This post shows a small selection of the piscivorous (fish eating) birds that you can see travelling up the Chobe River along Botswana’s northern border with Namibia. The Chobe River is perennial and flows past the main town Kasane on its way to join the mighty Zambezi River at Kazungula. The level of the Chobe river varies significantly according to the season with the highest water level usually seen in March to June period each year.

“In all things of nature there is something of the marvelous.” ~ Aristotle

Many piscivorous birds are nomadic moving according to the level of the water and abundance of food. I have seen African darters every time I have visited the Chobe river regardless of the level of water. The African darter is affectionately called the “snake bird” because of the way it swims through the water with just its head and long thin ‘snake-like’ neck above the water. The darter is a spear fisherman. It uses its large webbed feet feet to dive underwater before stabbing its targeted fish with its beak. It swims more slowly than a cormorant, but compensates by being more stealthy. The next image shows an adult darter throwing a Blackspotted squeaker up in the air to reorient it to be able to swallow it.

Unlike many other waterbirds, the feathers of the African darter do not contain any oil and are therefore not waterproof. This feature makes the bird less positively buoyant which enhances its diving ability. After diving for fish, the feathers become waterlogged reducing its ability to fly and maintain heat insulation. Its need to dry its feathers dictates that we regularly see the African darter sitting along the waterside wings spread, drying its feathers in the wind and the sun.

Reed cormorants share their watery habitat with African darters and I have never seen the two species fighting over territory. Both species are nomadic according the availability of food. Reed cormorants are diurnal and inhabit most freshwater rivers, where it is quiet, relatively shallow and well vegetated. This species mainly hunts fish but will feed on frogs, crustaceans and molluscs, when available. Reed cormorants can feed on fish up to 20 centimetres in length. In the next image, a Reed cormorant had caught a Blackspotted squeaker among the water lilies.

This species of cormorant feeds mainly on fish which it catches by diving under the water and chasing them. It is quick and lithe and can stay submerged for up to 40 seconds. To reduce its buoyancy, this species and African darters are known to swallow a few small stones or pebbles to increase its weight so it can dive faster. The larger fish caught are brought back to the surface to kill by biting them and suffocating them out of the water. Similar to many fish eating waterbirds, the Reed cormorant throws its catch into the air to reposition so it can swallow it head first. This manoeuver is needed to prevent the spines on the fish’s dorsal fin from getting caught in its throat while swallowing.

Faansie Peacock’s excellent app, Firefinch, notes that cormorants lack a preening gland, making them less waterproof and less buoyant, which enhances their underwater hunting capability, similar to African darters. This also explains why they spend much of their time out of the water with wings spread to dry.

“The soul never thinks without a picture.” ~ Aristotle

In a fascinating article on “Why do birds have different eye colours?” by Eamon Corbett, on the British Ornithologist’s Union (BOU’s) website, notes that cormorants are not the most colourful bird family – until you look at their eyes. When considering this incredible variation, the obvious question to ask is “why?” Explanations focus on a different levels of causation: from mechanistic, to genetic, to evolutionary. In bird feathers, the predominant pigments are melanins (black, brown, grey, rufous), and carotenoids (red, yellow, orange). Both are also present in bird eyes, but two other types of pigments – pteridines and purines – account for many brightly coloured bird eyes. Other contributing factors beyond pigments, include blood vessels, collagen fibres, cholesterol crystals, lipid droplets, and structural elements. What advantages do colourful eyes provide? One possibility is that they affect birds’ survival, through effects on their vision or camouflage. The idea that eye colour could influence visual acuity, perhaps by reducing glare, has yet to be tested. In many birds, signalling may play a greater role in the evolution of eye colour. This is a fascinating but still not well understood feature of birds.

The ability of a piscivorous bird to cope with light refraction at the air/water interface is intriguing, especially as light bends down when entering water from air. The capture rate of small stationary underwater prey by both egrets and herons is high from a variety of angles, implying a well honed ability to correct for refraction. Herons and egrets employ various methods to hunt fish. Some stand and wait for prey to come within striking distance, some walk very slowly towards their prey. Others use their feet, head, wings or full body in various ways to access particular prey opportunities. Herons and egrets usually catch their prey with a “beak stab”.

“Nearness to nature… keeps the spirit sensitive to impressions not commonly felt and in touch with the unseen powers.” ~ Charles Eastman

Head posture is an important component of egret and heron feeding behaviour. Egrets and herons hold their head in one of several ways, usually to make optimal use of its narrow binocular field of vision. It may extend its neck and tip its head so that its beak points straight down towards the prey. This ‘peering over’ head posture reduces glare and distortion. It also provides a binocular view of a potential prey item by looking down the tip of the beak rather than beneath it. When ‘neck swaying’, the neck and sometimes the body is moved from side to side, the head tending to be almost stationary. Neck swaying while feeding may be a method of enticing small movements of prey, while the bird’s head remains ready to strike.

