Introduction to Owls

All owls are predators and the size of the prey is generally reflected by the size of the owl. The Elf and Pygmy Owls prey on a variety of insects, spiders and other invertebrates, although, some are quite voracious and can take birds as large as themselves. Eagle Owls, at the other extreme have been known to take such formidable prey as Golden Eagles and a Roe Deer of 13kg as well as foxes, herons, domestic dogs and there is apparently a report of a large Siberian Eagle Owl taking a ¾ grown wolf!

Hunting, normally, is performed in two different ways. The first is perch hunting, where the bird literally sits and waits on a suitable perch until a prey item is located. The other technique is flight hunting where the owl slowly quarters the ground from a low altitude, looking and listening for prey and diving down when food is detected. The length of the wings is normally a good indication of the preferred method of hunting - short wings for perch hunting and long wings for flight hunting.

Most owls are opportunists and virtually anything that moves is fair game. Some species are more specialised feeders, in particular the Fish and Fishing Owls. These are two genera of owls, one from Asia and the other from Africa whose members feed mainly on fish, amphibians and aquatic invertebrates, snatched from beneath the water surface.

No other species are quite so specialised but some have a preference for particular prey. The Milky Eagle Owl of Africa feeds on a wide variety of prey, but where the ranges overlap it seems particularly partial to hedgehogs. The Eurasian Eagle Owl would appear to have a vendetta against other birds of prey, especially other owls. In some areas they form 10% of the bird’s diet, which is much greater than one would expect from chance. We must point out the majority of their food is Rabbit.

Much is known about the dining habits of owls because of certain inefficiencies of their digestive process. More than 300 species of bird in several different orders are known to regurgitate pellets of indigestible material. This figure includes all owl species. Owl pellets are so informative for several reasons. Firstly owls have comparatively weak bills and often prey that isn’t too large is swallowed whole which leaves the skeleton of the prey, including the skull, intact. Unlike most other birds, owls have no crop, and the food passes straight into the foregut (they do not posses a true stomach). The acid in the owl’s gut is rather weak with a pH of 2.2 - 2.5 which is the same as vinegar, this compares to diurnal birds of prey which have a pH of 1.3 - 1.8, which is approaching the pH of concentrated hydrochloric acid. This means that owls can only digest the soft tissues. The bones, fur and feathers remain virtually intact. The opening from the foregut into the rest of the digestive tract is small and prevents any undigested material from passing through. Instead, it remains behind where it is compacted into an oval pellet and is then actively regurgitated back up through the oesophagus. Pellets, therefore, contain bones including intact skulls, fur, feathers, the chitinous exoskeletons of insects and even the chaetae (bristles) from earthworms and so discovering what owls have been eating is quite straightforward.

Regurgitating a pellet is a voluntary act on the owl’s part and in the wild most birds will produce one prior to leaving the roost for hunting. Often, another smaller pellet will be produced during the night before the second main period of hunting around dawn. The size, shape and appearance of the pellet is normally characteristic of the owl species, certainly amongst British Owls.

One interesting fact relating to owl pellets is that the clothes moth, whose larvae sometimes attack your best woollen suit hanging in the wardrobe, actually evolved from a moth whose larvae feed on the fur and feathers in old owl pellets.

Because of their predominantly nocturnal tendencies, owls have evolved several physical adaptations which facilitate catching prey in the dark. All owls have large forward facing eyes giving good stereoscopic vision, vital for judging distances. Indeed, owls have the most forward facing eyes and hence the best stereoscopic vision of all birds. In smaller species the head often appears flattened so that the eyes can be as widely spaced as possible to increase the stereoscopic effect. This is often further enhanced by bobbing or weaving the head to give a differing perspective known as the parallax effect.

The eyes are very large, those of a Snowy Owl weighing as much as our own. They are modified in nocturnal species to improve sensitivity in low light intensities. They are tubular, rather than round, giving a relatively large cornea in proportion to the overall size of the eye and enabling more light to enter the eye. The light passes through the pupil (which can be closed by the iris to a small pinprick in bright light or opened so wide that virtually no iris is visible at night) to the lens. This is large and convex, causing the image to be focused nearer to the lens hence retaining maximum brightness. One drawback is that owls are long sighted and cannot focus on objects which are too close. Tactile bristles around the beak partially compensate for this. The tubular shape also gives a comparatively large retina size which is packed full of light sensitive rods, about 56,000/square mm. in the Tawny Owl. These rods are far more sensitive than cones at low light levels. The phenomenal light gathering properties of the owls eye is further enhanced in many species by a reflective layer behind the retina, called the tapetum lucidum, which reflects back onto the rods any light that may have passed through the retina without hitting one the first time. Tawny owls would appear to have the best developed eyes of all the owls, indeed of all vertebrates, being probably about 100 times more sensitive at low light levels than our own.

