Nictitating Membranes
Sometimes referred to as a “third eyelid,” the translucent nictitating membrane visible across this juvenile Black-crowned Night Heron’s eye serves to protect it from foreign objects and to moisten the eye while at the same time allowing the bird to retain some degree of visibility. It extends from the inner corner of the eye to the outer corner, and is drawn across the eye much like a windshield wiper. The membrane is thinner and more transparent than the fleshier upper and lower eyelids and is used periodically by birds when foraging, flying, diving, feeding young, gathering nesting material, etc. In this case, the heron’s nictitating membrane was drawn across its eye seconds before it plunged beneath the surface of the water to capture a crayfish.
Birds aren’t the only animals that possess nictitating membranes – it’s relatively common in fish, amphibians, reptiles, and mammals (but they are rare in primates).
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Black-crowned Night Herons Feeding
Black-crowned Night Herons employ a number of techniques for capturing a wide variety of food, from standing stock still for a considerable amount of time waiting for prey to swim by to securing prey while swimming. They feed mainly from evening until early morning. Because they are crepuscular (feeding at dawn and dusk) as well as nocturnal (as their name implies), feeding competition from other (diurnal) herons is minimized.
The diet of opportunistic Black-crowned Night Herons includes earthworms, insects, crayfish, mussels, fish, amphibians, snakes, turtles, birds, small mammals and plant material. They have been observed actively manipulating bait (bread in one case, dragonflies in another) to attract and catch fish. As can be seen in the inset photograph, they grasp, rather than stab, their prey.
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Black-crowned Night Herons: Not Your Typical Heron
When you think of a heron, you usually think of a diurnal wading bird that has long legs, a long neck and a long bill. Black-crowned Night Herons don’t possess any of these familial characteristics. Stocky and relatively short-legged and short-billed, these herons typically rest during the day and start actively hunting at dusk, continuing through the night.
Prey includes fish (half their diet) plus a wide range of other creatures including leeches, earthworms, insects, crayfish, snakes, turtles, small mammals, birds and frogs. The manner in which a Black-crowned Night Heron lures and captures its prey varies. Two of its most intriguing fishing techniques include bill vibrating (opening and closing its bill rapidly in the water to attract prey) and bait fishing – using bait to attract fish.
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