The head of a bird of prey contains its senses for locating its prey and its equipment to eat it.
The eyes of a diurnal (daytime) raptor are its main tool when it comes to locating prey. Such is their construction that they are estimated to have a sharpness of vision at between 3-5 times better than our own to the front (binocular) and two times better to the side (peripheral). In addition, the eyes and brain are able to process more pictures per second than us which further aids their vision. For instance, our brain can process 24 pictures per second and when we sit in front of the TV to watch our favourite soaps the picture changes 25 times per second so we can make sense of it and movement appears normal. A raptor can process between 70-100 pictures per second and is therefore more able to react instantly to any change in circumstances, such as the slightest movement of potential quarry.
What you see of a raptor’s eyes is only about an eighth of their actual size. They are so large that they occupy most of the skull and almost meet at the top of the head. Because of the amount of skull space they occupy there is not much room left for muscle to move them and consequently they have very limited ability to swivel their eyes in their sockets, around 10º or so. In fact, the falcon’s eyes occupy a percentage of its overall body weight such that if our eyes were proportionately the same, they’d each be the size of a soccer ball!
A raptor has three eyelids, an upper, lower and a third called the nictitating membrane. When a raptor closes its eyes it is only the lower lid which moves to cover the eyeball – the upper one remains fixed. The nictitating membrane moves from the front to the back of the eye and is a tough sheath which the bird automatically draws across the eye for protection when there is movement close to its head.
Above each eye is a predominant ridge. Made of a sort of cartilege, it acts not only as a shade for the eye but also offers some degree of protection when grappling with its quarry or chasing it through undergrowth.
If we humans are suddenly exposed to a different light level we have to wait for our pupils to react before we are able to see. Raptors on the other hand have voluntary control over pupil dilation and can react, and therefore see, immediately. Furthermore they can adjust each pupil independently, so if strong sunlight is coming in from one side only, the pupil on that side will dilate more than the other.
Raptors can also see into the ultraviolet spectrum which is handy for them but not for small mammalian prey. When it is hovering looking out for any movement in the grass below, a kestrel is also able to see the ultraviolet traces given off in the urine trail that the short-tailed vole leaves to help find its way back to its nest, thus giving it another weapon in its armoury.
Other than in the case of falcons and vultures, birds of prey rarely kill with their beaks, which is there to tear off pieces of food. Falcons, however, have a notch in the upper mandible, with a corresponding notch in the lower, which acts as a fulcrum or pressure point and enables it to snap the neck of small prey. Unlike other raptors, vultures only use their feet to stand on but they do have amazingly powerful beaks. American Black vultures are capable of killing prey up to the size of skunks and piglets, but they do it as a group and use their beaks.
The beak is made of the same stuff as our fingernails – keratin. The beak continually grows and is kept to a manageable length by everyday wear and tear and also by feaking, where the bird cleans its beak against a rough surface after eating.
The ear openings are small and set to either side of the head. Despite the fact that their hearing is better than ours raptors do not generally use their hearing to locate their prey.
Raptors don’t have one as such, but the nostrils are to be found in the cere which forms the top part of the hard area of beak. While there is much research going on into the subject, it is not generally believed that birds of prey have much by way of sense of smell (with one notable exception) and they certainly do not use it to locate their prey. The nostrils are usually just two openings in the cere but falcons have an additional feature – a small cone in the centre of each. This cone is believed to assist the bird to breathe normally in a high-speed dive by slowing down the flow of air into the lungs. Try sticking your head out of a car window, facing into the wind and then try to breathe normally at speeds over 30mph. You can’t. You catch your breath when so much air tries to get in. Imagine, then, being a falcon stooping at around 150mph.
The notable exception mentioned above is the Turkey Vulture which has an astounding sense of smell. Its nostrils are large and actually go right through the beak to meet in the middle, forming a cavity. This set-up enables the bird to detect rotting meat from over two miles away through a canopy of trees. In fact the American Black and King vultures which share this bird’s range will often circle above it, keeping a close watch in case it should suddenly make off purposefully towards a potential meal which it may have smelled but they haven’t seen.