August Bird of the Month: Horsfield's Bronze Cuckoo


They Fail as Husbands and As Wives

Ogden Nash, in his poem The Cuckoo, characterised  these birds as leading Bohemian lives – failing as husbands and as wives while cynically disparaging others' marriages.  He was much better as a poet than an ornithologist. 

Horsfield's Bronze Cuckoos, this month's featured bird species, are very successful serial monogamist husbands and wives.  And, as we shall see, their skills improve just that bit more year by year.   

Gladstone's Notebook

Image courtesy Wayne Weatherstone.
This article is number 8 in a 12 part series.  Each month we look at one of the birds recorded in Gladstone Weatherstone's notebook between 1962 and 1981, see if anything is different today and, if so, try to explain why.

 Gladstone was a dedicated and knowledgeable amateur naturalist who lived on Lyndfield Park near Gunning between 1941 and 1996. 

Horsfield's Bronze Cuckoo 

On 24 August 1980, Gladstone wrote "First Horsfield Bronze Cuckoo for season arrived today and also the first Pallid Cuckoo. Haven't recorded them in the immediate area for two years but this year earlier than usual."

Both these birds are regular spring and summer visitors to our district.  This article confines itself to Horsfield's Bronze Cuckoo.  To us, Gladstone's cuckoos would look and sound identical to those here today.  But there is an evolutionary arms race going on unperceived around us.  There are probably subtle but telling differences between the birds of 1980 and today.  These evolutionary changes are even more likely to have affected bird behaviour since 1941 when Gladstone started farming at Oolong. What's going on here?  And why weren't these cuckoos seen in the previous two years?  We tackle both these questions below.  Let's start with an introduction to the bird.

Photo courtesy Frederic Pelsy/Macaulay Library
 at the Cornell Lab of Ornithology (ML206185821)
Horsfield's Bronze Cuckoo, like most other cuckoos in Australia, deposits its eggs in the nests of unwitting host species.  It also usually removes one already laid by the conscript foster parent at the same time.  After that, the host bears the entire time and effort of rearing the young cuckoo.  

The female cuckoo typically goes on to repeat the process elsewhere, often with a new partner in a different territory.  If all goes according to plan, soon after emerging the cuckoo hatchling will ditch its foster siblings' eggs overboard or, if they manage to hatch, ejects them from the nest.

Thomas Horsfield MD (1775-1854)
Portrait by J.Erxleben.  Wikimedia Commons.

This cuckoo is a summer breeding migrant to southern Australia, arriving in our district around August-September each year.  It departs for places like northern Queensland, Indonesia and the Malay Peninsula in Autumn.  

Dr Thomas Horsfield first described the species in 1821 from a bird collected in Java.  He was a medical doctor born and educated in America, who worked as a surgeon for the Dutch colonial forces before moving to the British East India Company.  His real and consuming passion was in the flora, fauna and geology of what is now Indonesia.  

On his return to London, he became director of the British East India Company museum, describing and classifying many plants and animals.  His moniker features in the names of at least 15 other species as well as this cuckoo.

Superb Blue Wren drawn
by Gladstone Weatherstone

The primary host for Horsfield's Bronze Cuckoos is the Superb Blue Wren.  It also uses the dome-shaped nests of other small birds such as thornbills and gerygones as well as some open cup nests containing speckled eggs such as those of the Scarlet and Red-capped Robins.  

Female fairywrens do all the nest making and egg brooding themselves.  They are aided by up to four non-breeders which help in defending the nest and bringing food to the nesting female.

The Co-Evolutionary Arms Race 

Horsfield's Bronze Cuckoos begin arriving in late winter and pair up for breeding.  They inhabit a wide variety of lightly wooded habitats, where they often perch on a fence-post or exposed branch of a shrub.  We hear them more often than we see them.  Their persistent, descending whistled call continues throughout the day and sometimes into the night as well.  

Superb Blue Wrens and Horsfield's Bronze Cuckoos have been engaging in a co-evolutionary arms race over a very long time.  The cuckoo has been adapting to improve its ability to breach the hosts' defences and the fairywren strengthening its capacity to identify and repel attackers.  The 2020 models of both species are likely to be improvements, albeit slight ones, on their 1980 editions.  They are even more likely to be sharper, more formidable birds than their 1941 forebears.

The two competitors in the evolutionary arms race- Horsfield's Bronze Cuckoo
(above) and Superb Blue Wren (right).
Both photos courtesy Duade Paton.


Each year the ongoing war between host and parasite plays out over three stages of battle. Let's look at the fairywren's defence strategies and the cuckoo's responses at each of these stages.  

1 Prevention is Better Than Cure

The cuckoo aims to leave her egg in a host nest.  The fairywren's goal is to prevent the cuckoo from getting into the nest at all, thereby saving a lot of trouble and wasted energy later.  It does this by vigorously mobbing any cuckoo coming into view in the hope of making the interloper give up and try its luck elsewhere.  

