If you’ve ever seen a kingfisher home in on and catch a fish, then you’ll be aware that this brightly coloured bird truly lives up to its name.
Kingfishers have evolved a remarkable ability to fish. They eat mainly small fish such as minnows and sticklebacks as well as aquatic insects and newts, and need to eat their own bodyweight in food each day which is about 5,000 fish during the summer.
Before it dives into the water a kingfisher will sit on a perch for some time, bobbing its heads backwards and forwards to gauge the exact position of a fish, while keeping its body perfectly still.
It spots a fish, and with a few wingbeats dives headfirst at a speed of up to 25 mph, folding its wings tight against the sides of its body as it plunges into the water and catches a fish in the blink of an eye.
Like many birds, kingfishers have two foveae in its eyes. The fovea is the area of the retina which has the greatest density of light receptors. Kingfishers are able to switch from the main central fovea to the auxiliary fovea as they enter the water, so when the egg-shaped lens of the eye points towards it they can maintain visual acuity, or sharpness of vision.
In other words, the kingfisher has monocular vision in the air and binocular vision underwater, which helps it overcome the challenges of the change in refraction between air and water. Although the underwater vision is not as sharp as it is in the air, it is more important for a kingfisher to be able to judge the distance of the moving prey then have a clear image.
To enhance their vision further kingfishers have high numbers of red pigments in the oil droplets contained in the cone cells of their retinas. It is not fully understood why they have evolved like this, but the droplets may help reduce the glare or dispersion of light from matter in the water, so kingfishers can spot a potential meal better.
When the kingfisher is underwater, it protects its eyes by closing its third eyelid, known as the nictating membrane, although it is not actually blind as the eyelid is transparent.
After a successful catch the kingfisher emerges beak first and flies back to its perch. It will then wriggle the fish about until it is held by its tail, before bashing it against the perch several times to kill it. This makes the fish easier to swallow and in the case of sticklebacks relaxes the spines to stop it become trapped in the kingfisher’s throat
The kingfisher then turns the fish around to position it lengthways and swallows it headfirst.
Several times a day kingfishers will regurgitate small pellets of fish bones and other indigestible remains. Kingfisher chicks are fed fish but do not expel pellets as they have acidic digestive systems which dissolve everything. Once they have fledged at about 3 weeks their digestive system becomes alkaline.
The behaviour we have described above is that of the common kingfisher, the only kingfisher native to the UK. But there are over 100 species of kingfisher found all over the world and although they are most famous for hunting and eating fish they will supplement their diet with crustaceans, amphibians, reptiles, and even small birds and mammals.
In the tropical regions of Asia, Africa and Australasia, 90% of kingfishers don’t actually fish at all but hunt in woodlands and forests. Kookaburras, for example, which are terrestrial tree kingfishers, native to Australia and New Zealand, eat snakes up to 3 feet long, mice, and the young of other species of birds. They will even accept handouts from humans.