Heron in hiding

As I was walking around Lake Temescal in the Berkeley hills the other day, I noticed a very sedentary Black-crowned Night Heron, that was doing its best to hide among the cattail vegetation.  Always keeping a stray cattail leaf between me and a good shot of its head and eyes, the bird moved as I moved, making a good shot pretty tough.

As their name implies, these herons are active at night, beginning at dusk and continuing through the night until early morning hours, thus avoiding competition from other daytime-foraging herons and egrets.  So, it makes sense that the bird would be looking for a nice hiding place in the daytime.

It was so dark down in the cattails, I decided to use a flash to illuminate the bird a little better. I was surprised to see the amount of reflection from the bird’s eye — a bright red eyeshine.

A closer view really shows off the red iris surrounding the enlarged pupil, and the red eyeshine produced by light reflecting back from a layer in the back of the eye in front of the retina.

Eyeshine occurs in a variety of mammals, birds, fish, and spiders, most of whom are active at night.  Birds usually produce red eyeshine in response to the light from a flash or a flashlight; horses have blue eyeshine, and cats and dogs often exhibit green eyeshine.  This is not the same thing as the “red-eye” effect that a flash produces when photographing people whose pupils are dilated in a dim room and fail to constrict before the flash goes off to capture the picture.  “Red-eye” in this instance is a view of the very vascular back of the eye, and is not really eyeshine per se.

Without the aid of the flash, the heron’s eyes look dark in the center, with a red iris that is only found in mature adult herons.

In conditions of low light, the highly reflective layer in the back of their eyes (called a tapetum lucidum) gathers light and bounces it back to the light detecting photoreceptors in the retina, thus enhancing the animal’s perception of shapes in a darkened environment.

This enables Night Herons to prey upon nesting gulls and terns, stealing chicks or eggs right off the nest at night.  A variety of critters may find their way down a Night Heron’s gullet, including worms, insects, clams, fish, amphibians, reptiles, including hatchling turtles, and even small mammals.  It’s a real “night stalker”…