This first line of defence is now failing more oftern.  Why?  Because the cuckoo is now very skilled at remaining unobserved and has got egg-laying in the host nest down to a fine art.  The cuckoo generally makes its attack at times in the morning when the female fairywren is temporarily off the nest.  It also seems to target first time mums who lack experience of the looming problem.

Laying speed is vital if the cuckoo is to do her work unobserved.  She deposits her egg into the host nest in as little as 1.5 seconds and just under 6 seconds at most – giving her a good chance of getting away with it.  Cuckoos have probably become much speedier layers over time.

2 Reject  Bad Eggs.  Accept Genuine Products Only.

The fairywren's first line of defence is now failing more often.  Cuckoos are better at depositing their eggs undetected.  But the fairywren can still take remedial action if it sees an imposter's egg.  The cuckoo's substitute has to pass muster in comparison to those of several different species. It is a serviceable all-purpose fake rather than a perfect counterfeit of its primary host.  In the dull light of the domed fairywren nest, the phoney is still good enough to pass the scrutiny of the host bird. 
Egg colouring is not the only giveaway.  Any alert fairywren might become suspicious if it sees a cuckoo lurking nearby and an unusually large egg in the nest will ring loud alarm bells.  But cuckoos have been working on this problem.  The cuckoo, although twice the size of the wren, now lays disproportionately small eggs which often pass for those of fairywrens – particularly in poor light.  

Superb Blue Wren female. 
 Image courtesy  Mark Lethlean

A third and very cunning defence developed by the fairywren is for the mother to teach her chicks a secret code while they are still in the egg.  This is a short snatch of song which she sings to the eggs and can expect to hear replied if they contain her progeny.  At this stage of the evolutionary arms race, the cuckoo has not learned how to give the response needed when first challenged – which can bring it unmasked.  Nevertheless, many still get through.

3 Cull Counterfeit Chicks

Until recently, if the cuckoo egg was accepted, the contest was usually over - the cuckoo had won, and the host would brood the imposter's egg with its own.  After emerging from the egg, the cuckoo hatchling would, sooner or later, despatch its foster siblings and so enjoy the undivided attention of its conscript parents.

Now there is evidence that some fairywrens see that the large, insistently begging hatchling in their nest is bad news.  A recent study* showed female fairywrens abandoned newly hatched cuckoo chicks within 2-6 days in 38% of cases observed.  While cuckoos hatch able to sound like a begging Superb Blue Wren, their mimicry is no longer fooling some fairywren mothers.  The cuckoo's call may not be good enough.  And it may also arouse the fairywren's suspicions if there is only one chick left in the nest.  Second time mothers, in particular,  are more likely to be a wake-up and abandon the nest with its unwanted foundling.

The evolutionary battle is unlikely to end at this stage.  Cuckoo chicks are quick learners and determined runners in the race for survival.  Although they emerge from the egg mimicking the Superb Blue Wren, they can speedily change their begging call.  If they find themselves in, say, a thornbill's nest or they just aren't getting a positive response to their begging they can quickly change their tune – often with success.  Future generations of cuckoo chicks will likely get better at imitating their hosts.

Too late for any defensive tactics.  A Superb Blue Wren saddled with an insatiable and insistent Horsfield's Bronze Cuckoo                             Image courtesy Mark Lethlean birdlife

New Improved Models?  Are Today's Wrens and Cuckoos Different to the 1980 Editions?

Professor Naomi Langmore is a  co-author of this study on the co-evolutionary arms race.  She has confirmed to me that today's cuckoos and fairywrens are likely to be just that little bit different to those Gladstone saw in 1980.  The 40 generations that have passed since then are certainly enough for both species to modify their behaviours.  Cuckoo egg-laying speed will on average be a few nanoseconds faster, eggs just a bit smaller and chicks even better mimics.  Fairyrens will be relying more on the third line of defence in giving newly hatched cuckoo chicks the heave-ho and getting more skilled at identifying them.

Why Did Cuckoos Stay Away for Two Years?

Gladstone's 1980 sighting followed two years in which he saw neither Horsefield's Bronze nor Pallid Cuckoos in the area.  This is almost certainly because his place did not carry enough Superb Blue Wrens and other potential hosts to make it attractive to cuckoos.  

Image courtesy Mark Lethlean.  
  A study at one site**  showed that a female Horsfield's     Bronze   Cuckoo would not parasitise a territory with fewer   than 23   breeding pairs of its primary host.  If this holds more   widely,   the fairywren pictured left shares a productive   breeding   environment with many others – otherwise, she   would not be   troubled by the changeling confronting her here.

 Landscapes lacking trees and understorey shrubs, as     Gladstone's place was in 1980, are not ones in which many of     our woodland birds can flourish.  

