This article explores the evolution and breeding strategy of the Common Cuckoo (Cuculus canorus)
Species: Common Cuckoo (Cuculus canorus)
Family: Cuculidae
Animal behaviors designed to raise young and ensure the survival of offspring are fundamental to the survival and evolution of most species. For birds, Raising young is a demanding process requiring significant parental investment. It starts with finding a mate, building a nest, and incubating eggs. Once hatched, the nestlings need constant feeding, and eventually, the fledglings need to be taught how to hunt for themselves.
Some species have evolved to bypass parental requirements using a process called brood parasitism. Perhaps the most famous of which is the Common Cuckoo. The Common Cuckoo lays its eggs in the nest of another species, leaving the host to incubate and raise the offspring. This behavior enables the Cuckoo to save energy and resources that would otherwise be spent on parental care by outsourcing the tasks of incubation and feeding.
History Of Brood Parasitism
The Common Cuckoo may be the most famous brood parasite, but the capability has independently evolved 7 times in birds across 5 clades. These are Honeyguides, Common Cuckoos, Striped Cuckoos, Indigobirds, Pheasant Cuckoos, Cowbirds, and the Black-Headed Duck.
However, there are estimated to be between 11,000 and 18,000 bird species on the planet. Out of these, only 100 are known to be obligate brood parasites. So why is this strategy for species survival only utilized by less than 1% of bird species?
The primary reason is possibly that host species have, throughout millennia, developed strong defenses against brood parasites. Brood Parasites like the Common Cuckoo had to evolve a series of specialized behaviors to be able to exploit this niche survival strategy. An ongoing battle persists between them and their hosts, with each side having to develop strategies and counterstrategies. This battle encompasses the complete breeding cycle, let’s take a look at each element.
Breeding Lifecycle
Frontline Defense:
During the nest-building phase, the Common Cuckoo has to select a suitable host and then monitor their nest-building activities. However, the cuckoo needs to do this secretly as hosts have learned to be cautious, they will locate Cuckoos and try to nest far away from them, keeping their nests concealed.
Some host birds have evolved to build their nests in a way that isn’t suitable for the Common Cuckoo to access. Forexample, keeping nests as small as possible, or building completely enclosed nests with small access points, or nesting in communal areas.
Enclosed nests and communal sites are not suitable for Cuckoos, but many birds haven’t evolved this capability, and have to rely on active defense of the nest site. This can take the form of the host parents attacking the Common Cuckoo when they see them, or the female permanently sitting in the nest.
The Common Cuckoo has learned these behaviors, and the male will frequently lure the hosts away from their nests using its physical presence and also by mimicking the calls of sparrowhawks and other raptors; so that the female can gain free access to the nest site, which they will typically aim to do when the host has just started incubating.
Laying an egg typically takes 20 minutes or more. But Common Cuckoos cannot afford this luxury and have evolved to lay eggs within 5 to 41 seconds. They have also evolved to lay small eggs comparatively for their size.
Incubation Stage:
During the incubation stage, hosts have developed the ability to help recognize their eggs from that of the Common Cuckoo and will reject the cuckoo egg if they do so.
There are a few strategies that enable them to do this and vary across species, but this includes varying the egg markings within their clutch either to a lesser or greater extent. The hosts also increase the variability of eggs across the overall population of their species to make the eggs harder for the cuckoo to mimic.
The Common Cuckoo has in turn evolved to mimic the eggs of the hosts and also to add varyability so that their eggs don’t always look the same. They can even camouflage their eggs to disguise them in the nest site so that they blend in.
The Cuckoo has also adapted the thickness of its eggs so that they are thicker and stronger to resist any attempts by the host to destroy the parasite eggs.
The battle between the Common Cuckoo and its host does not stop once the eggs are laid. The Cuckoo continues to monitor the nest site and use predator-like calls to keep the host on alert. They even employ mafia-type techniques to bully the hosts into keeping the Cuckoo egg safe, whereby they will destroy a nest site if the host destroys a cuckoo egg.
Nestling Stage:
Upon hatching, the Common Cuckoo nestling displays remarkable mimicry of the behavior and vocalizations of the host’s nestlings. The host parents instinctively care for the louder, more demanding offspring.
Cuckoos have evolved to push out the host’s eggs or even hatchlings from the nest to eliminate competition for sustenance and parental care. This parasitic behavior highlights the cunning and resourcefulness of the Common Cuckoo in ensuring its own survival at the expense of the host species.
Fledgling Stage:
The Common cuckoo manipulation needs to continue once fledged, as they are also dependent on the host for a short period. During this stage, they are significantly larger than their host. The Common Cuckoos strategy ensures the cuckoo’s reproductive success with minimal parental investment.
So what were the evolutionary origins of obligate brood parasitism? There are two evolutionary theories, and despite extensive research over a number of decades, the subject remains a topic of scientific debate.
Evolutionary Origins Of Brood Parasitism
Interspecific Parasitism from Intraspecific Origins
So far, we have discussed the form of parasitism that evolved in species like the Common Cuckoo called Obligate or Interspecific Brood Parasitism; a breeding strategy in which the brood parasite female evades all parental care by laying its eggs in host nests.
There is another form of parasitism called Intraspecific Brood Parasitism, and it is where birds lay eggs in each other’s nests within the boundary of a single species. It’s a ploy frequently used by birds that nest in colonies.
The evolutionary theory is that brood parasitism between species evolved from parasitism within the same species. The limitation of this theory is that there is a lack of any scientific correlation between the two breeding strategies, and is therefore suggested to be less likely.
Direct Evolution from Offspring Care
The second evolutionary theory postulates that obligate brood parasitism evolved directly from ancestors that cared for their young, but due to competition for nest sites, they took to taking over the nests of other birds. Habitat expansion and increased migration could have facilitated this evolution by increasing breeding ranges and food sources, reducing reproductive investment needs.
In the long term, the egg war between parasites and hosts is ongoing. The adaptive strategies between the parasites and hosts are continually evolving in order to maintain a dynamic balance. The co-evolutionary process is a shining example of natural selection in action.
References
- Soler, Manuel. (ed.) (2017) Avian Brood Parasitism : Behaviour, Ecology, Evolution and Coevolution. 1st ed. 2017. Cham: Springer International Publishing. [Accessed 13/06/2025]
- Antonson, N.D. et al. (2020) ‘Ecological uncertainty favors the diversification of host use in avian brood parasites’, Nature Communications, 11(1). [Accessed 13/06/2025]
- Hargitai, R. et al. (2010) ‘Eggshell characteristics and yolk composition in the common cuckoo Cuculus canorus: are they adapted to brood parasitism’, Journal of Avian Biology, 41(2), pp. 177–185. [Accessed 13/06/2025]
Images Used Under Creative Commons (CC)
- Marsh Warbler Eggs | By Klaus Rassinger und Gerhard Cammerer, Museum Wiesbaden – Own work, CC BY-SA 3.0 [Accessed 13/06/2025]
- Common Cuckoo Eggs | By Roger Culos – Own work, CC BY-SA 4.0. [Accessed 13/06/2025]
