Cuckoo Logic: The Alien World of the Brood Parasite

The cuckoo is at the emotional heart of evolutionary biology. Whilst puzzles such as altruistic behaviour and the peacock’s tail can be solved by a tweaking of evolutionary theory and a realigning of our image of nature; the behaviour of the European cuckoo, Cuculus canorus, is different. The cuckoo’s behaviour cannot be dressed up with human ideals of the family and morality. In the figure of the brood parasite, we see the bones of natural selection lain bare, naked and unavoidable.

A cuckoo egg mimicking the two reed warbler eggs in a reed warbler nest

The behaviour of the female cuckoo contradicts cultural ideas about what parenting, particularly mothering, is. There is no concept of parental love for the cuckoo; the mother will slip into the nest of the selected host, pluck out a host’s egg, quickly deposit her own egg and leave. She pays no part in nurturing the offspring; she is the proverbial bad mother. But the more paternal aspect of parenting is also absent in cuckoos. Most human cultures value parents passing down the knowledge and values they accumulate in addition to their genetic material to their children. Familial traditions have no meaning for the cuckoo, for it receives nothing from it’s parents except their gametes and the home range they return to in the breeding season.

The cuckoo hatchling comes of age in an environment very different from our bustling human world. They are fed by their hosts, each subspecies or gentes of cuckoo is host species- specific, and are unlikely to see any other cuckoos before they fledge. Despite this, by adulthood they must have an idea that only European cuckoos, not the host species, are sexual partners. The cuckoo chick must on some level know that the creature caring for them is not ‘their kind’, one of the first things they must do is understand themselves as a stranger.

A cuckoo chick ejecting reed warbler eggs

When the cuckoo chick hatches, sightless in a nest full of host eggs, it behaves in a way chilling to see, a puppet governed by pure evolutionary logic. Like a murderous sleepwalker, the naked red cuckoo will hoist the hosts’ eggs (or hatchlings) into a hollow of its back, shuffle to the edge of the nest and discard the hosts’ progeny, to die outside the nest. This grisly scene is repeated until all rivals are gone. It is left lord of the nest, and can enjoy the food the hosts brings unhindered. We may recoil from the cuckoo chick and it’s “odious instinct”, horrified by a babe whose first action is sin. But this exemplifies the error of applying human morals to all animals indiscriminately. They were never innocent so cannot fall from grace – they just act out a strategy honed by their ancestors.

See a short, if sentimental, video of this here.

Illustration by Laura Cooper of a cuckoo chick being fed by a reed warbler host

What baffles most about the cuckoo’s world is the behaviour of the host. Here “host” refers to the parasitic relationship between the cuckoo who exploits the resources of the unrelated pair who raise it, just as I may be the host exploited by Plasmodium in malaria. But the case of the cuckoo suggests a different meaning of “host”. The host pair appear to welcome the cuckoo chick; they are passive as it ejects their offspring and feed it even as it grows to monstrous sizes. The sight of a reed warbler contorting itself to ram an insect down the throat of a cuckoo twice its size suggests a relationship that has passed hospitality and become subservient.

This can lead us to see the host as either foolish or manipulated by the Machiavellian chick. Both of these descriptions are partially true. But the host is only temporarily foolish. In evolutionary time the species will become wise and the cuckoo will lose the evolutionary arms race and move onto a naive host species. The blackbird still has vestiges of its victory over the cuckoos, its chicks will eject strange eggs from the nest though cuckoos must have long given up parasitising this species.

The cuckoo chick does manipulate – it’s calls mimic a whole brood of the hosts’ chicks to trick the hosts to give it enough food to fill several host chick bellies. But as Nick Davies in the book Cuckoo: cheating by nature suggests, manipulation by the chick is not the sole reason for hosts’ care. Davies reports that reed warbler parents will accept odd chicks of many other species, it is not due to the skills of the cuckoo. It is instead due to the breeding patterns of the host itself.

The reed warblers have evolved great skill at recognizing cuckoos attempting to parasitise nests and rejecting cuckoo eggs. This is because at this stage there is still a significant chance they can have another brood before they migrate. But once a given pair has had a cuckoo hatch in the nest, it is too late to have another brood before they migrate, and only a 50% chance of making it to the next breeding season. Therefore, the hosts carry on as if they had their own offspring, as they can do nothing now to increase their reproductive success.

It is likely that it would benefit the host pair to reject cuckoo chicks and save the energy spent caring for them on preparing for the next breeding season. But this strategy cannot evolve. The low probability that a cuckoo-rejecting pair will ever breed means that the genes associated with this behaviour can’t pass to the next generation and so meet an evolutionary dead end.

Instead of the foolishness or manipulation, what is really seen when the reed warbler feeds the stranger in its nest is something that exists because of an absence of positive or negative selection pressures, as selection cannot touch it. The reed warbler responds to the cuckoo chick by replicating the behaviour it uses to feed its offspring as no counter-strike against the cuckoo can be evolved.

Adult male cuckoo