Finland – Video shows a common cuckoo laying an egg from the cavity entrance of a redstart nest inside a nest box in Finland.

The footage is part of a new scientific study that directly records how common cuckoos place their eggs inside host nests located in enclosed cavities.

Researchers say the findings overturn a long-standing belief in natural history that cuckoos carry their eggs in their beaks.

The study was led by Associate Professor Robert Thomson of the University of Cape Town and Professor Tomáš Grim of the University of Ostrava.

Using video monitoring, the team recorded more than 60 egg-laying events across four breeding seasons in Finland.

The observations focused on interactions between common cuckoos and common redstarts, a species that nests in enclosed tree cavities rather than open nests.

One researcher involved in the study said the recordings reveal what actually happens during egg laying in difficult nesting conditions, noting that speculation had filled this gap for centuries.

The research identified two different egg-laying strategies used by cuckoos in cavity nests.

In some cases, females lay eggs from outside the nest entrance, effectively placing the egg into the cavity. In other cases, they enter the nest chamber before laying.

Each method carries different risks and benefits, including the chance of missing the nest or becoming trapped inside.

Researchers noted that entering the cavity leads to higher success in egg placement but also increases potential risks for the bird.

The study also highlights an evolutionary interaction between brood parasites and their hosts, showing how both sides adapt over time.

Scientists say cavity nesting may offer protection to host species, but cuckoos have developed flexible behavior to overcome these defenses.

The findings also reject the long-held idea that cuckoos transport eggs in their beaks, a belief that dates back to early naturalists.

Researchers confirmed that all observed cuckoos laid eggs directly into nests, even in situations where carrying eggs might seem more convenient.

The use of long-term video monitoring across hundreds of nest boxes was key to capturing the behavior in detail.