The Ballet of the Unhatched Chicks

A not-so-preposterous notion

In April I listened with pleasure to the Taku Winds’ performance of Moussorgsky’s Pictures at an Exhibition. One of the ‘pictures’ illustrated in music is called (in English) the Ballet of the Unhatched Chicks. At first glance, that would seem to a preposterous notion—chick dancing inside their eggs!?

Although I don’t know what Moussorgsky envisioned, and the actual picture that may have prompted the musical image was perhaps a cartoon, it is not as preposterous as it may seem. When a chick has used up the yolk inside its egg and now fills up its eggshell, it starts to move around, shifting into position for breaking out of the shell. Then the chick punctures the membrane that creates an air chamber at the blunt end of the egg and starts pecking at the shell itself. As it pecks, it rotates (reportedly counterclockwise) by shoving with its legs. The process of pecking and shoving takes a day or two. With the help of a temporary ‘egg tooth’ on its bill and a special, temporary hatching muscle on its neck, the chick eventually hacks open the egg shell and emerges into the larger world of its nest. Sometimes assisted by a parent bird, it is a very functional ballet!

Incubating parent birds may also ‘talk’ to their eggs, as the eggs approach hatching time. In at least one case, the unhatched chicks learn these parental calls. A detailed study in Australia (yes, far from Alaska, but a very thought-provoking study) with superb fairy-wrens is a fascinating and complex example.

Superb fairy-wrens live in scrubby habitats in southeastern Australia. They are subject to brood parasitism by a species of bronze-cuckoo, which lays its eggs in nests of this fairy-wren, leaving the host birds to rear the cuckoo chicks. The fairy-wrens pay a high cost if there is a cuckoo chick in their nest: the cuckoo chick eventually evicts all the fairy-wren chicks!

So it is advantageous for the fairy-wrens to recognize their own chicks, discriminating them from cuckoo chicks. Female fairy-wrens have a special incubation call, and they talk to their embryonic chicks in the eggs. The chicks learn this call; the more often they hear it, the better they learn it. They use this call after they hatch, when they beg food from a parent. Chicks that use the ‘password” are fed more often than those that don’t. In contrast, cuckoo chicks apparently do not learn this call, making it relatively easy for host parents to tell their own offspring from an interloper. When the host parents don’t hear the right password from chicks in the nest, they can simply abandon the whole brood, which would include a cuckoo chick (and soon consist only of a cuckoo chick). Then the parents can start over with a new nest, hoping not to be parasitized again.

However, there is a price to pay for a frequent incubation call: predators hear it and often raid the nest. So female superb fairy-wrens use the call most frequently when the risk of cuckoo parasitism is high, and apparently that risk varies considerably from year to year. Thus, in some years, the risk of brood parasitism is high enough to outweigh the risk of predation, while in other years, the reverse is true.

I would not be surprised to learn that embryos of other species are capable of selectively learning certain calls and even behaviors. A fascinating area of research!

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