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Third-hatched nestling in broods of the laughing kookaburra (Dacelo novaeguineae) are often killed by aggressive attacks from their older siblings within days of hatching. By installing surveillance cameras inside nest hollows, we examined nestling aggression over the "siblicidal" period, in particular to identify whether parental behavior and competitive disparities between nestlings affected aggression, and hence the likelihood of siblicide. Aggression decreased as nestlings aged and dominance hierarchies became established. The first-hatched nestling was the most aggressive. Fighting between the first-hatched nestling and its closest rival (second-hatched nestling) increased when the hatch interval between them was short, when the size difference between them at hatching was small, and when the second nestling was female. Female nestlings are faster-growing than males, so young sisters may be an incipient threat requiring preemptive action by older siblings. When the second-hatched nestling was female, the first-hatched also attacked the third-hatched nestling more frequently. Thus the third-hatched nestling seems to experience some of the "overflow" of aggression occurring between its two older siblings. Nestlings in siblicidal broods were not fed less compared to nonsiblicidal broods; this is unsurprising because siblicide occurs when feeding rates are comparatively low. However, siblicidal nestlings were brooded less, and in shorter bouts, which allowed them more time to fight.

Nestling aggression in broods of a siblicidal kingfisher, the laughing kookaburra