Secret life of the Yellow Bittern
Posted by BESG on 16 January 08, Wednesday
Contributed by Lin Yangchen
On 6th January 2008 Lin Yangchen released his study of a juvenile Yellow Bittern’s (Ixobrychus sinensis) first venture out of the safety of pond bank vegetation to explore the new world (below left) and the hunting skills of an adult.
“Before it could get very far on the narrow stem, it freaked out and hurried back in, losing its footing in the process and flapping its wings to regain it. A couple of days later, it had become more confident and made its way to the reeds farther out.
“It stayed still for quite some time, its feet straddling the reeds, not knowing what to do next (above right). Its predicament was quite calamitous, with nothing but water below. I was late for work so I could not stay to find out what happened next. The parent was never seen with it.
“About a week later, when the water level had receded due to dry weather, an adult was seen hunting.
“It is not certain whether this was the juvenile that had grown up, or another bird. It has two modes of prey approach - the slow prowl in the photo on the left (top) and a brisk walk to quickly make up the distance when needed. When there are more underwater obstructions like leaf litter, it lifts its feet higher up when walking, its body leaning more towards the supporting foot (left bottom).
“On 20 Feb 07, there was a post on the blog saying that the bittern needs to move the head left/right and front/back during prey tracking in order for the two eyes on different sides of the head to see the prey. However, my particular bittern did not move its head at all when tracking prey. Maybe it moved its head when walking fast towards the prey area, like most birds bob their heads when they walk, but in the last stages before prey capture, there was no such movement.
“This photo below (left) shows that the monocular camera lens is able to capture both eyes in the same picture, suggesting that the bittern has stereoscopic vision without having to move the head.
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“It seems from the picture that the long bill does not bisect the stereo vision. Indeed, the long bill might increase the accuracy of the stereoscopic system, as any deviation from the correct firing angle will cause the bill to block one of the eyes from seeing the prey. In other words, the straight-line distance between the two eyes may have evolved to be of a particular fraction of the distance between the eyes and the tip of the bill, so as to optimise the accuracy of the aiming system.
“When the bittern has locked on to the target, the neck extends fully (or to the extent required) in a split second, catapulting the bill towards the target (above right).
“Notice that the nictitating membrane is closed during firing to maintain the watertight integrity of the eye socket. The accuracy of the correction for the refraction of light is indeed amazing, as it has only one chance to catch the fish.
“After catching the fish in the tip of its bill, it flips the fish nearer to the base of the bill in preparation for ingestion. There is a transition moment at which the fish (still alive, with fins flapping) is in midair (right).
“All this happens extremely fast, and before you know it, it has swallowed the fish. It has to happen this fast because the fish would otherwise have a higher chance of wriggling free.
“This bird is a skilled fisherman.”
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Categories: Feeding-vertebrates, Heron, Egret, Bittern
Comment by Amila Salgado
Made Thursday, 17 of January , 2008 at 12:43 pm
I enjoyed read this one thouroughly. Fantastic photo-work and interpretation.
