Birds as physicists

Ever noticed that when you put a pencil into a glass of water, it appears to be broken? Why is this so? As seen in this image of the pencil in water, light first travels through the air – which is easy to move through – and then through the water, where it slows down. This change in speed causes the light to bend, or refract, meaning that the part of the pencil that is in the water will appear shifted.

So if a bird is looking at a fish through a layer of air and then through a layer of water, the fish will not actually be in the same location as it appears any more than the pencil is in the location it appears.

So…..if the fish is not actually in the position that the bird sees it looking through the water, how is it possible for the Snowy egret seen here to drive his bill into the water and snag a very small target that is not where it appears to be? The bird has to play “physicist” and compensate for the phenomenon of “diffraction’.

The egret knows enough (perhaps through many misses and trial and error) that to overcome the disadvantage it faces due to diffraction, it must position itself as close to directly over the target fish as possible. If he is not at an angle, the image of the fish will not be bent and will in fact be close to where it appears leading to a higher “hit rate” like this bird I photographed today.

Other birds like ospreys have also learned this approach to catching prey. When you watch an osprey “fishing” from the air, you will notice that it hovers before it plunges. The hovering manuever allows it to position itself as close to directly over the fish reducing the angle of diffraction and placing the target close to where it appears below the water’s surface. Avian physicists? Maybe stretching it a bit, but it works for the birds.

Tech Tip - To capture the very fast movement of the egret’s act of catching this little fish, I had to make two adjustments in the camera’s settings. First of all, the shutter speed has to be fast enough to freeze the motion and capture the image without blur. I used a speed of 1/2000 second which successfully stopped the bird’s action in mid-strike. Secondly, trying to shoot the precise photo by shooting one frame would be a daunting task to say the least. By shooting at a high frame rate (in this case 12 frames/second), I was able to capture a series of consecutive images and then pick the ones that were closest to what I was hoping for. These images show the moment of impact in the water and the moment of pulling the fish from the water…two images from a series of about 25.