An international team of ornithologists has overturned one of the oldest assumptions in natural history by directly documenting how common cuckoos lay their eggs in host nests located inside cavities. The findings provide definitive evidence that cuckoos do not carry eggs in their beaks; a theory that has persisted since ancient times.
Led by Associate Professor Robert Thomson (University of Cape Town) and Professor Tomáš Grim (University of Ostrava), the research team used video monitoring to capture over 60 egg-laying events across four breeding seasons in Finland. The study focused on interactions between common cuckoos and their host species, the common redstart, which nests in enclosed tree cavities rather than open cup nests.
These observations finally allow us to see what actually happens during egg laying in a challenging nesting environment,” said Grim. “For centuries, speculation filled this gap in knowledge. Now we have direct evidence.”
Two distinct egg-laying strategies
The study revealed that cuckoos employ two distinct strategies when parasitising cavity nests. In some cases, females lay eggs from outside the nest entrance, effectively projecting the egg into the cavity. In others, they enter the nest chamber directly before laying.
Each strategy involves trade-offs. Laying from outside reduces the risk of becoming trapped inside the cavity but increases the likelihood of missing the nest entirely. Entering the cavity ensures accurate placement of the egg but carries risks, including disturbing the nest structure or becoming stuck.
“We found that direct laying inside the cavity has a higher success rate, but also greater potential costs,” explained Grim. “This balance likely explains why both strategies persist within the population.”
Shedding light on an evolutionary arms race
The findings provide new insight into the evolutionary “arms race” between brood parasites and their hosts. Cavity-nesting species like the redstart may benefit from increased protection against parasitism, yet cuckoos have evolved behavioural flexibility to overcome these barriers.
“This is a rare example of clear behavioural variation within a brood parasite species,” said Thomson. “It highlights just how adaptable these birds are when faced with different ecological constraints.”
Debunking a centuries-old myth
One of the most significant outcomes of the study is the definitive rejection of the long-standing belief that cuckoos may transport eggs in their beaks to place them in host nests.
“In older literature, egg-carrying was often presented as fact, despite the lack of evidence,” said lead author Michal Kysučan. “Our recordings show conclusively that cuckoos lay directly into nests, even in situations where carrying might seem easier.”
The myth, which dates back to early naturalists, including Aristotle, persisted largely because of the difficulty of observing egg-laying behaviour in concealed nest sites.
Why it matters
Understanding how brood parasites exploit different nesting environments is central to broader ecological and evolutionary research. The study demonstrates that even well-studied species can harbour previously unknown behavioural complexity.
The use of extensive video monitoring across hundreds of nest boxes was critical to the success of the project, underscoring the value of long-term, high-resolution behavioural data in ecology.
“These kinds of detailed observations are essential if we want to fully understand species interactions and their consequences for biodiversity,” said Thomson.
Notes
The study was conducted by researchers from the Czech Republic, Finland, the United Kingdom, and South Africa, and is published in the journal Animal Behaviour (https://doi.org/10.1016/j.anbehav.2026.123548).
Animal Behaviour
Observational study
Animals
Cuckoos overcome cavity-nesting host defences with alternative egg deposition modes
13-Apr-2026