Most people are familiar with how the common cuckoo’s (Cuculus canorus) life cycle works, and as a result, it is widely regarded as one of the villains of the animal kingdom. This bird is an obligate brood parasite, which reproduces solely by having other species raise its offspring. A female cuckoo lays an egg in the nest of another bird; once the baby cuckoo hatches, it throws the other eggs or chicks out of the nest and takes up all the attention of the foster parents, who then slave away to raise a chick that isn’t even their own species. In our eyes, such behaviour by the cuckoo appears downright underhanded, not to mention lazy. But personally, I find this behaviour – hijacking the life cycle of another species – to be really interesting.
There are some obvious advantages: for example, without the burden of raising their own chicks, cuckoos are able to produce a relatively high number of offspring – up to 25 eggs have been recorded for one cuckoo in a single year. Also, adult cuckoos – which spend the majority of their year in Africa, migrating north each spring to breed – are able to head south again earlier than other migrants, as early as July or August. Meanwhile, being a brood parasite isn’t as easy as simply dropping an egg into the first convenient nest and calling it a day. Birds which get the most attention from cuckoos respond by evolving more vigilant and discriminatory behaviours, so they can hopefully avoid wasting their breeding season on somebody else’s chick. In turn, the cuckoo evolves to try and get around these defences.
First, the right target is required. Different lineages of cuckoo specialise in different hosts – in Britain, dunnocks, meadow pipits and reed warblers are favourites – and most of these lineages have evolved so that their egg visually matches that of their particular host. (Dunnocks are an exception as unlike other hosts, they haven’t yet learned to reject unfamiliar eggs.) The female cuckoo has to defend a territory containing a suitable number of host nests, and she has to closely monitor when her targets start laying. If she jumps the gun and lays her egg in a fresh, empty nest, the owners will definitely reject it – but she can’t leave it too late either, or the resident eggs will hatch before hers. When she does make her move, she also has to be as fast as possible, replacing an existing egg with her own in seconds; again, parent birds are more likely to abandon their nest if they actually see a cuckoo in the vicinity.
If the cuckoo egg survives long enough to hatch, however, the foster parents won’t desert the chick, and won’t even stop it from throwing out their own clutch. Alone in the nest, the cuckoo uses a secret weapon to ensure that it gets enough to eat. Its begging call has evolved to sound like a whole nest of chicks – so the parents are compelled to bring in enough insects for a whole nest of chicks. Through their misplaced efforts, the young cuckoo quickly grows to be many times their size, while taking significantly longer to leave the nest and become independent than their real offspring would have done. Once it is big enough, it will make its first migration to Africa, relying on instinct since all the adult cuckoos will have already gone by then.
One thing that has puzzled me, since seeing a video on YouTube, is what happens if a cuckoo lays its egg in a nestbox, where the chick will have a harder time pushing away the host eggs. In fact, a long-term study in Finland has examined this, looking at how cuckoos parasitise common redstarts, which naturally make their nests in cavities and will make use of nestboxes. According to Samas et al.’s 2016 paper on the project, just over a third of cuckoos that targeted a nestbox managed to successfully place their egg in the central nest cup. Although a cuckoo chick in a nestbox could eject at least some eggs from the cup, it did sometimes find itself having to share with redstart chicks, though this didn’t significantly affect whether the cuckoos survived. Another paper by Grim et al. in 2014 describes one incidence of great tits using a nestbox parasitised by a cuckoo; one great tit chick managed to stay in the nest cup and, despite spending most of its time stuffed underneath the larger cuckoo, survived to fledge.
Unfortunately, while the common cuckoo is globally widespread – with a breeding range across Europe and Asia – it has suffered significant declines in Britain over the past few decades, possibly due to a reduction in the caterpillars which make up most of its diet.
If you would like to learn more about cuckoos, I recommend Nick Davies’s book Cuckoo: Cheating by Nature, which goes into great detail about the stages of the cuckoo’s annual reproduction, and the observations and experiments involved in learning about it.
Davies, N. (2015), Cuckoo: Cheating by Nature. New York: Bloomsbury USA.
Grim, T., Samaš, P., Procházka, P. & Rutila, J. (2014), Are tits really unsuitable hosts for the common cuckoo? Ornis Fennica 91: 166–177.
Samaš P., Rutila, J., & Grim, T. (2016), The common redstart as a suitable model to study cuckoo-host coevolution in a unique ecological context. BMC Evolutionary Biology 16: 255.