Siblicide in the cooperatively breeding laughing kookaburra (Dacelo novaeguineae) |
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Authors: | S Legge |
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Institution: | (1) Evolutionary Ecology Group, Botany and Zoology, Australian National University, Canberra, A.C.T. 0200, Australia e-mail: Sarah.Legge@anu.edu.au Tel.: +61-2-62798434, Fax: +61-2-62495573, AU |
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Abstract: | I describe siblicide in the laughing kookaburra (Dacelo novaeguineae), a reverse size-dimorphic, cooperatively breeding kingfisher. Clutches were usually of three eggs, and nestlings hatched
asynchronously, with intervals of 2–72 h between successive eggs. Siblicide occurred in two temporally and mechanistically
distinct episodes. The youngest nestling died in one-third of all nests within days of hatching as a result of aggression
from its elders. Kookaburra nestlings attacked each other using a hook on their upper beak – a rare example of a morphological
specialisation for sibling rivalry. In one-fifth of all nests, the youngest nestling starved to death much later, without
overt aggression, when nestling growth rates were highest. I examined the effects of food availability and competitive disparities
between nestlings on the incidence of both types of siblicide. The probability of late, starvation-mediated mortality was
negatively correlated with the number of male helpers. Early, aggressively mediated siblicide occurred in nests characterised
by a suite of correlated variables that I call the ”kookaburra siblicide syndrome”: (1) no male helpers attended the nest,
(2) the third-hatched nestling was much smaller than the second-hatched nestling, (3) the first and second nestling to hatch
were male and female, respectively, and (4) there was a short hatch interval between the first two nestlings. The kookaburra
siblicide syndrome variables could be inter-correlated if they were all related to the female’s condition at the onset of
incubation. Females in poorer condition may be less likely to have male helpers, more likely to lay small third eggs, and
more likely to hatch the first two eggs relatively synchronously because of nutritional constraints during the onset of incubation.
These females may further promote siblicide by modifying the sexes of the first two nestlings. If a female hatches soon after
an older but eventually smaller brother, dominance between the first two nestlings could be destabilised. I suggest this leads
to escalated aggression in the nest and the death of the third nestling, which is least able to defend itself.
Received: 17 December 1999 / Received in revised form: 8 May 2000 / Accepted: 20 May 2000 |
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Keywords: | Siblicide Hatching asynchrony Egg size Biased sex allocation Cooperative breeding |
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