Tytonidae – Barn & Grass Owls

The Tytonidae (Barn & Grass Owls) family are Strigiformes, along with Strigidae (true Owls or typical Owls). It includes just two genera; the barn owls Tyto and the bay owls Phodilu. The two modern genera are thought to have originated from a common ancestor from the Oligocene period. It is believed the modern genus Tyto descended from large nocturnal birds in the West Indies during the Quaternary.

The main characteristic of the barn owls is the heart-shaped facial disc, formed by stiff feathers which serve to amplify and locate the source of sounds when hunting. Further adaptations in the wing feathers eliminate sound caused by flying, aiding both the hearing of the owl listening for hidden prey and keeping the prey unaware of the owl. Barn owls overall are darker on the back than the front, usually an orange-brown colour, the front being a paler version of the back or mottled, although considerable variation is seen even within species. Bay owls closely resemble the Tyto owls, but have a divided facial disc, ear tufts, and tend to be smaller. They are medium to large owls with large heads long, strong legs with powerful talons. They also differ from the Strigidae in structural details relating in particular to the sternum and feet.

Sri Lanka Bay Owl Phodilus assimilis – ©Thomson Saburaj CC BY-SA 4.0 via Wikimedia Commons

The family is wide-ranging, although they are not very tolerant of severe winter cold, so are absent from northern areas of Europe, Asia, and North America; they are also absent from driest desert regions. They live in a wide range of habitats from semi-deserts to forests, and from temperate latitudes to the tropics. Within these habitats, they live near agricultural areas with high amounts of human activity. The majority of the 17 living species of barn owls are poorly known. Some, like the red owl, have barely been seen or studied since their discovery, in contrast to the western barn owl Tyto alba, which is one of the best-known owl species in the world. However, some subspecies of the western barn owl possibly deserve to be separate species, but are very poorly known.

Five species of barn owl are threatened, and some island species went extinct during the Holocene or earlier. Barn owls are mostly nocturnal and generally non-migratory, living in pairs or singly.

Some island subspecies are occasionally treated as distinct species, a move which should await further research into barn owl phylogeography as mitochondrial DNA shows little variation in many cases. Twenty to thirty subspecies are usually recognised, varying mainly in body proportions, size, and colour. Barn Owls range in colour from the almost beige-and-white nominate subspecies alba, erlangeri, and niveicauda, to the nearly black-and-brown contempta. Island forms are mostly smaller than mainland ones, and those inhabiting forests have darker plumage and shorter wings than those living in open grasslands. Several subspecies are generally considered to be intergrades between more distinct populations.

Greater Sooty Owl Tyto tenebricosa – ©JJ Harrison CC BY-SA 3.0 via Wikimedia Commons

The Barn Owl is the most widespread landbird species in the world, occurring on every continent except Antarctica. Its range includes all of Europe (except Fennoscandia and Malta), most of Africa apart from the Sahara, the Indian subcontinent, Southeast Asia, Australia, many Pacific Islands, and North-, Central-, and South America. In general, it is considered to be sedentary, and, indeed, many individuals, having taken up residence in a particular location, remain there even when better nearby foraging areas are available. In the British Isles, the young seem largely to disperse along river corridors, and the distance travelled from their natal site averages about 9 km (5.6 miles). In continental Europe, the dispersal distance is greater, commonly somewhere between 50 and 100 kilometres (31 and 62 miles) but exceptionally 1,500 km (932 mi), with ringed birds from the Netherlands ending up in Spain and in Ukraine.

In the United States, dispersal is typically over distances of 80 and 320 km (50 and 199 miles), with the most travelled individuals ending up some 1,760 km (1,094 miles) from their points of origin. Dispersal movements in the African continent include 1,000 km (621 miles), from Senegambia to Sierra Leone, and up to 579 km (360 miles) within South Africa. In Australia, there is some migration, as the birds move towards the northern coast in the dry season and southward in the wet season, as well as nomadic movements in association with rodent plagues. Occasionally, some of these birds turn up on Norfolk Island, Lord Howe Island, or New Zealand, showing that crossing the ocean is within their capabilities. In 2008, Barn Owls were recorded for the first time breeding in New Zealand. The barn owl has been successfully introduced into the Hawaiian island of Kauai in an attempt to control rodents; distressingly, it has been found to also feed on native birds

Hunting in twilight or at night, the barn owl can target its prey and dive to the ground. Its legs and toes are long and slender, which improves its ability to forage among dense foliage or beneath the snow and gives it a wide spread of talons when attacking prey. They hunt by flying slowly, quartering the ground and hovering over spots that may conceal prey. Their long, broad wings enable abrupt turns. They have acute hearing, with ears placed asymmetrically, which improves detection of sound position and distance; so, they do not require sight to hunt. The facial disc helps with the bird’s hearing, as is shown by the fact that, with the ruff feathers removed, the bird can still determine a sound source’s direction, although without the disc it cannot determine the source’s height. It may perch on branches, fence posts, or other lookouts to scan its surroundings.

