Lecture 14: Brood Paratism

APS209 Animal Behaviour

Brood Parasites


1. To present the concept of a coevolutionary arms race between brood parasites and their hosts.

2. To describe the biology of cuckoos and experimental tests of the function of behaviour
3. To examine whether the host-parasite arms race is ongoing or in evolutionary equilibrium.



1. To understand the principles underlying coevolutionary arms races
2. To understand how adaptations and counter-adaptations may be tested in field experiments
3. To understand the importance of studying mechanisms in the study of behaviour

Brood parasites
Brood parasites effectively parasitise the parental care of their hosts. Such parasitism may occur among members of the same species (intra-specific brood parasitism, which is a fairly widespread behaviour among birds) or different species (inter-specific brood parasitism). About 1% of all bird species are obligate brood parasites.

Coevolutionary ‘arms race’
Parasites and their hosts (and equally, predators and prey) are expected to be in a coevolutionary arms race whereby host defences select for adaptations in the parasite, which in turn result in counter-adaptations from the hosts, and so on. Such arms races may reach a stable evolutionary equilibrium of adaptation and counter-adaptation, or may remain evolutionarily dynamic with continuous adaptive change occurring, of which we can observe only a snapshot.

Common cuckoos Cuculus canorus and their hosts
The common cuckoo has about 10 regular host species in the UK. Various features of female egg-laying behaviour have been shown, by experiments, to be adaptive responses to host egg-rejection defences. Among these adaptations is the production of eggs that mimic the host eggs. Thus each female will specialise on a particular host species, with those specialising on a particular host being referred to as a ‘gens’ (plural ‘gentes’), as in ‘reed warbler gens’, ‘meadow pipit gens’, etc.

Hosts have also evolved in response to parasitism, exhibiting a range of egg rejection abilities. Suitable hosts are more discriminating than unsuitable hosts. Furthermore, in places where cuckoos and hosts are sympatric, hosts are more discriminatory than when hosts are not parasitized.

Continuing arms race or evolutionary equilibrium?
Variation among species in egg rejection ability suggest that hosts and their parasites are in a continuing arms race. However, it may not always pay to be a rejector if the costs of parasitism are low, or if the costs of rejection are high. Alternatively, the ability of a host to discriminate may depend on the rejection mechanism.

Reading: see Chapter 12 in John Alcock’s Animal Behavior (2009: pp. 379-419); also Nick Davies’ book: Cuckoos, cowbirds and other cheats (2000; T & AD Poyser, London).
Davies NB & Brooke ML (1988) Cuckoo versus reed warblers: adaptations and counter-adaptations. Animal Behaviour 36: 262-284.
Davies NB & Brooke ML (1988) An experimental study of coevolution between the cuckoo, Cuculus canorus, and its hosts I Host egg discrimination. J. Animal Ecology 58: 207-224.