As well as rods, all owls possess colour sensitive cones in their eyes. Although having fewer light sensitive cones than humans, they can probably detect colours to some extent. They are certainly not blind in daylight and some, like the Eagle Owl, have better day time vision than us. Our night time vision, however, is better than some diurnal Pygmy Owls. Owls are unable to move their eyes in the sockets because of the size and tubular shape. To compensate, they have a deceptively long flexible neck which enables them to turn their head 270° in either direction horizontally and at least 90° vertically.

The plumage of owls is adapted in several ways. Being mainly nocturnal, owls have no need to display visually and so their markings are normally mottled shades of black, brown and grey, patterned to conceal them during the daytime from possible predators and other birds which might mob them. The snowy owl, although distinctive with its white plumage, blends perfectly with the arctic snow which is often still on the ground when the female is incubating her eggs. The Barn Owl’s plumage may appear more puzzling. If viewed from above, the most likely direction from attack whilst hunting over its favourite habitat of long rank grassland with its brown dead stems of long grass, it almost disappears from sight as it does in its original nesting sites in sandstone cliffs. The white under-parts may make it less visible against the sky when it hunts in daylight which it does when food is in short supply or when there are chicks in the nest.

The ear tufts possessed by some species are just feathers and nothing to do with the ears. They can be used as camouflage, by breaking up the owls outline and their posture communicates their mood.

Most owls have a defensive posture which is used to intimidate potential enemies. It varies from species to species but usually involves the wings being spread out around the body with the head lowered down and the feathers fluffed up. This all gives the impression that the owl is much bigger and when accompanied by “bill clicking”, hissing and swaying the body from side to side is normally enough to drive off most predators.

The feathers on owls’ wings have several unique features. Firstly, they have a downy up per surface, which reduces the noise as they move over each other as the wing beats. Secondly the outer primaries have a stiff comb like fringe on their leading edge which modifies the airflow over them as they cut through the air which reduces noise. Finally, on the trailing edge of the wing, the feathers have a soft hair like fringe, which reduces air turbulence behind the wing. The net result of these modifications is totally silent flight even at ultrasonic level which means that prey are not forewarned of attack and the bird can use its amazing hearing at its best without the noisy wing beats possessed by other birds. Also the plumage of owls is very thick and downy, retaining vital body heat when hunting at night. This makes owls look allot larger than they actually are. Most owls are deceptively light for their size, also most have quite large and broad wings, which with the low body weight gives them a very low wing loading which is why their flight is so buoyant and moth like. This allows them to fly at a very low speed, which improves the chances of locating prey, and it also lets them carry quite heavy loads back to the nest or roost.

The thick soft plumage insulates the owl whilst it is exposed to cold night time temperatures, and in most species (certainly all northerly species) the leg feathers extend right down to the feet and sometimes over the toes as well.

Owl legs and feet vary considerably between species, depending on its ecology. All species have surprisingly long legs, most of which is hidden in the soft belly feathers. The Barn Owl has long sparsely feathered legs using them to snatch prey from long vegetation. Despite ranging into temperate zones the minimal feathering probably prevents the bird’s legs becoming sodden if it hunts in damp grass. The Grass Owls from Africa, Asia and Australia hunt in even longer grass and their legs are even longer.

The European sub-species of Eagle Owl is found in Northern and Central Europe often at quite high altitudes, and its feet and toes are covered in feathers. The Snowy Owl from around the Arctic Circle has the most heavily covered of all. These feathers also help protect the owl from being bitten by prey. At the other end of the scale, most of the Fish and Fishing Owls have totally bare legs and feet, again feathers in this situation would become waterlogged. The Burrowing Owl from North and South America lives on the Prairies and Pampas in the burrows of Prairie Marmots and Viscaschas, occasionally it even does some digging of its own using its very long legs. It is extremely terrestrial, often chasing invertebrates on foot and the extra height gained from its legs gives its a better view of predators on the flat grass plains.

