The Evolutionary Gamble: Why Cuckoos Lay Eggs in Other Birds’ Nests

One of nature’s most fascinating deceptions plays out in the nests of unsuspecting birds. A cuckoo, rather than raising its own chicks, sneaks its egg into the nest of another bird, outsourcing the labor of parenting. But what makes this trick even more ruthless is what happens after the egg hatches—the cuckoo chick instinctively pushes out the host’s eggs or hatchlings, securing all resources for itself. This double-layered behavior—egg parasitism and chick eviction—raises an intriguing question. Did one behavior lead to the other? And how did this evolutionary strategy emerge across different cuckoo species?

Brood parasitism—the act of laying eggs in another bird’s nest—is not exclusive to cuckoos. Cowbirds, honeyguides, and some finches employ similar tactics. But cuckoos have perfected the art. Their eggs often mimic those of the host bird, reducing the likelihood of rejection. This mimicry itself is a product of co-evolution, where host birds evolve to recognize imposters, and cuckoos, in turn, evolve better disguises. However, just laying an egg isn’t enough. If the host bird’s own chicks hatch, they would compete for food, reducing the cuckoo chick’s survival chances. Enter the second evolutionary step—the instinct to eliminate competition.

This action is a long-term evolutionary process that even shows up with further variations. As shown in Figure, obtained from 'Cuckoo adaptations: trickery and tuning' by N.B. Davies, it shows three different forms - (a) The host‐race of the common cuckoo Cuculus canorus specializing on reed warblers Acrocephalus scirpaceus lays a mimetic egg (cuckoo egg on right, with three reed warbler eggs), whereas (b) the host‐race specializing on dunnocks Prunella modularis lays a non‐mimetic egg (cuckoo egg with four dunnock eggs). (c) Jacobin cuckoos Clamator jacobinus lay a non‐mimetic white egg in the nest of the cape bulbul Pycnonotus capensis. (d) The shining bronze‐cuckoo Chalcites lucidus plagosus lays a dark, cryptic egg in the dark, domed nest of its host, the yellow‐rumped thornbill Acanthiza chrysorrhoa, which has pale eggs.

Within hours of hatching, a young cuckoo chick performs a behavior so deeply ingrained that it seems almost mechanical—it uses its back to heave unhatched eggs or even newly hatched nestlings over the edge. This instinct is universal among several parasitic cuckoo species, including the common cuckoo (Cuculus canorus) and the great spotted cuckoo (Clamator glandarius). BBC Earth has an excellent video capture of the egg/hatchling eviction behavior narrated by Sir Richard Attenborough.

But why does this behavior develop so early?

A newly hatched cuckoo chick is blind, featherless, and utterly helpless—except for one crucial instinct. Without hesitation, it begins pushing any other eggs or hatchlings out of the nest. This behavior isn’t learned; it’s encoded through generations of evolutionary refinement. Chicks that successfully eliminated competition had a higher chance of survival, ensuring that this trait became deeply ingrained over time. While the precise neurological and genetic mechanisms behind this instinct remain an open question, natural selection has clearly favored cuckoo chicks that act decisively from the moment they hatch.

It’s possible that the two behaviors—laying eggs in foreign nests and chick eviction—evolved sequentially. Initially, a cuckoo species might have relied on brood parasitism alone, but competition from host chicks reduced their success rate. Over time, natural selection could have favored chicks that instinctively removed their rivals. Interestingly, not all parasitic birds exhibit this second behavior. Some cowbird species lay their eggs in host nests but do not actively eliminate competing chicks. Instead, they rely on faster growth or host feeding preferences to dominate. This suggests that while brood parasitism evolved first, eviction behavior emerged in certain lineages where competition remained high.

Behind these behaviors lies a deeper complexity—genes and biochemical pathways that influence instinctive actions. While the exact genetic basis of chick eviction remains an open question, research in related avian behaviors suggests that a combination of early hormone exposure and neural development plays a role. Some studies indicate that stress-related hormones, such as corticosterone, could be involved in triggering aggressive behaviors in nestlings. Additionally, genetic factors influencing motor coordination and early hatchling reflexes may contribute to this instinct, though the precise mechanisms remain unclear. Rather than being a consciously learned behavior, the cuckoo chick’s actions appear to be a deeply ingrained response shaped by natural selection over generations.

Nature doesn’t operate on morality; it operates on survival.

Cuckoos have evolved a strategy that ensures their genetic success, not through nurturing but through deception and elimination.

Their behavior isn’t a conscious decision, it’s an instinct honed over millions of years, encoded in their DNA.

And in this evolutionary arms race between parasitic birds and their hosts, the battle is far from over.