Conservationists using camera traps have obtained evidence that shows mona monkeys on the island of Príncipe off Central Africa are pinching the eggs of the Príncipe thrush, a critically endangered species whose entire population is estimated at less than 250.

Although it was thought to be likely, until now there was no firm evidence to prove the monkeys (Cercopithecus mona) were robbing the nests of the Príncipe thrush (Turdus xanthorhynchus), a starling-sized bird with an olive-brown back, white-flecked breast and bright yellow bill.

Mona monkeys came to Príncipe and its sister island of São Tomé centuries ago, possibly as Portuguese sailors’ pets. The islands, now the Democratic Republic of São Tomé and Príncipe, lie in the Gulf of Guinea.

Originally uninhabited and covered with evergreen forest, São Tomé and Principe were colonized by Portugal in the 1470s. In addition to the monkeys, the sailors also brought with them dogs, cats, pigs, rats, mice and African civet cats (Civettictis civetta) over the centuries.

“This is indeed a very global issue — the invasive species — and there is so much to find out and do that I feel we are only starting to see the tip of the iceberg,” said Estrela Matilde, executive director of Fundaçao Príncipe, a local conservation NGO that’s studying the impact of these introduced mammals on the Príncipe thrush and other native species.

The camera traps were set up to monitor 55 artificial thrush nests baited with quail eggs during a survey carried out from September 2020 through August 2021 in Príncipe Natural Park, the bird’s last stronghold in the south of the island.

Of the 55 nests, 42% were pilfered by animals within six days, a team led by Fundação Príncipe wrote in a brief report published in the journal Oryx. Mona monkeys were responsible for 18% of those events. Other predators could not be identified. Matilde told Mongabay this was because the images were overexposed, or the cameras malfunctioned.

One of the active thrush nests monitored by the team was visited five times by mona monkeys until the parent birds abandoned it on July 6.

These findings offer “compelling evidence that breeding activity of this Critically Endangered bird is being disrupted by mona monkeys,” the authors state.

Before this study, only one Príncipe thrush nest had ever been found. The Fundação Príncipe team found four active thrush nests, which are woven from moss and plant roots and tucked into rock and tree cavities. They saw no eggs or chicks, though the team did spot two young thrushes at the end of 2020.

Monkeys were likely not the only ones responsible for robbing the nests. A 2019 survey found other potential predators in thrush breeding sites.

“We have footage of the highest peaks, and there are cats there and dogs there and there are rats everywhere,” Matilde said, noting that rats are often stealthy enough to avoid triggering camera traps and may be responsible for a large number of nest robberies.

Mona monkeys are a popular game species with local hunters, but this pressure has driven them into Príncipe Natural Park where hunting is banned, and their numbers have increased inside the park.

Matilde says plans are being considered to allow some controlled hunting within the park to reduce their numbers to try to protect the thrush. But every intervention has to be carefully thought out.

“We do not understand yet the dynamics between invasive and native and it may be that there is some balance that can be disrupted if we start hunting monkeys,” she said.

Support from the island’s hunters may help the researchers in another way, by shedding more light on exactly what the animals are eating. Forty-three samples were taken from the stomachs of monkeys shot by hunters in the buffer zone and forests around the park and sent to a laboratory in Portugal for testing.

The stomach contents of three civet cats were also sent. The results are not yet available.

Martim Melo, from the University of Porto’s Research Centre in Biodiversity and Genetic Resources, in Portugal, said that while controlled hunting of the monkeys within the park may reduce pressure on the thrush and benefit several other species, total eradication of invasive mammals is usually the only practical solution.

However Melo, who was not involved with the Oryx study, said, that this is rarely possible. In Príncipe, where monkey meat provides both food and income to islanders, it would also be unpopular.

Melo and colleagues carried out a study in 2007 that got the Príncipe thrush classified as critically endangered on the IUCN Red List. They also used analyses of the thrush’s songs, DNA and physical traits to confirm it as a separate species from the São Tomé thrush (Turdus olivaceofuscus).

The two had been lumped together as a single species since 1924, initially masking the threats facing the Príncipe thrush.

Its struggle for survival, compared to that of the flourishing São Tomé thrush just 150 kilometers (93 miles) away, has sparked much speculation among scientists.

“We always wondered if the introduction of mona monkeys led to the extinction of several species before their discovery for science and if the native bird community we see now represents those species that were able to withstand their pressure — with the Príncipe thrush being a case of a species struggling to do so,” Melo said.

In addition to having different songs and different plumage, one key trait that sets the Príncipe thrush apart from its São Tomé relative is its tameness.

This is something that Fundação Principe’s Matilde witnessed nine years ago after she had just moved to the island from Portugal.

She and a group of friends were on a camping trip in Pico Mesa, a large mountain in Príncipe Natural Park. Matilde had lugged heavy cameras, binoculars and even a telescope to the summit in the hope of seeing a Príncipe thrush. She didn’t spot one there, but back at the campsite the next day one of the birds approached her friend out of curiosity while she was crouched in the undergrowth, taking a pee. Matilde told her friend not to move while she photographed the bird, much to her friend’s frustration.

After that encounter, the Príncipe thrush claimed a “very special place in my heart,” Matilde said. “They seem not to see badness in the world.”

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Banner image: A Príncipe thrush. Image courtesy of Fundaçao Príncipe.

Citations:

Guedes, P., Dos Santos, Y., De Lima, R. F., & Bird, T. L. (2021). Introduced mona monkey is a key threat to the critically endangered Príncipe thrush. Oryx, 55(6), 809-809. doi:10.1017/s0030605321001198

Dallimer, M., Melo, M., Collar, N. J., & Jones, P. J. (2010). The Príncipe thrush Turdus xanthorhynchus: A newly split, ‘critically endangered’, forest flagship species. Bird Conservation International, 20(4), 375-381. doi:10.1017/s0959270910000390

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