Cuckoos: Masters of Trickery in the Avian World

The monsoon season, with its abundance of food and favorable conditions, signals the onset of the breeding period for a wide variety of bird species in India. Among them, cuckoos stand out—not just for their melodious calls, but for their remarkable and somewhat deceptive breeding strategy. Unlike most birds that raise their own young, cuckoos are brood parasites: they lay their eggs in the nests of other birds, leaving the unsuspecting hosts to incubate the eggs and care for the chicks. This fascinating behavior, though seemingly ruthless, is a brilliant evolutionary adaptation that has made cuckoos one of the most intriguing groups in the avian world.

                           Currently, 141 species of cuckoos (family Cuculidae) are recognized worldwide, and interestingly, a majority—around 60%—exhibit brood parasitism, a remarkable and intriguing reproductive strategy. In India, most members of the cuckoo family (Cuculidae) are known for their remarkable strategy of brood parasitism, where they lay their eggs in the nests of other bird species, leaving the responsibility of incubation and chick-rearing to the unsuspecting hosts. However, there are a few notable exceptions, such as the Greater Coucal (Centropus sinensis), which is a non-parasitic species that builds and tends to its own nest. Among the brood parasites, several well-known species include the Asian Koel (Eudynamys scolopaceus), which typically targets crows; the Common Hawk-Cuckoo (Hierococcyx varius), famous for its chick's mimicry of host begging calls; the Indian Cuckoo (Cuculus micropterus), which parasitizes insectivorous birds like Drongos; the Pied Cuckoo (Clamator jacobinus), traditionally associated with the arrival of the monsoon and known to target babblers; and the Grey-bellied Cuckoo (Cacomantis passerinus), which often chooses tailorbirds and prinias as hosts. 

                      The cuckoo’s parasitic adaptations can be broadly categorized into two types: ‘trickery’ and ‘tuning’. Trickery refers to the ways in which adult cuckoos, along with their eggs and chicks, evade the defences of host birds—this involves intricate adaptations that have co-evolved alongside the host species’ own counter-strategies. On the other hand, tuning involves how, once the cuckoo's egg is accepted, the development of the egg and chick is finely adjusted to match the host’s incubation period and feeding behavior. Unlike trickery, tuning may not always lead to co-evolutionary changes in the host. Cuckoo trickery involves adaptations to counter successive lines of host defences and includes: tricks for gaining access to host nests, egg trickery and chick trickery. Cuckoo tuning includes adaptations involving: host choice and monitoring of host nests, efficient incubation of the cuckoo egg, efficient provisioning and protection of the cuckoo chick, and adaptations to avoid mis imprinting on the wrong species (Davies, 2011). 

                    Raising a cuckoo chick is clearly costly for the host, as the cuckoo often destroys or displaces the host's own eggs or chicks, drastically reducing—or even completely eliminating—the host's reproductive success for that breeding attempt. In theory, such a high cost should drive the evolution of strong defenses in host species to detect and reject parasitic eggs or chicks. Indeed, in many host-cuckoo systems, we find remarkable adaptations by cuckoos that have evolved specifically to overcome these defences. One well-known example is host-egg mimicry, where cuckoo eggs closely resemble the host’s eggs in colour and pattern, helping them avoid detection and rejection. Cuckoos typically lay three types of eggs—non-mimetic, mimetic, and cryptic—each representing a unique strategy to avoid detection by host birds. Non-mimetic eggs do not resemble the host’s eggs and are often accepted by hosts that lack egg discrimination. Mimetic eggs closely imitate the host’s eggs in colour and pattern, helping avoid rejection in species with strong defences. Cryptic eggs, on the other hand, are dull or camouflaged, blending into the nest to escape both host detection and predation. These strategies highlight the diverse adaptations cuckoos have evolved in their parasitic lifestyle. In response to successive layers of host defences, cuckoos employ a variety of trickery strategies other than host-egg mimicry to ensure the success of their parasitism like –

1.     Monitoring hosts to find concealed nests

2.     Male cuckoos’ lures host away during laying

3.     Hawk mimicry to avoid high detection from host

4.     Rapid laying to counter egg rejection

5.     Host chick mimicry to prevent foreign chick rejection

Jungle Babbler feeding Common Hawk cuckoo Hatchling

               Similarly, Tuning strategies are largely consistent with the life cycles of their host species. Mainly, host selection is an essential stage in tuning tactics. Different parasitic cuckoo species may select suitable hosts based on host size, diet, nesting place, egg-laying season, and nest accessibility. Cuckoo tuning mechanisms include egg tuning, which synchronies egg development with the host's incubation, and chick tuning, which modifies chick behavior and growth to take advantage of host care effectively.

                 We now have a potential concern with ‘mis-tuning'. What is a cuckoo's way of recognizing itself? In theory, brood parasites may avoid mis imprinting by delaying social learning and using a species-specific password-like cue to direct their learning towards conspecifics rather than hosts. Brown-headed cowbirds utilize a 'chatter' cry to associate with other cowbirds after fledging, and presumably learn more about their own species. Brood parasites do imprint on their hosts as a guide to future host choice.

References: -

1.     N. B Davies. (2011). Cuckoo adaptations: trickery and tuning. Journal of Zoology, 284(1), 1-14.

2.     Praveen, J., & Lowther, P. (2020). Avian brood parasitism in South Asia. Indian Birds, 16(4), 103-119.

3.     Image credit – Google