Owls are not closely related to the birds of prey, they are more closely related to night-jars. Many owls are nocturnal hunters, overnight occupying the niche in the food chain held by the other raptors.
Owls range in size from the Elf Owls (Micrathene whitneyi), Least-Pygmy Owls (Glaucidium minutissimum) & Long-Whiskered Owlets (Xenoglaux loweryi), which are of similar size to a sparrow, up to the Eurasian Eagle Owl which can weigh up to 7lbs & have a wing span of over to 6 foot.
Owls have several distinctive features not possessed by other raptors, some of these specifically aid the nocturnal hunting habits of some of the owls.
- Wings :
The feathers on the front edge of the wings are serrated, this breaks up wind flowing over the wings and enables silent flight when gliding. The silent flight gives the advantage of surprise attack on the prey, but is also of major importance in very low light conditions, when hunting is done mainly by sound & any additional noise could mean missed prey.
- Eyes :
The eyes are very large & egg shaped, occupying over 50% head space, this means that owls have a relatively small brain. Because of the shape, the eyes are fixed in position in the head, this means that owls must move their head rather than eyes (see neck). Because of the fixed eyes, the beak is positioned relatively low down on the birds face, keeping out of the field of view of the bird.
There is still some debate as to whether or not owls can see in color, some suggest because they often hunt in low light conditions, where seeing colour would be of no advantage, owls do not see in colour. They back this argument up with the fact that their retina is densely packed with rods rather than cones, & it is the cones which are associated with colour vision. Others suggest that their colour vision is at least as good & in some species better than humans, though I haven't found any that give evidence for this opinion. What is most likely is that in very good light, owls are able to distinguish colours, though not in lower light conditions. This was borne out by experiments on Little owls, which were able to distinguish between yellow, green & blue, though it did have problems with red & greys.
I have found several references to the fact that owls are the only bird that can see the colour blue, but nothing to back this up. I have also found reference to the fact that a female Japanese Quail that sees another female mating with a male with a red or blue spot on his breast is most likely to choose a male with matching colour spot to mate with - this would seem to indicate that they can see blue, so disproving the comment that only owls can see blue.
Owls cannot see in total darkness, their ability to hunt in very low light is due to their hearing rather than their sight. They do have very good eyesight in very low light conditions though, this is helped by having a reflective layer behind the retina, called the tapetum. It is suggested that if humans had vision as sensitive as Tawny owls, they would be able to clearly see an object half a mile away lit only by the flame from a candle. The tapetum is not unique to owls, other noturnal hunting animals, such as cats, have the same membrane, which is the cause of their glowing eyes at night time.
Their sight is also very sensitive to movement. Although they have excellent long vision, they have poor eyesight close-up.
There are a lot of people (including myself until recently, thanks to Prof. Carl Jones of the Durrell Wildlife Conservation Trust for pointing out my error) suggesting that the colour of the eyes gives indication of when the owl hunts :
Having taken the time to check the facts, there is no reall correlation between hunting time & iris colour. All dark eyed owls will hunt nocturnally with a few having crepuscular and/or diurnal habits. With the exception of 1 diurnal hunting owl with orange eyes, the rest are strictly nocturnal or nocturnal & crepuscular. Yellow eyed owls have the most wide spread hunting habits, some being either strictly nocturnal, crepuscular or diurnal and others with various combinations of preference.
Unlike man, owls' irises are able to dilate & contract independantly in the two eyes. Often when sitting sideways in the sun, the pupil of the eye facing the sun will be very small, while the pupil of the eye in the shade will be very large. This is most noticeable in the diurnal owls, where one eye will appear as totally yellow with small black dot in the centre & the other totally black with a very thin yellow border.
during daytime (diurnal)
- Dark :
night time (nocturnal)
- Orange :
dusk/dawn (crepuscular) or daytime
Like other birds, have third eyelid - called the nictitating membrane - which is below the normal eyelids. Unlike other birds of prey, both of the normal eyelids close when an owl blinks. In order to protect its eyes, owls eat with the eyelids shut.
