Cuckoos are remarkable for their habit of laying their eggs in the nests of other species, so that the cuckoo chicks are then raised by foreign parents. Cuckoos are often called ‘brood parasites’ because in this way they parasitise parental care provided by other species, and so avoid the considerable effort associated with raising offspring themselves. Just as with any other sort of parasitism, hosts defend themselves against exploitation by cuckoos by: mobbing adult cuckoos who come near the nest, rejecting odd eggs and rejecting foreign chicks. This sets in motion a coevolutionary arms race between cuckoos and their hosts, in which better host defences select for counter-adaptations in cuckoos, to overcome them (Kilner & Langmore 2011).
About 1% of all birds are brood parasites, and it has evolved at least 7 different times within the birds as a whole. Brood parasitism is also known in other animals, such as insects and fish (Kilner and Langmore 2011).
Most of our work on brood parasites has focused on the Horsfield’s bronze-cuckoo Chalcites basalis, through a longstanding collaboration with Naomi Langmore at the Australian National University. This Australian species targets many different host species, but specialises on parasitising fairy-wrens (Malurids).
We have found that this cuckoo is sequentially monogamous (Langmore et al 2007), with males holding territories that encompass 10-20 host territories. Females stay faithful to one male and then lay their clutch of roughly 8 eggs, with one egg per nest every other day among each of these hosts, before leaving and being replaced by another female.
The female lays a ‘one size fits all’ egg – which approximately mimics the majority of its many hosts, so enabling the cuckoo egg to survive, unrejected in the host nest (Feeney et al 2014).
After hatching, the cuckoo chick kills host chicks by evicting them from the nest. It then flexibly adjusts its begging call to tune into the particular existing communication system in that host for soliciting food from parents – making a long rasping begging call, characteristic of young thornbills, when soliciting food from thornbill parents but a purer sounding note, which resembles the calls of superb fairy-wrens when raised in a fairy-wren nest (Langmore et al 2008). How they achieve this, having first killed all the host young and thus removed any templates for learning, is still a mystery.
At 40% of nests, hosts defend themselves against parasitism by rejecting the cuckoo chick. The female simply stops feeding it and starts building a new nest and the cuckoo starves to death (Langmore et al 2003). Female fairy-wrens recognise a cuckoo chick because it is alone in the nest (usually they raise 3 or 4 chicks in a brood0, and they are more likely to reject cuckoos that make a wrong-sounding begging call (Langmore et al 2003). They get better at spotting a cuckoo chick in their nest as they gain experience in raising their own offspring (Langmore et al 2009).
Feeney W. E. et al 2014 ‘Jack-of-all-trades’ egg mimicry in the brood parasitic Horsfield’s bronze-cuckoo? Behav Ecol 25: 1365-1373
Kilner, R. M. and Langmore, N. E. 2011 Cuckoos versus hosts in insects and birds: adaptations, counter-adaptations and outcomes. Biological Reviews 86:836-852
Langmore, N. E. et al 2003 Escalation of a co-evolutionary arms race through host rejection of brood parasitic young. Nature 422:157-160
Langmore, N. E. et al 2007 The spatial organisation and mating system of Horsfield’s bronze-cuckoos Chalcites basalis Anim Behav 74:403-412
Langmore, N. E. et al 2008 Socially acquired host-specific mimicry and the evolution of host races in the Horsfield’s Bronze-Cuckoo Evolution 62:1689-1699
Langmore, N. E. et al 2009 Flexible cuckoo chick rejection rules in the superb fairy-wren Behav Ecol 20:978-984