Owl

Introduction

Owls are a world-wide order of birds known as Strigiformes and the 216 species range in size from the tiny, sparrow size, Elf Owl to the Eurasian Eagle Owl which has a wing span of nearly 2 metres and can weigh almost 5kg. They are a group of predatory birds, characterised by large forward facing eyes surrounded by a facial disk of short stiff feathers and an upright posture. A large proportion of owls are nocturnal. They occupy an equivalent niche to the diurnal birds of prey such as hawks, falcons, eagles and buzzards but they are not actually related. The resemblance to the day time birds of prey is an example of convergent evolution, where both groups have independently evolved several features such as the hooked beak and talons. Owls are all very closely related to each other, much more so than, for instance, the diurnal raptors which include birds as dissimilar as vultures, secretary birds, falcons etc. Even so, owls are separated into two fairly distinct families.

The first family is the Tytonidae which is made up of 17 species of barn and grass owl, and one species of Bay Owl. Members of this family are quite distinct from other owls and possess several differences. The most obvious external ones being the heart-shaped rather than round facial disk, the longer skull and beak, longer legs, longer and more pointed wings and a forked tail. Grass Owls come from Africa, South East Asia and Australia and are very similar to Barn Owls but have longer legs.

All of the other 198 owls are in the family Strigidae.

The collective noun for owls is a “Parliament”.

The Barn Owl is protected in the same way as any other wild bird, in that it is illegal to injure, kill or take one from the wild. It also has added protection in that, during the breeding season, it is illegal to disturb the nest sites. That means that if a bird is actually sitting on eggs or there are unfledged babies in the nest site it is illegal to disturb them in any way. If that nest site is in a building that is going to be developed, building work has to be delayed until the young have fledged.

Food and Hunting

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.

Eyesight

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.

Breeding

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 its self 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 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.