The four species of kingfisher found along the Chobe river are piscivores (fish-eaters), the Malachite, Pied, Half-collared and Giant. The Striped, Woodlands, Brown-hooded and Pygmy kingfishers eat invertebrates such as insects, lizards, frogs and even small snakes and scorpions. The Malachite kingfisher is the second smallest kingfisher in southern Africa after the Pygmy kingfisher. The Malachite is like a brightly collared jewel which is normally seen perched on a reed or stem of grass. The Malachite, Pied and Giant kingfishers are residents but can be nomadic if water conditions change. These are diurnal fishermen relying on sight to find prey. The Giant and Pied kingfishers are dimorphic, meaning that the male and female have different plumage colouring, while both sexes of the Malachite have the same coloured and patterned plumage.

Several piscivorous raptors can be seen when travelling along the Chobe river. These are the African fish eagle which is resident and territorial. Migratory piscivorous raptors include the Yellow-billed and Black kite and I am told that there are Ospreys along the Chobe river, but have never seen one. Fish eagles hunt from a perch high in a large tree overlooking the river and also from a sandbank which gives an elevated view of the adjacent water.

“Nature is not a place to visit, it is home.” ~ Gary Snyder

I am always intrigued by how African fish eagles can see fish below the water’s surface. In Timoty Cornwell’s Blog, ‘quailandaardvark.workpress.com’, he explains several of the unique features of birds in general and raptors eyes in particular. The adaptations that make avian eyes so remarkable are more impressive when you understand how they are are different from the human eye. There is a small depression in the retina called the fovea which is more densely packed with colour detecting cone cells than other parts of the retina. This is where most of our sharply focused highly detailed vision occurs. Raptors have a second fovea which makes for a larger field of sharp focus. Avian eyes have evolved a structure called the pecten which resides in the vitreous humour (inner eye fluid) of the eye and bathes the tissues in nutrients allowing for fewer blood vessels across the surface of the retina resulting in higher visual resolution. Birds also have far more photoreceptor cells than other animals. The sclerotic ring is a series of small bone plates encircling the eyes of birds and reptiles. This ring provides protection and support for the large eyes most birds have and creates additional anchors for muscles that help with focusing and blinking. Raptors have lenses which can see extremely far and use unique Crampton’s muscles to apply pressure to reshape the cornea and aid in close up vision. These muscles allow them to have a much deeper field of sharp focus than we do.

How do birds of prey see fish in the water? They use all of aforementioned adaptive advantages to hunt for distant prey but birds hunting for fish have one other behaviour that is important when catching prey that is underwater and it is to attack from a steeper angle than they would when hunting prey on land. This is vitally important because it cuts down on the refraction caused by the water and means the fish they see are closer than where they appear to be. The Pied kingfisher is a master hoverer and uses this steep angle of attack to dive for its underwater prey.

“Fresh air is good for the mind as for the body. Nature always seems to be trying to talk to us as if she had some great secret to tell. And so she has.” John Lubbock

Like many birds, kingfishers have two foveae in their eyes. Kingfishers are able to switch from the main central fovea to the auxiliary fovea as they enter the water, so they can maintain visual acuity, or sharpness of vision. 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.

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 relaxes the fish’s dorsal fin spines to stop it become trapped in the kingfisher’s throat. Several times a day kingfishers will regurgitate small pellets of fish bones and other indigestible remains.

Once I have taken a photograph of a wild animal or bird and begin to edit the image I get to see the subject close up which, more often than not, pricks my curiosity as to why it’s beak or wing is structured in a particular way, what role does the colour of the eye play and so on. Birds have some remarkable adapted features. Increasingly scientist and engineers are using nature’s best ideas and imitating natural designs and processes to solve human problems in a discipline called biomimicry. One example pertaining to kingfishers is in the aerodynamics of the nose of bullet trains.

“Biomimicry is the conscious emulation of life’s genius.” ~ Janine Benyus

In the 1990s, train engineers in Japan built bullet trains able to travel over 300 kilometres per hour. They used the shape of a bullet to make the nose of the train more aerodynamically efficient. The problem was that when the train exited a tunnel, the air in front of their bullet-shaped nose expanded rapidly, creating a loud “tunnel boom.” Looking for a solution, engineers began studying why kingfishers diving into the water at high speed did so with almost no splash. Analysis revealed the linear flow of water past a kingfisher’s beak which reduced the splash when entering the water at speed. The nose cone of the bullet train was changed to the shape of a kingfisher’s beak and the train moved through the tunnel at high speed without a boom and was 10% to 15% more energy efficient.

When Albert Einstein was five, his father gave him a compass. He noticed that the needle always pointed north. This was his first encounter with the idea that something invisible underlies our reality. This is the sense I get when I look more closely at my wildlife photographs. The closer I look at them the more my curiosity is pricked. The more I look, the more curious I become, the more I see and the better I understand.

“If you look at nature carefully, and really pay attention and you are lucky, you will catch a glimpse of something deeply hidden.” ~ Albert Einstein

Explore, seek to understand, marvel at its interconnectedness and let it be.

Have fun, Mike