 Horsfield's Bronze Cuckoo is one of many species which,   although not necessarily critically endangered yet, are   decreasing in number due to the loss of our woodlands.  Over   the last few decades land managers, governments and organisations like Greening Australia have been working to turn this around.  There is clear evidence that integrating trees and shrubs into the farming landscape can make for healthy bird communities in productive farms.  When we see and hear these birds, we know we are on the right track.

What's the Cuckoo Ever Done for Us? 

O blithe New-comer! I have heard,
I hear thee and rejoice.

In his poem "To the Cuckoo" William Wordsworth was in no doubt that the cuckoo (to him the "Thrice welcome, darling of the Spring") was of almost inexpressible spiritual value to humankind.  Perhaps so, but there are more practical reasons why we should welcome them.

 In November 1894 the Victorian Government Entomologist, Mr C French F.L.S., F,R.H.S, visited Rutherglen in response to pleas from the Vinegrowers' Association for help in dealing with the damaging and seemingly intractable Vine Moth.  Caterpillars of this moth were destroying their vines.   Mr French had several remedies for them.  One was to use a weak solution of a spray called Paris Green.  He also praised the worth of Horsfield's Bronze Cuckoo, and the Pallid Cuckoo, telling the growers:

"These birds are very fond of caterpillars generally, but particularly those of the vine moth: the singular fact being that although many of our domestic birds will hardly touch this caterpillar these two cuckoos would appear to be singularly fond of them and destroy enormous numbers.  Bronze cuckoos, when for scientific purposes have been killed, upon examination of their stomachs were found to be crammed with caterpillars of this vine moth."

He did not go on to recommend they integrate trees and shrubs into their vineyards.  However, if Greening Australia been operating then, I think he would have suggested this.

These cuckoos are also very fond of what many of us know as Spitfire Grubs – the larvae of a species of sawfly.  Spitfires are commonly found clustering in large numbers on eucalypts, melaleucas, callistemons and other native trees.  In the absence of controlling predators like cuckoos,  the larvae can do considerable damage to these trees.

Image:  Department of Environment and Water South Austrlia

As well as being great pest controllers, these birds are reliable indicator species to tell us that we have an excellent suite of small birds present.  Cuckoos only come and stay if there are good numbers of fairywrens, thornbills and the like – all of which are part of a healthy and productive ecosystem. 

Thank You

Thank you to:

Nicki Taws (Program Specialist – Ecologist, Greening Australia);
Dr Tony Saunders (ornithologist and President of the Crookwell Native Flora and Fauna Club);
Mark Clayton (former Senior Technical Officer, CSIRO Wildlife and Ecology).  

Mark, Tony and Nicki review drafts of these articles before publication.  I appreciate their help.

Thank you also to:

 John Weatherstone for providing a transcription of Gladstone's Notebook and fielding lots of follow up questions about Gladstone related matters.
Professor Naomi Langmore for advice about the surprisingly rapid evolution of host defences and cuckoo counter adaptations over time.

Further Information

Lyndfield Park; looking back moving forward.  John Weatherstone, 2003.  Tells the story of Gladstone's former property as it moved from conventional to restorative farming.

Call of the Reed Warbler:  A New Agriculture – A  New Earth.  Charles Massy, 2017.  A book about regenerative agriculture, including a complete chapter on Lyndfield Park.

You can obtain a short biography of Gladstone Weatherstone as a PDF file by emailing  


The main sources used in writing this article were: 

"The Australian Bird Guide" Peter Menkhorst et al.  CSIRO Publishing
"Birds of the Upper Lachlan Shire" Dr Tony Saunders
"Cuckoo Notes" C. L. Barrett (1905), Emu - Austral Ornithology, 5:1, 20-23, DOI:
"The Caterpillar Pest" Corowa Free Press (NSW : 1875 - 1954), Friday 30 November 1894
"Australian cuckoos and their adaptations for brood parasitism" Naomi Langmore.  Chinese Birds 10.5122/cbirds.2013.0007
N. E. Langmore & R. M. Kilner (2010) The co-evolutionary arms race between
Horsfield's Bronze-Cuckoos and Superb Fairy-wrens, Emu - Austral Ornithology, 110:1, 32-38, DOI:10.1071/MU09032
"Bringing Back Birds. A Glovebox Guide" Nicki Taws, Greening Australia Capital Region. 2007

*  N. E. Langmore & R. M. Kilner (2010) The coevolutionary arms race between
Horsfield's Bronze-Cuckoos and Superb Fairy-wrens, Emu - Austral Ornithology, 110:1, 32-38, DOI:
To link to this article:

**  Langmore, Naomi & Kilner, Rebecca. (2007). Breeding site and host selection by Horsfield's bronze-cuckoos, Chalcites basalis. Animal Behaviour. 74. 995-1004. 10.1016/j.anbehav.2007.02.028.