The cuckoo is known for laying its eggs in other people’s nests and letting other birds raise its own offspring. Females usually prefer open, easily accessible nests. But some cuckoos have specialized in a host species that breeds in tree cavities: the redstart. The nesting holes of this small songbird are actually too narrow for the much larger cuckoo. Nevertheless, the brood parasite manages to smuggle its eggs into the clutches. Now researchers have discovered the tricks of the female cuckoos.
Cuckoos do not build their own nests, but lay their eggs in the nests of other species. The female cuckoos are each adapted to a specific host species. The color, pattern and size of their eggs are designed so that they fit almost perfectly into the clutches of their respective hosts. While most cuckoos parasitize species whose nests are easily accessible, some have specialized in the common redstart, which prefers to breed in narrow tree cavities. More than 2,000 years ago, the Greek polymath Aristotle speculated about how female cuckoos managed to transport eggs into such nests. He speculated that they might transport their eggs to their destination in their beaks. Since egg laying in hidden nesting sites is difficult to observe, this misconception persisted for many centuries.
Strategies with risk
In order to uncover the true strategies of female cuckoos, biologists led by Michal Kysučan from Palacký University in the Czech Republic placed numerous nesting boxes for redstarts in Finnish forests and observed them via video. Within four years, they managed to film 63 cuckoo egg layings on 53 different nesting boxes. “These observations allow us to finally see what actually happens during egg laying in a difficult nesting environment,” says co-author Tomáš Grim from the University of Ostrava in the Czech Republic. “For centuries, speculation filled this gap in knowledge. Now we have direct evidence.”
Contrary to what Aristotle had suspected, none of the cuckoos observed used their beaks to transport eggs. Instead, the researchers documented two other strategies, each with their own advantages and disadvantages: “Three quarters of the female cuckoos laid their eggs while sitting at the entrance to the cave,” the team reports. The birds dropped the egg into the unfamiliar nest without forcing themselves into the nest cavity. But the female cuckoos did not always manage to aim correctly: many eggs landed next to the actual nest and were subsequently ignored by the host bird parents. Others didn’t even hit the nest box but fell to the ground. “As a result, this strategy had low reproductive success,” write Kysučan and his colleagues.
Evolutionary arms race
The other cuckoo mothers tried harder to place their eggs correctly: they squeezed themselves into the narrow nest cavity. “However, this strategy was associated with a higher risk of the female cuckoo getting stuck in the cave,” report the researchers. Even in cases where the cuckoos were able to free themselves again, they often messed up the original nest so much that the host birds became suspicious and gave up their clutch. Nevertheless, more chicks survived using this method than if they had laid eggs from the edge of the cave. “We found that laying directly inside the cave has a higher success rate, but also greater potential costs,” explains Grim. “This balance likely explains why both strategies persist within the population.”
From the researchers’ perspective, the example of the cuckoo and redstart illustrates an evolutionary arms race between brood parasites and their hosts. “This is a rare example of significant behavioral variation within a brood parasite species,” says co-author Robert Thomson from the University of Turku in Finland. “It highlights how adaptable these birds are when faced with different ecological constraints.”
Source: Michal Kysučan (Palacký University, Olomouc, Czech Republic) et al., Animal Behavior, doi: 10.1016/j.anbehav.2026.123548