been the subjects of poetry and prose. Yet, they frequently elicit fear and superstition and have been considered bad omens throughout the world. In many cultures, owls were believed to foretell illness or death to people who encountered them. In Africa, it is still widely believed that owls are messengers of death, and to hear one is a sure sign of impending misfortune. In parts of China and Southeast Asia, legend says that owls relish the blood of newborn babies, and several Native American groups have stories of children being spirited away and eaten by owls.

Whether real or perceived, some owls have become a problem at airports because they are attracted to open areas surrounding runways that offer excellent habitat for small rodents. Hunting owls sometimes collide with aircraft, causing serious damage to both parties, but thus far no crashes.

On the positive side, educational efforts by scientists and conservationists have enabled many people to appreciate the potential for owls and other predators to help control rodent numbers. As a result, farmers and ranchers have installed specially designed boxes in the eaves of barns or placed them in forests to encourage owls to nest. Owls are also highly sought after by birdwatchers, who make special efforts to observe them throughout the world.

Resources

Books

BirdLife International. Threatened Birds of the World. Cambridge: BirdLife International, 2000.

del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 5, Barn-owls to Hummingbirds. Barcelona: Lynx Edicions, 1999.

Hume, R., and T. Boyer. Owls of the World. Limpsfield: Dragon's World, 1991.

König, C., F. Weick, and J.-H. Becking. Owls: A Guide to the Owls of the World. New Haven: Yale University Press, 1999.

Periodicals

Galeotti, P., and R. Sacchi. "Turnover of Territorial Scops Owls Otus scops as Estimated by Spectrographic Analyses of Male Hoots." Journal of Avian Biology 32 (2001): 256–262.

Marks, J.S., J.L. Dickinson, and J. Haydock. "Genetic Monogamy in Long-Eared Owls." Condor 101 (1999): 854–859.

Mueller W., J.T. Epplen, and T. Lubjuhn. "Genetic Paternity Analyses in Little Owls (Athene noctua): Does the High Rate of Paternal Care Select Against Extra-Pair Young?" Journal für Ornithologie 142 (2001): 195–203.

Rasmussen, P.C. "A New Species of Hawk-Owl from North Sulawesi, Indonesia." Wilson Bulletin 111 (1999): 457–464.

Roulin, A., C. Riols, C. Dijkstra, and A.-L. Ducrest. "Female Plumage Spottiness Signals Parasite Resistance in the Barn Owl (Tyto alba)." Behavioral Ecology 12 (2001): 103–110.

Seamans, M.E., R.J. Gutiérrez, C.A. Moen, and M.Z. Peery. "Spotted Owl Demography in the Central Sierra Nevada." Journal of Wildlife Management 65 (2001): 425–431.

Jeffrey S. Marks, PhD