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Masters of Disguise

A fairy-wren feeds a hungry Horsfield’s bronze-cuckoo fledgling.

A fairy-wren feeds a hungry Horsfield’s bronze-cuckoo fledgling.

By Naomi Langmore

To avoid rejection by their hosts, Australian bronze-cuckoo chicks are near-perfect visual and vocal mimics that can quickly modify their call to match the species they are parasitising.

For millenia, Australia’s bronze-cuckoos have tricked other birds into rearing their young. The female cuckoo quietly removes a single egg from the nest of the host and replaces it with one of her own, and the unsuspecting host incubates the egg along with her own clutch.

Upon hatching, the cuckoo chick is naked and blind, and no bigger than your thumb nail. Yet despite its helpless appearance, it is a systematic and ruthless killer. Within hours of hatching, the tiny cuckoo sets about pushing the host’s own eggs and chicks out of the nest, one by one. Then, secure in its position as the sole occupant of the nest, it sets up an incessant demand for food and grows at such a rate that within 3 weeks it is double the size of its foster parents (Fig. 1).

This extraordinary behaviour is called brood parasitism. Although cuckoos are the most famous of the brood parasites, this behaviour also occurs in other birds such as ducks and finches, as well as some species of insects and fish.

Australasia is a particular hot-spot for cuckoos – there are more species of parasitic cuckoos here than in the rest of the world put together. Thus cuckoos are a major factor in the life history of many Australasian birds. For example, ten species of cuckoo breed in Australia, and between them they parasitise around 65% of Australia’s songbirds, with 28% being major cuckoo hosts.

Studying cuckoos brings you close to some of the most impressive interactions in the animal world. The sight of a newly hatched cuckoo chick (Fig. 2) evicting its nestmates from the nest, or a tiny host bird working from dawn until dusk in an attempt to satisfy the insatiable hunger of an enormous cuckoo chick leaves you in no doubt about the vast costs involved in rearing a cuckoo chick.

In the face of such high costs, it’s not surprising that host birds have evolved strategies to defend themselves against cuckoos. The main defence adopted by cuckoo hosts around the world is to throw odd-looking eggs out of their nests. Hosts have evolved remarkably refined abilities to detect subtle differences between eggs and reject any that are not their own.

However, the hosts of Australian bronze-cuckoos are faced with a problem – their nests are dome-shaped and very dark inside (Fig. 3), making it difficult to detect differences in egg colour and pattern. Instead, bronze-cuckoo hosts have by-passed egg rejection and evolved an entirely novel defence against cuckoos – rejection of hatched cuckoo chicks.

We first discovered hosts rejecting cuckoo chicks during our study of the Horsfield’s bronze-cuckoo (Chalcites basalis) and its host, the superb fairy-wren (Malurus cyaneus) in Canberra’s woodlands. Superb fairy-wrens are cooperative breeders; the breeding pair is assisted in rearing their offspring by “helper” males– the sons from previous broods.

The breeding female is the first to detect the cuckoo chick in the nest. She stops feeding her adopted offspring ~2–6 days after hatching, and immediately begins building a new nest nearby. The males in the group are a bit slower on the uptake and carry on feeding the cuckoo for a few more hours, but eventually they realise what the female is up to and they too abandon the chick, leaving it to die in the nest.

Recently, chick rejection has also been discovered in two hosts of the little bronze-cuckoo (Chalcites minutillus) in northern Australia. These hosts, the large-billed gerygone

(Gerygone magnirostris) and the mangrove gerygone (Gerygone laevigaster), are even more efficient chick rejecters; they throw the cuckoo chick out of the nest within hours of hatching, sometimes in time to save their own brood of young.

If there’s one rule about cuckoo–host interactions, it’s that for every defence evolved by a host, the cuckoo will retaliate with an equally cunning new trick to circumvent the host’s defence. For example, in response to egg rejection by hosts, many cuckoo species have evolved exquisite mimicry of host eggs. This led us to wonder whether bronze-cuckoos had evolved comparable tricks to prevent chick rejection by their hosts. Perhaps, just as cuckoos have evolved egg mimicry to prevent egg rejection, they might also have evolved chick mimicry to prevent chick rejection?

To test this idea we quantified the appearance and begging calls of three bronze-cuckoo species, the Horsfield’s, shining and little bronze-cuckoos, and compared them to their hosts.

To our considerable amazement, we found that the cuckoo chicks are spectacular visual mimics of their hosts (Fig. 4). In order to match their hosts, the three cuckoo species have evolved pink, yellow and black skin, respectively. They not only mimic skin colour, but also the colour of the mouth and the broad border around the mouths of nestlings. To make the match perfect, the little bronze-cuckoo has even evolved fluffy white nestling down, a trait that is common in songbirds but unique among cuckoos.

This superb visual mimicry is all the more striking when you consider that it is useful for all of 7 days, when the cuckoo begins to sprout feathers. It seems that this is long enough to convince the host parents that the chick is theirs, because the feathered cuckoo is not such a close match to the host chicks.

The cuckoos have also evolved begging calls that match their host’s calls. For example, Horsfield’s bronze-cuckoos produce a soft “peep peep” like their fairy-wren hosts, whereas shining bronze-cuckoos produce a long, rasping whine like their thornbill hosts.

Perhaps most remarkable of all is the versatility of the Horsfield’s bronze-cuckoo, which can adapt its mimicry to enable it to exploit more than one host species. When reared by a fairy-wren (the primary host), Horsfield’s bronze-cuckoos produce a “peep” call, but when reared by a thornbill (a secondary host) it produces a whining call.

We were intrigued to know how the cuckoo knows which call to produce. Are there separate races of the Horsfield’s bronze-cuckoo, each of which specialises on a different host, or does the cuckoo change its call depending on which host rears it? To test this, we transferred some cuckoo eggs from fairy-wren nests to thornbill nests. If the eggs belonged to a “fairy-wren” race of the cuckoo, we predicted that they should produce a fairy-wren call when they hatch. On the other hand, if cuckoos can modify their calls after hatching, they should produce a thornbill call.

We waited in great anticipation for the eggs to hatch, microphones and tape recorders at the ready. On the day the chicks hatched they produced a fairy-wren type call, but within 2–3 days they began to modify their calls, gradually changing them from a peep to a whine (Fig. 5). This was an astonishing feat, because the cuckoos were alone in the nest and had never heard a thornbill begging call.

We suspect that the cuckoos switch to an alternative call if they aren’t receving enough food. Either they try out different sounds to see which one elicits a parental response (the same way that human babies develop words from babbling), or the thornbill call may be a pre-programmed back-up call if the fairy-wren call doesn’t work.

The puzzle of why host birds rear cuckoos has intrigued naturalists and scientists for more than 2000 years, ever since Aristotle (384–322 BC) observed that “it lays its eggs in the nest of smaller birds after devouring these birds’ eggs”. Our observations and experiments are inching us ever close to resolving this puzzle, and the Australasian cuckoos are adding a wealth of new information to this field.

But there is still much to find out!

Naomi Langmore is an Australian Research Fellow in the Research School of Biology at the Australian National University.