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

Third-hatched nestling in broods of the laughing kookaburra(Dacelo novaeguineae) are often killed by aggressive attacksfrom their older siblings within days of hatching. By installingsurveillance cameras inside nest hollows, we examined nestlingaggression over the "siblicidal" period, in particular to identifywhether parental behavior and competitive disparities betweennestlings affected aggression, and hence the likelihood ofsiblicide. Aggression decreased as nestlings aged and dominancehierarchies became established. The first-hatched nestling was the most aggressive. Fighting between the first-hatched nestlingand its closest rival (second-hatched nestling) increased whenthe hatch interval between them was short, when the size differencebetween them at hatching was small, and when the second nestlingwas female. Female nestlings are faster-growing than males,so young sisters may be an incipient threat requiring preemptiveaction by older siblings. When the second-hatched nestlingwas female, the first-hatched also attacked the third-hatched nestling more frequently. Thus the third-hatched nestling seemsto experience some of the "overflow" of aggression occurringbetween its two older siblings. Nestlings in siblicidal broodswere not fed less compared to nonsiblicidal broods; this isunsurprising because siblicide occurs when feeding rates arecomparatively low. However, siblicidal nestlings were broodedless, and in shorter bouts, which allowed them more time tofight.