It is well known that females will usually lay two eggs, but only one chick will survive. It is not known if or how often a pair may in fact successfully fledge two chicks. From many observations over the years, I’ve never seen a pair with more than one dependent young.

The second egg is generally smaller in size and weight compared to the first egg. Usually only one chick survives despite two eggs often being laid. The second and smaller chick is often out-competed by its older sibling. This is a common reproductive strategy for many Australian parrots where the second egg functions as an “insurance policy” if the first egg fails (1) .

During September 2021 a pair of Yellow-tailed Black-Cockatoos appeared to have two dependent young. Only four birds were present for approximately 30 minutes, with the following observations being made:

  • The appearance of both young birds noted and photographed (one male with pink eye ring in development which is not unheard of in first year males, and the other likely female) and both birds vocalising and behaving as first year YTBC chicks do (e.g. clumsily feeding on Banksia, body position and posture). The other two birds were clearly an adult male and female.

The only other explanations would be if one young was last year’s chick; however the behaviour of either bird didn’t fit this. Alternatively, if one young bird has somehow become seperated or lost its parents and has latched on to another family; this seems rather implausible though.

References
1. Way, S. L. and van Weenen, J. (2008) Eyre Peninsula Yellow-tailed Black-Cockato (Calyptorhynchus funereus whitei) Regional Recovery Plan. Department for Environment and Heritage, South Australia.

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