Conservationists are trying to save this endangered bird. But their efforts may be producing too many males

A conservation programme designed to protect one of Indonesia’s most threatened birds may have an unexpected problem. It could be producing too many male chicks. Scientists who are looking at the maleo a bird that only lives on the island of Sulawesi have discovered that the way the eggs are kept warm in special places where they are raised may be changing the number of male and female baby birds.

The finding does not mean the conservation programmes are failing. In fact, protecting maleo eggs from people and predators has helped improve breeding success at some sites. But researchers say the new evidence raises an important question: could an intervention designed to save the species inadvertently make its recovery more difficult?

Why the maleo is already in trouble

The maleo is different from birds in how it raises its young. Adult maleo do not sit on maleo eggs. Instead female maleo bury maleo eggs deep in soil that is warmed by the sun or by heat. The maleo eggs are large about five to six times the size of a chicken egg. A female maleo can lay between eight and twelve maleo eggs during a period of two to three months. Once the maleo eggs are buried they are left to develop. When the maleo chick hatches it must dig its way to the surface and begins life without maleo parents. That unusual breeding system also makes the maleo especially vulnerable to people who collect maleo eggs.

The species is classified as Critically Endangered. Decades of egg collection and disturbance at grounds have led to a sharp decline in populations. A 2025 range-wide assessment looked at 228 current maleo nesting grounds across Sulawesi. The study found that 55% of sites that were active in 1980 were no longer active by 2019. By 2019 85% of all known grounds were either abandoned or had no more, than two pairs visiting per day during the peak season.

Hatcheries were meant to give the birds a better chance

To protect eggs from poaching and predators, conservation projects have been moving maleo eggs into protected hatchery facilities for decades. 19 conservation projects have used some kind of hatchery system since the 1970s. The eggs are moved to areas where they can grow and develop. When the baby birds come out they are set free, in nature.

The approach has produced encouraging results in some locations. Protecting eggs from human collection has been linked to population recoveries, and the 2025 assessment concluded that ending egg-taking is one of the most important steps for preventing the maleo’s extinction.

But researchers now suspect that changing the conditions in which the eggs develop could have another consequence.

The surprising sex-ratio problem

A June 2026 study published in Animal Conservation examined whether managed incubation could be affecting the sex of maleo chicks.

Researchers compared chicks from naturally incubated eggs with those produced in a hatchery at the Tambun Maleo Sanctuary in Indonesia. The difference was striking, although the sample was small. The naturally incubated group had an even split between males and females: six males and six females. In the shaded hatchery, researchers found nine males and only three females, a ratio of three males for every female.

Scientists stress that the sample is too small to prove that hatchery incubation is causing the imbalance. But they say the pattern is consistent with what has been observed in other megapode birds.

Why temperature could be the key

Maleos belong to a group of birds known as megapodes. Unlike most birds, megapodes rely on environmental heat rather than their parents’ bodies to incubate their eggs. That makes temperature particularly important. Previous research on the Australian brush-turkey has shown that relatively small changes in incubation temperature can affect embryo survival differently in males and females. Lower temperatures were associated with more male chicks, while higher temperatures produced more females under experimental conditions.

Maleos appear to be highly sensitive to temperature when choosing where to bury their eggs. They can select soil at suitable temperatures and may bury eggs at considerable depths to provide the right conditions for development. But conservation hatcheries can look very different from natural nesting grounds. At Tambun, maleo eggs are moved from a large, sun-exposed nesting area into a covered facility. They may also be buried at shallower depths. That could mean the eggs experience different temperatures from those selected by the birds themselves.

The researchers are not suggesting that hatcheries should simply be abandoned. Instead, they want conservation programmes to investigate whether incubation temperature is responsible for the apparent male bias and whether incubation conditions can be adjusted. There is already a relatively simple way to investigate the problem. Scientists can find out if a maleo egg has an female baby by using DNA from the inside of the eggshell. This helps researchers check how many males and females are being born. They don't have to depend on looking at the grown-up birds. This method could help people who protect maleo birds understand if the issue is happening everywhere or, in some places.

A conservation success story with a new warning

The maleo’s story is not simply one of decline. The 2025 range-wide assessment found that some nesting sites have recovered after communities began protecting eggs from collection. At three nesting grounds in the Tompotika region, for example, leaving eggs in place while guarding them against poaching resulted in four- to seven-fold increases in maleo numbers since 2006. That makes the new finding less a criticism of conservation work and more a warning about how complicated species recovery can be. Saving more eggs is clearly important. But for a species already struggling to find enough breeding birds, the researchers say conservationists also need to know whether those eggs are producing enough females to sustain future generations. For the maleo, the next step may therefore be as simple and as difficult as getting the temperature right.