All owls have a flexible outer toe. At rest it points roughly forward sticking out slightly to the side, but it can be angled backwards giving an increased area to catch prey and a better grip.

No owl can be said to build its own nest in the proper sense of the word. Burrowing Owls may extend an already existing burrow, and some of the large owls may scratch a slight depression or scrape in loose earth but nothing that compares with the marvels produced by some other birds. Nest sites vary but tree hollows and cavities are the preferred site for many species. The Elf Owl nests in holes in Sanguaro cactuses made by the Gila Woodpecker, the Burrowing Owl as mentioned in animal burrows, Grass owls trample tunnels and nest chambers in long grass, whilst larger species scratch a scrape or use old stick nests of crows, pigeons or diurnal birds of prey. The American Great Horned Owl often drives out quite large raptors such as red-tailed buzzards and takes over their nests. Tawny Owls often use old squirrel drays, and Barn Owls in Africa use the giant stick structures made by Hammerkops (a stork like bird) occasionally nesting communally. Many species can be encouraged to use nest boxes and they may even be used in preference over natural sites.

Many owls are quite territorial, especially during the breeding season, sometimes even other owls species are not tolerated, at the other extreme some species will defend the nest site itself but will share hunting habitat.

Breeding seasons are always geared so that rearing chicks coincides with the peak availability of food, this is not always when prey is most numerous but when it is able to be caught in increased numbers. This may be as a result of decreased vegetation cover, or when the prey are more active or vocal say in defence of their territories allowing them to be caught more easily. In some species this often means that the eggs are laid very early, some times with snow still on the ground, this also gives the relatively slow growing young more time to learn how to hunt more efficiently before the following winter.

All owls lay white eggs, which suggests that they all evolved from a hole nesting ancestor. Elaborate markings to conceal the eggs from predators aren't needed in dark holes indeed the whiteness may make them more visible to the parents. The egg above is being Candled. A light is shone through the egg so we can see the developing embryo. (The solid Pink Blob in the Middle) The blood vessels are forming to feed the developing chick and to allow Oxygen to be absorbed into the blood stream. The Yolk inside the egg isn't visible but is there to feed the baby as it grows.

The number of eggs laid varies from species to species, year to year and between individual birds. In general larger birds lay fewer eggs, and birds from tropical regions lay fewer than birds from more extreme latitudes. In temperate and sub-arctic regions some species like the Snowy Owl the Short-eared Owl and the Barn Owl can increase the size of the clutch as prey availability increases, in years when the lemming or vole populations crash, breeding may be abandoned totally.

Owl eggs are relatively spherical to a greater or lesser extent. In most species the female starts incubating as soon as the first egg is laid. The eggs are laid at intervals of at least a day, often more, resulting in what is known as asynchronised hatching, where the eldest chick can be up to 2 weeks older than the youngest. The means that each chick reaches its peak food demand period at different times and so spreading the load. In lean years the oldest and therefore strongest chicks at least will survive, the younger chicks, once dead, may even be utilised to feed the others.

During incubation, and until the smallest chick is large enough to maintain its own body temperature, all the food is provided by the male, the female rarely leaving the nest site. She dispatches the food and feeds the chicks small slivers until they can swallow the prey whole, she then helps the male with the hunting.

The age of fledging varies greatly and some species even remain in the area until the following year. Gradually the young learn to hunt, often starting on insects or food brought in by the parents, which may still be alive. Most species are independent by their first winter and in many cases the young are actively driven away by their parents prior to this.

Like most birds, owls sing to attract a mate and defend their territories although perhaps not as musical as the songs of thrushes and warblers, the owl song functions in exactly the same way. The noises made by owls include the melodic trills of pygmy owls, the bell like chimes of the European Scops Owl, the classic hoot of the Tawny Owl (and it’s not so musical shriek), the strangulated screech of the Barn Owl and the deep resonant hoots of Eagle Owls.

In general the pitch and volume of the call reflect the size of the territory of the owl and thus how far the call has to travel. Of course, there are exceptions. Many forest species have loud calls for the size of their territories but this is because of the sound-deadening effect of the trees. The Flammulated Owl from the western United States is a similar size to the Western Screech Owl, which inhabits the same part of the world, but its territory is much larger and hence it has a loud deep call, more becoming of a much larger bird.

Diurnal species are far less vocal than nocturnal ones, being able to use visual displays to advertise for partners and defend their territories.

Don't forget, Owls don't breed very well when its Raining because it's Too Wet to Woo...