Horsfield's bronze cuckoo

The Horsfield's bronze cuckoo is one of five Australian species in the genus Chalcites (formerly Chrysococcyx) a type of parasitic bird, ==Diet and behaviour==

The main diet of the Horsfield's bronze cuckoo is insects and they are nomadic, travelling to different regions of Australia to breed and find food. The Horsfield's bronze cuckoo is known as a brood parasite, this means that they lay their eggs in a host species nest. They mainly parasitize the fairy-wrens in the genus Malurus. 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 bear 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. 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. ==Breeding==

The Horsfield's bronze cuckoos 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. 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. ==Range==

The species is native to Australia and South East Asia, found in Indonesia, Papua New Guinea, Malaysia, and southern Thailand. In 2024, the first known specimen was recorded in New Zealand. ==Parasitism==

As a brood parasite, the Horsfield's bronze cuckoo does not build its own nest but will use a host species' nest to lay its eggs. The breeding season for the Horsfield's bronze cuckoo relies on their host and they will lay one to mimic that of the fairy wren or thornbill's egg, an elongated pinkish-white egg, that is speckled with red-brown spots. The breeding season for the superb fairy-wren is between September and February and a female may have three consecutive broods in this time, allowing the cuckoo multiple attempts to parasitize this species. The female cuckoo may choose a breeding site with a high density of hosts, which allows extra opportunity for her success in parasitizing a nest successfully. Studies have shown at one site a female did not parasitize a territory with less than 23 breeding pairs of their primary host (Malurus cyaneus). Egg laying is very fast for the Horsfield's bronze cuckoo; it is able to lay an egg in under 6 seconds typically in the morning shortly after the host has laid. 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. As the nestling grows it will be fed by the host parent and possibly the group, growing more rapidly until fledged. ==Coevolutionary arms race==

Counter-adaptations have been documented for host species and cuckoos alike, as each adapts to the other in a coevolutionary "arms race". Studies show that coevolutions happen at all stages of the growth cycle, not just the early stages. Fairy-wrens have adapted some host defences to reduce parasitism. The high cost of hosting a parasitic species, in energy and genetics, drives the host to improve its defences, which in turn drive the parasite to improve its offences. Among the host's defensive adaptations: • Hosts like to nest in large colonies, to cooperate in spotting cuckoos and chasing them away. • Hosts learn and recognise their own eggs, and abandon any that are in the nest before they have started their own. • Eggs that are cryptic and therefore unable to be seen in the dark nest. • Thickened egg shells. • Efficiency in laying the egg, being able to lay an egg secretly and quickly while the host is absent. ==Media==