Nazca Booby Siblicide

The Galápagos Islands

Genovesa (general)
Genovesa (birds)

Obligate Sibling Murder in Nazca Boobies

(view pics at larger size)

Nazca adult and chicks - 30Kb

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.

Older chick pushing younger chick away - 30kb

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 older chick pushing younger chick out of nest - 25Kb

Close-up detail of the photo above.

Older chick making sure younger chick is gone - 29Kb

The older chick seemed to want to make sure that the younger one wouldn't try to get back.

Older chick returns to parent - 29Kb

The older chick then returned to tuck under its parent.
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'.


Abondoned younger chick - 27Kb

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.

Almost the end - 29Kb

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.

The Islands







 > Murder

San Cristobal

Santa Cruz

Santa Fé


Seymour Norte


   Farewell dinner '05

The Wildlife

Diary 2004

Diary 2005

Gal. Index


Bartolomé  ¦   Española  ¦   Fernandina  ¦   Floreana  ¦   Genovesa  ¦
   Isabela  ¦   North Seymour  ¦   Santa Cruz  ¦   Santa Fé  ¦   Santiago

Text and images © Liz Leyden
Email: liz [at] lizworld [dot] com
Don't hotlink to my images.

Many of my best Galapagos images are available, royalty-free, at:

View My Portfolio