Kookaburra sibling rivalry gets rough

Kookaburras may have the most famous laugh in the bird world, but life for their nestlings doesn’t sound particularly funny.

The chunky Australian kingfishers usually lay three eggs in a nest. The older two hatchlings commonly kill the youngest, reports Sarah Legge of the Australian National University in Canberra. In the September Behavioral Ecology and Sociobiology, she describes two ways that older sibs do in the youngest.

First, Legge observed the two elder siblings attacking the third sibling outright. They hacked at the youngster with a hook on their upper beaks, which Legge calls “a rare example of a morphological specialization for sibling rivalry.” The youngest birds died of such injuries in a third of all the nests she observed. The other big danger for the youngest kookaburra comes from slow starvation. In a fifth of Legge’s nests, the older siblings hogged the food.

In the starvation scenario, one risky circumstance was hatching into a nest where there were no extra adult males pitching in on child-rearing duties. Moreover, an eldest brother followed by a sister made an especially dangerous nest for a third hatchling, Legge reports.

Kookaburra females quickly grow larger than males, and when sis surpasses big brother, it destabilizes the nestling dominance hierarchy. Legge blames starvation in the nest, in part, on troubled kookaburra mothers. Females in poor condition are less likely than thriving females to have extra male helpers hanging around. Also, a stressed mom may be unlikely to create a healthy third egg and incubate it for fast hatching.

Because the circumstances of starving junior nestlings fit such a consistent pattern, Legge calls it “kookaburra siblicide syndrome.”