- Neck :
Because the eyes are unable to move in the sockets & having a very short neck, owls need to be able to rotate their heads much further than other animals to see all around them. The extra movement that this requires is allowed by the fact that owls have 14 neck vertebrae, compared to humans (& Giraffes) 7. Some owls can rotate their head almost 270 degrees in either direction (by rotating left they can look right), though rarely go more than 180 degrees (i.e. looking directly behind themselves).
In English folklore, it is believed that if you walked around an owl sitting in a tree, it would watch you as you went round, eventually wringing itself to death. No owl can turn its head more than 3/4 the way round, so this is not possible.
- Facial Disc :
The front of an owls face is almost circular & slightly hollowed, the shape of the facial disc helps funnel sound to the ears. The marsh harrier, which also depends on sound for hunting, has a similar facial disc. The feathers around the facial disc often stand out slightly from the rest of the feathers on the head, absorbing sound from other directions, ensuring better resolution of sounds directly in front. (I also saw a comment somewhere that the facial disc also reflects light towards where the owl is looking - but I thinks that's very dubious).
- Ears :
Extremely acute hearing, much better than most other birds of prey. Are able to locate prey through sound alone. Test have been done by placing owls & prey in a totally dark room & by hiding prey under cover (e.g. snow), in all cases the owls have been able to accurately locate & catch the prey.
The tufts of feather at the top of the head are for display & signalling purposes only (much more like eyebrows) & are nothing at all to do with the ears. When raised the tufts may indicate that the bird feels threatened, when laid back they may indicate aggression. The ear tufts also act as part of their camouflage, breaing up the round outline of the head.
The ears are on either side of the head. Unlike ours (well most of us), the ears are placed asymmetrically, one ear higher than the other. I have found various descriptions that say that in all owls the left ear is higher than the right, also descriptions that suggest it is the other way round for all owls. I have also found species specific descriptions that say that the Barn Owl's left ear is higher than the right and for Great Gray Owls & Northern Barred Owls (Strix varia) the right ear is higher than the left. Also the shape & direction (up/down) of the opening of the ears is slightly different. In addition to the positioning & shape of the ears, the facial disc of the owls serves to funnel sound directly ahead of them towards the ears. All of this gives rise to much better spatial location of sound. It is also believed that owls can filter out sound that they do not wish to hear, being able to focus in on the sounds liable to be made by potential prey. (Despite this being suggested as an ability of owls in particular, I don't think it is unique, people can do it - you can focus on a single persons voice in a noisy environment, like a factory or party, so why not other animals).
- Crines :
Around the beak, owls tend to have a group of short hair like feathers, called crines. As the birds are very much dependent on sound for locating prey & have very poor close eyesight, once caught & dead losing the prey is a distinct possibility (they cant see it clearly & its quiet), the crines are touch sensitive & used for locating dead prey. The feathers are the first to develop, so they can be used to locate food offered by the parent. Secondarily, they help disperse dried blood away from the mouth & beak, avoiding disease.
- Crop :
Unlike the other birds of prey, owls do not have a crop, the food going straight to the stomach.
The eggs of all species of owl are white & laid over a period of several days. Unlike many other birds, owls start incubating the eggs from the time of laying the first egg. This means that the eggs are also hatched over a period of days, giving a better chance of survival to the first hatched. Both parents care for the young, which have enormous appetites, being able to eat their own weight in food over night. If the eggs are removed from the owls prior to hatching, then often more eggs will be laid as a replacement. For breeding pairs in captivity, to avoid babies, the technique often used is to remove the eggs, hard-boil & replace them, incubation will continue as normal & no further eggs will be laid.
The plumage of many owls, especially from cold regions, is very thick, making the birds look very much larger than their body size, but giving very good insulation. Owls from the colder regions also are feathered extensively down their legs & feet, giving insulation & also some degree of protection from bites from their prey.
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