Environment – Sharp eyed kōtare

Kōtare (sacred kingfishers) are brutal and highly effective predators. We have a pair nesting on our section, and I am continually amazed at the diversity and abundance of prey they bring the chicks. Walking sticks, mud crabs, wētā, silvereyes, and rainbow and ornate skinks are all on the menu. Right now, they mainly catch cicadas, but I did see dad with a freshly killed mouse the other day. 

Kingfishers have amazing evolutionary adaptations to help them hunt, in particular their incredible eyesight. They have massive eyeballs, oval shaped and about as large as can be allowed by their head size. If a kingfisher were the size of a human (scary), their eyes would be the size of grapefruit. Large eyes in predatory birds increase their visual acuity. They effectively have bigger image sensors which form higher resolution representations of the world around them.

When we look at something and need a detailed view, we orient our eyes so that the light from the target hits the focal point of our retina. You are doing this right now, using your focal point (or fovea) to read these words. However, kingfishers have not one, but two, fovea in each eye. Both fovea have extremely high concentrations of light receiving cells. The first fovea are forward facing and provide binocular vision to accurately judge the distance to a target. The second fovea face to the sides and occur in little depressions in the retina that allow even more light receiving cells to be packed in. Effectively they get such high-resolution images at their fovea, that it functions like a telescope. 

Kōtare are ‘sit-and-wait’ predators. They perch from a vantage point and wait for potential prey to reveal themselves. They detect prey through their motion, and they have incredibly well-developed motion-detection abilities. 

Although their large eyes and multiple focal points can potentially provide huge amounts of information to the brain, the birds filter out a lot of this information with specialised motion detecting cells just behind the retina. These cells basically start processing the image, highlighting things that are moving, before the sensory data gets delivered to the brain.

The above adaptations depend on one last trick to work effectively: advanced image stabilisation tech. We have this too – if you move this column around, you can keep reading it as you unconsciously stabilise your gaze by moving your eyes to track the words. Because kōtare have such large, oval shaped eyes though, they have limited eyeball mobility. However, they are masters at keeping their head completely stationary while they are hovering, or perched on a swaying branch. 

Next time you see a kōtare on a moving perch, watch as their body moves around and their head remain motionless. It is extraordinary!