In defence of cuckoos

by Sarah Lloyd - 21 July 2009

Pallid Cuckoo

I'm always defending cuckoos! Some people consider that birds that lay their eggs in another bird's nest and leave the rearing of their young to foster parents are so despicable that they shouldn't be allowed to exist. But cuckoos have several redeeming features that make them worth defending: their songs herald spring's arrival, their plumage is either toned in camouflaging hues or is strikingly iridescent and they are among the few birds that control populations of hairy caterpillars that most other birds find distasteful and avoid.

All four cuckoos in Tasmania are migratory and after arriving in about mid September they sing their songs (which admittedly lack the varied repertoire of songbirds') to attract a mate. Fan-tailed and Pallid Cuckoos often sing from an exposed perch such as a dead branch, power line or fence. The Fan-tailed sings a mournful descending trill, the Pallid a rising octave of microtones. The beautifully coloured iridescent green Horsfield's and Shining Bronze-cuckoos usually keep well hidden in dense vegetation, their incessant singing the only indication of their presence. That is, unless you're fortunate, as I was recently, to encounter a trio of displaying males.

At the Kate Reed Reserve I watched three Shining Bronze-cuckoos as they preceded my walk along the track. They displayed with spreading wings, sang repeatedly and searched for insects amongst the foliage of the sheoaks and native cherries. But they were not after just any insects. On three occasions in the space of ten minutes I watched the cuckoos capture caterpillars – two hairy, one smooth.

Caterpillars have a number of defences that make them unpalatable. Firstly, toxins from their food plant accumulate in their bodies; and secondly, many are coated in hairs, some of which are urticating. Urticating hairs (like those on stinging nettles Urtica spp.) come in two types: some are tubular an contain venom which is injected into the unwary; others simply break off and are small enough to penetrate clothing or skin and cause irritation (something akin to handling fibreglass insulation). Cuckoos cope with the caterpillars' gut contents by first biting off their heads then shaking them about to expel the toxic innards. After this the caterpillar is swallowed whole – hairs and all. The caterpillars' hairs form a felted mat on the stomach lining which is eventually expelled as a pellet.

Further on in the reserve I noticed a Black-headed Honeyeater moving between a silver wattle and a eucalypt. While this is a reasonably common sight, an intermittent 'cheep' that emanated from the wattle prompted closer examination. I had walked right past a young Pallid Cuckoo whose colouring so closely resembled the mottled wattle bark that I'd failed to see it. Though still quite young, it was twice the size of its black-headed honeyeater foster parents who were kept extremely busy ramming food into the mouth of the hungry chick.

A month or so later I witnessed a similar event. The Pallid Cuckoo was a substantial size, almost fully fledged and flying around. It was being dive bombed by Dusky Woodswallows, among other birds, who obviously regard any cuckoos as a threat to their own reproductive success. Undeterred, the surrogate parents, in this instance Strong-billed Honeyeaters, were diligently feeding their enormous offspring.

As mentioned above, all four cuckoos that breed in Tasmania are brood parasites. They lay their eggs in the nests of other birds and leave the rearing of young to the foster parents. Horsfield's and Shining Bronze-cuckoos use the nests of thornbills, fairy-wrens, scrubwrens, flycatchers, silvereyes or honeyeaters; Pallid Cuckoos lay their eggs and leave their young in the care of honeyeaters, flycatchers, woodswallows or cuckoo-shrikes; Fan-tailed Cuckoos lay their eggs in the domed nests of fairy-wrens, thornbills or scrubwrens and sometimes in the cup nests of honeyeaters and flycatchers.

Before laying her eggs the unobtrusive female cuckoo will closely watch the nest building activities of a potential host and memorise the locations of the nests in an area. While the conspicuous male cuckoo sings loudly and distracts the chosen host, the female will fly to the nest, remove the host's egg and lay (or place) one of her own. Unlike most birds whose egg laying can take minutes, she can lay her egg (which has evolved to closely resemble the hosts' eggs) in seconds. After hatching, the cuckoo chick will use its flattened back to eject any other eggs or chicks that happen to be in the nest. Cuckoos are often maligned for their parasitic nesting habits, but this anthropomorphism ignores their value.

A month ago I saw a Horsfield's Bronze-cuckoo eating hairy leaf skeletonisers; more recently I watched a young Shining Bronze-cuckoo, while waiting for its Brown Thornbill parents to bring food, eat the pear and cherry slugs that cause unsightly damage to fruit trees during summer. Surely reasons enough for their existence.

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