Horsfield's bronze cuckoo

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Horsfield's bronze cuckoo
Horsfield's Bronze-Cuckoo Newhaven.jpg
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Cuculiformes
Family: Cuculidae
Genus: Chrysococcyx
Species: C. basalis
Binomial name
Chrysococcyx basalis
(Horsfield, 1821)

The Horsfield’s Bronze-cuckoo (Chrysococcyx basalis) is a small cuckoo in the family Cuculidae. Its size averages 22g[2] and is distinguished by its green and bronze iridescent colouring on its back and incomplete brown barring from neck to tail. What distinguishes the Horsfield’s Bronze-cuckoo from other bronze cuckoos is its white eyebrow and brown eye stripe.[3]The Horsfield’s Bronze-cuckoo is common throughout Australia preferring the drier open woodlands away from forested areas.[4]


The Horsfield’s Bronze-cuckoo is one of five Australian species in the genus Chrysococcyx (formally Chalcites) a type of parasitic bird,[3] that parasitises fairy-wrens primarily to raise their young.[5][2]

The Five species of Cuckoo in the genus Chrysococcyx;

- Black-eared Cuckoo (Chrysococcyx osculans)

- Horsfield’s Bronze-cuckoo (Chrysococcyx basalis)

- Shining Bronze-cuckoo (Chrysococcyx lucidus)

- Little Bronze-cuckoo (Chrysococcyx minutillus)

- Gould’s Bronze-cuckoo (Chrysococcyx russatus)

Diet and Behaviour[edit]

Photographed at Capertee Valley, NSW, Australia

The main diet of the Horsfield’s Bronze-cuckoo are insects and they are nomadic, travelling to different regions of Australia to breed and find food.[5]Small insects are taken from leaves, branches, caught on the wing and in breeding season, Horsfield’s Bronze-cuckoos feed each other in a courtship ritual.[6]

The Horsfield’s Bronze-cuckoo is known as a brood parasite, this means that they lay their eggs in a host species nest.[7]They mainly parasitise the fairy-wrens in the genus (malarus sp). It has been well documented that the Superb Fairy-wren (Malurus cyaneus) and the Splendid Fairy-wren (Malurus splendens) are the two main species to bare host to the Horsfield’s Bronze-cuckoo, although they may also parasitise other small Passeriformes including Thornbills, Warblers and Scrub-wrens that can be utilised as a secondary host in certain locations.[2][8][5][9] Although the behavioural attributes of a host species may play a role in parasitism, it is thought that the female selects its host through imprinting, remembering the species that it was raised by and ultimately using that species to raise its brood.[10] [11]


The Horsfield’s Bronze-cuckoo’s are known to form monogamous pairs in the breeding season and occupy the same breeding territories as their host species, however partnerships are short lived as a female will only occupy the breeding territory for a few weeks, as another female takes her place, she may form a pairing with the same male.[5]Females that leave a breeding site after several weeks may move to another site and continue to breed with another male, forming another bond in a new breeding territory. Breeding territories of the Horsfield’s Bronze-cuckoo generally do not overlap giving rise to the possibility that a pair will defend an area through the season.[9]


As a brood parasite the Horsfield’s Bronze-cuckoo does not build its own nest but will utilise a host species nest (Malurus sp) to lay their eggs. The breeding season for the Horsfield’s Bronze-cuckoo relies on their host and they will lay one elongated pinkish-white egg, that is speckled with red-brown spots to mimic that of the fairy wren or thornbills egg.[2] [12]The breeding season for the Superb Fairy-wren is between September – February and a female may have three consecutive broods in this time, allowing the Cuckoo multiple attempts to parasitise this species.[13][14]The female cuckoo may choose a breeding site with a high density of hosts, which allows extra opportunity for her success in parasitising a nest successfully. Studies have shown at one site a female did not parasitise a territory with less than 23 breeding pairs of their primary host (Malurus cyaneus).[5]

The egg of a Horsfield’s Bronze-cuckoo is small for its size, evolving overtime to mimic those of their host in what can be described as an evolutionary arms race between parasite and host, also the smaller the host for the cuckoo, the likelihood of successfully raising multiple broods thus the energy and nutrients needed to produce more smaller eggs than few larger eggs can be utilised more efficiently.[15][16]Egg laying is very fast for the Horsfield’s Bronze-cuckoo, being able to lay an egg in under 6 seconds typically in the morning shortly after the host has laid,[17]the adult cuckoo removes one egg each time she lays, only laying one egg per nest and replacing one host egg with one of her own.[14]

Younger semi-experienced females were generally selected over new and novice breeding females due to their success and experience. The Horsfield’s Bronze-cuckoo chose females that would choose similar breeding sites to previous years and were likely to raise several broods in one season.[15]Generally the superb fairy wren will not reject the cuckoos egg, fairy wrens make oval dome nests that can be dark inside, meaning it is harder for the fairy wren to distinguish between their own egg and the host’s egg, furthermore the mimicry in eggs from the Horsfield’s Bronze-cuckoo have evolved over time and are hard to distinguish besides their slight elongation and glossier finish.[2][9][12]

The cuckoo chick hatches within 12 days of incubation, 2 days before the host egg, ejecting other eggs in the nests within two days of hatching, leaving the cuckoo the sole chick. As newly hatched cuckoo chicks eject host eggs they do not get to learn the host’s begging called but can possess begging call polymorphism, where nestlings produced the calls of their primary host.[8][12][18][19]As the nestling grows it will be fed by the host parent and possibly the group growing more rapidly until fledged.

Co-Evolutionary Arms Race[edit]

Counter adaptations have been documented for host species and cuckoos alike. As each species evolves it will adapt to its environment and ultimately to its competition. In what is called a co-evolutionary arms race to better the species, parasite against host race.[20]Studies show that co-evolutions happens at all stages of the growth cycle, not just the early stages.[21]In the case of the fairy wrens, they have adapted some host defences to try and minimise parasitism. The cost of hosting a parasitic species is high, in energy and genetics, the defences from the host increase, in turn increasing the parasites defences in a co-evolutionary arms race between the species.[22]

Some of the adaptations that the host employs are:

-Adult hosts will drive cuckoos away in groups to reduce parasitism.[23]

-Hosts like to nest in large colonies to deter adult cuckoos then chasing them away if a cuckoo is spotted, increasing nesting success.[24]

-Helpers in large colonies provision the female so she can attend the nest more proficiently[25]

-The ability to learn and recognise their own eggs, and abandon any that are in the nest before they have started their own.[15][12]

As a counter move the cuckoo also evolves adaptations:

-Mimicry of host eggs.[26][21][27][28]

-Eggs that are cryptic and therefore unable to be seen in the dark nest.[29][30]

-Thickened egg shells[31][32]

-Efficiency in laying the egg, being able to lay an egg secretly and fast while the host is absent.[14][33]

The cost of parasitism is high for the Horsfield’s Bronze-cuckoo’s hosts, with both host and cuckoo evolving with each other, the cuckoo is extremely successful as a parasite.As hosts evolve defenses against parasitism by cuckoos, cuckoos evolve ever better means of tricking their hosts into rearing their young, which, in turn, promotes the evolution of improved host defenses.[34] The counter adaptations from the host species must keep evolving in order to reduce parasitism. Horsfield’s Bronze-cuckoo’s have been counter adapting to host defences in terms, ‘leading’ the co-evolutionary arms race by effective means in the early stages of parasitising a host nest and leaving the host with a smaller chance of detecting the eggs of the Horsfield’s Bronze-cuckoo.[14]


Typical call, SE Queensland, Australia


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