While I was photographing a Galápagos Mockingbird, so was bringing up the rear as usual, the vanguard of our group had seen the older chick virtually lift the younger bird by the neck and begin to push it out of the 'nest' area.
I had read about the Nazca Booby's "obligate siblicide" before my trip last year, so wasn't quite so shocked, but I hadn't seen it for myself.
Nazca Boobies lay two eggs, several days apart. If both hatch, the older chick kills the younger. The younger chick is an 'insurance' offspring in case the older one is predated or dies.
It didn't take long before it consolidated its one-sided victory by ensuring the tiny chick was pushed further away and unlikely to return.
Close-up detail of the photo above.
The older chick seemed to want to make sure that the younger one wouldn't try to get back.
The older chick then returned to tuck under its parent. chick.
What shocked all the mothers in the watching group was that the parent was totally uninterested in the goings on. It did seem strange that it's inborn 'instinct' about the obligate sibling murder 'trumped' its parenting instinct, but of course, it presumably had done exactly the same to its younger sibling when it was a chick, unless it was the 'insurance'.
The young chick now has no chance and will probably be picked off by a Frigatebird or even a Mockingbird. Certainly the Mockingbird I'd been photographing was very close by.
There were some kids passing through, and of course they wanted to push it back under the parent bird (they were very angry with the 'mother' bird, not the older chick) or to rescue it themselves: their guide had to work very hard to persuade them that they mustn't interfere with nature.
We walked along to the end of the trail, and when we got back, the older chick was under its roosting parent and the chick was lying prostrate, possibly dead.
I lurked behind as usual, and the younger chick (shown by the red arrow) raised its head and cheeped a couple of times - totally ignored by its parent and older sibling.
That's when it really 'got' to me.
Post Office Bay
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Giant Prickly Pear
Farewell dinner '05
Life on Board '04