Red Owl Tyto soumagnei – ©Dubi Shapiro

Rodents and other small mammals may constitute over ninety percent of the prey caught. Birds are also taken, as well as lizards, amphibians, fish, spiders, and insects. Even when they are plentiful, and other prey scarce, earthworms do not seem to be consumed. In North America and most of Europe, voles predominate in the diet, and shrews are the second most common food choice. In Ireland, the accidental introduction of the bank vole in the 1950s led to a major shift in the barn owl’s diet: where their ranges overlap, the vole is now by far the largest prey item. Mice and rats are the main foodstuffs in the Mediterranean region, the tropics, subtropics, and Australia. Gophers, muskrats, hares, rabbits, and bats are also preyed upon. Barn owls are usually specialist feeders in productive areas and generalists in areas where prey is scarce.

On the Cape Verde Islands, geckos are the mainstay of the diet, supplemented by birds such as plovers, godwits, turnstones, weavers, and pratincoles. On a rocky islet off the coast of California, a clutch of four young were being reared on a diet of Leach’s Storm Petrel Oceanodroma leucorhoa. On bird-rich islands, a barn owl might include birds as some fifteen to twenty percent of its diet, while in grassland it will gorge itself on swarming termites, or on Orthoptera such as Copiphorinae katydids, Jerusalem crickets Stenopelmatidae, or true crickets Gryllidae. Smaller prey is usually torn into chunks and eaten completely, including bones and fur, while prey larger than about 100 grams (3.5 oz) – such as baby rabbits, Cryptomys blesmols, or Otomys vlei rats – is usually dismembered and the inedible parts discarded.

Compared to other owls of similar size, the barn owl has a much higher metabolic rate, requiring relatively more food. Studies have shown that an individual barn owl may eat one or more voles (or their equivalent) per night, equivalent to about fourteen percent of the bird’s bodyweight. Excess food is often cached at roosting sites and can be used when food is scarce. This makes the barn owl one of the most economically valuable wildlife animals for agriculture. Farmers often find these owls more effective than poison in keeping down rodent pests, and they can encourage barn owl habitation by providing nesting sites.

Barn owls living in tropical regions can breed at any time of year, but some seasonality in nesting is still evident. Where there are distinct wet and dry seasons, egg-laying usually takes place during the dry season, with increased rodent prey becoming available to the birds as the vegetation dies off. In arid regions, such as parts of Australia, breeding may be irregular and may happen in wet periods, with the resultant temporary increase in the populations of small mammals. In temperate climates, nesting seasons become more distinct, and there are some seasons of the year when no egg-laying takes place. In Europe and North America, most nesting takes place between March and June, when temperatures are increasing. The actual dates of egg-laying vary by year and by location, being correlated with the amount of prey-rich foraging habitat around the nest site. An increase in rodent populations will usually stimulate the local barn owls to begin nesting, and, consequently, two broods are often raised in a good year, even in the cooler parts of the owl’s range.

African Grass Owl Tyto capensis – ©Derek Keats CC BY 2.0 via Wikimedia Commons

Feathers become abraded over time and all birds need to replace them at intervals. Barn owls are particularly dependent on their ability to fly quietly and manoeuvre efficiently. In temperate areas, the owls undergo a prolonged moult that lasts through three phases over a period of two years. The female starts to moult while incubating the eggs and brooding the chicks, a time when the male feeds her, so she does not need to fly much. The first primary feather to be shed is a central one, number 6, and it has regrown completely by the time the female resumes hunting. Feathers 4, 5, 7, and 8 are dropped at a similar time the following year and feathers 1, 2, 3, 9 and 10 in the bird’s third year of adulthood. The secondary and tail feathers are lost and replaced over a similar timescale, again starting while incubation is taking place. In the case of the tail, the two outermost tail feathers are first shed, followed by the two central ones, the other tail feathers being shed the following year.

The male owl moults rather later in the year than the female, at a time when there is an abundance of food, the female has recommenced hunting, and the demands of the chicks are lessening. Unmated males without family responsibilities often start losing feathers earlier in the year. Their moult follows a pattern similarly prolonged as that of the female. The first sign that the male is moulting is often when a tail feather has been dropped at the roost. A consequence of moulting is the loss of thermal insulation. This is of little importance in the tropics, and barn owls there usually moult a complete complement of flight feathers annually. The hot-climate moult may still take place over a long period but is usually concentrated at a particular time of year outside the breeding season

When disturbed at its roosting site, an angry barn owl lowers its head and sways it from side to side, or the head may be lowered and stretched forward and the wings outstretched and drooped while the bird emits hisses and makes snapping noises with its beak. Another defensive attitude involves lying flat on the ground or crouching with wings spread out.