Predators in the sky: Crows see tortoises as tasty snacks

Conservationists are racing to save South Africa’s remaining dwarf tortoise populations from being devoured by an airborne indigenous invader — the pied crow.

“What we’re seeing is very high impacts from crow predation on all the [tortoise] species, from the angular to the common species,” said Bonnie Schumann, the former Nama Karoo coordinator for the Endangered Wildlife Trust’s (EWT) Drylands Conservation Programme, who has since retired. 

“We find a lot of dead tent tortoises, which are also a small species, and then, of course, the Karoo dwarf tortoise and the speckled dwarf tortoise as well.” 

In the semi-arid Nama Karoo, climate change is playing a role in the increasing spread of pied crows. Temperatures have warmed by about 2°C and Birdlife SA has documented a shift in the pied crow populations moving into the region over the past 20 years, she said.

“It’s a largely treeless environment — they need trees or something to nest in — but we’ve put in power lines and roads. Every morning they’ve got breakfast served for them from road mortalities, so it’s essentially a subsidised species.”

The crows are clever, highly adaptable, opportunistic and eat virtually anything, “so the conditions are just right for them moving into an area where tortoise species haven’t adapted to them being there”, Schumann said.

“From the little glimpse that we’ve had in the work that we’ve done, bearing in mind it’s a vast area that we’re covering, what we’re seeing is very alarming because there doesn’t appear to be large, healthy populations. At any rate, if there are, we haven’t found them …

“Everywhere we’ve gone, we’ve found shells and remnants of shells but we’re really struggling to find live animals so this has been going on for some time … It is likely that probably in the last 30-odd years, they’ve been in decline and it’s not been properly documented.”

South Africa is home to two of the planet’s most well-known botanical biodiversity hotspots: the succulent Karoo and fynbos biomes. But few people are aware that the country holds the title of the tortoise capital of the world and is home to 13 species, most of which are endemic, Schumann said.

“Tortoises even predate dinosaurs in one form or another, and on our watch now, in the last three or four decades, we’re basically losing populations.”

Globally, more than 60% of the 357 recognised species have either become extinct or are threatened. About 52% of all assessed tortoise taxa have been classified as critically endangered, endangered or extinct.

Habitat loss, the degradation of remaining habitat, climate change and illegal trade are driving their global spiral towards extinction, coupled with uncontrolled fires and electric fencing, in South Africa, in particular. The death knell or “straw that broke the camel’s back, probably”, said Schumann, is aerial crow predation.

“You can imagine if you stand out in the Karoo somewhere, it’s dead quiet. There’s not a leaf moving and, if a crow is circling overhead, and anything moves down there, they spot it,” Schumann said.

The effects of hyperpredation by airborne predators, primarily crows and ravens, part of a group collectively known as corvids, is a relatively new threat that has been poorly documented in many countries. However, the evidence is mounting rapidly to show that without interventions, “we will see the extinction of many more tortoise species within our lifetime”, she said.

Crows and ravens have joined the ranks of what are known as “subsidised species”, Schumann said. “Simply put, they are thriving globally on the wide selection of resources human activity has inadvertently provided for them. Their populations are increasing exponentially and expanding into new areas, where they become native invaders.”

Hyperpredation happens when subsidised species exert excessive pressure on prey populations. For tortoises, this effect has been “particularly catastrophic”.

“They are very long-lived, but grow and mature slowly, and reproduce at a slow rate. Smaller species, like the dwarf tortoises, produce only one to three eggs a year from the age of around 12. “For a population to remain stable, mature individuals need to reproduce for decades to produce enough offspring that survive to adulthood.”

When it comes to crow predation, size matters, as in the case of dwarf tortoises, but even adult tortoises are highly vulnerable to crow predation, Schumann said.

“Crows use their powerful bills to peck through the shells of young tortoises and can fly up with adult dwarf tortoises and drop them onto rocks to break their shells open. The loss of adult tortoises make the recovery efforts of dwindling populations much more difficult.”

Two years ago, the EWT, in partnership with Dwarf Tortoise Conservation and the Turtle Conservancy, launched a dwarf tortoise project to locate viable populations and implement conservation action for the Karoo’s speckled dwarf tortoise and the Karoo dwarf tortoise. The Karoo is home to nine of the 13 tortoise species found in the country. 

The speckled dwarf tortoise, which is endemic to Namaqualand and the world’s smallest tortoise species, reaching a maximum length of about 10cm. “Dwarf tortoise conservation champion and researcher Victor Loehr has spent most of his life unravelling the secret life of these diminutive reptiles, contributing much of what is known about them to the scientific world,” she said.

Schumann said up to 20 years ago, scientists regularly surveyed tortoise populations and, during Loehr’s early work on speckled dwarf tortoises, they were abundant. But follow-up work at his study sites 10 years later, and his most recent research on the Karoo dwarf tortoise in 2019, demonstrated an “alarming decline” in population numbers for both species.

The EWT team has struggled to find a single viable population of the Karoo or the speckled dwarf tortoise over the last two years of extensive surveys. 

Over 20 surveys have yielded “depressingly few live animals” and, in most cases, only shell fragments were found. In April, during the most recent survey near Calvinia, the EWT team recorded the largest number of dead speckled dwarf tortoises at a single location.

A total of 45 carcasses were documented under and around a crow’s nest in a dead tree, with another 10 carcasses recorded on the adjacent slope. Five more were recorded elsewhere on the property, bringing the total to 60 dead speckled dwarf tortoises on one property, many of which were mature. Only two live ones were found.

“The crows carry them back to the nests and feed them to their chicks so, over the nesting season, they can have a massive impact. They are raising chicks that are hungry and growing fast.”

She thinks conservationists are seeing the “tail end” of the decline in tortoise populations, which like many other species, including invertebrates and reptiles, are data-deficient in South Africa.

“A lot of it has happened already because we’ve been to areas where they should be and where they’re not and all we found is old remnants of shells … not even intact shells in some cases,” she said, noting, however, that it’s hard to make concrete conclusions because there’s so little data to draw from.

Schumann said that although tortoises occur in some of the country’s protected areas, “this does not mean we can pack up and go home”. 

“If the causes of decline, including hyperpredation by crows, are not clearly understood and addressed, their survival in protected areas is not guaranteed and may have already been compromised …

“The question is, if they’re in a protected area like a nature reserve, the crows don’t know that. It doesn’t matter if they’re in a protected area and the cause of decline remains the same. If the crows are impacting outside a reserve, they’re impacting inside a reserve.”

The only species of tortoise which has a dedicated local conservation project is the critically endangered geometric tortoise. There is an urgent need to implement concerted conservation efforts for the other tortoise species, including all the dwarf tortoise species and other relatively small species, such as the tent tortoises.

“The outlook for all these species is grim without specific conservation interventions.”

The EWT is collaborating closely with local and international tortoise experts, landowners, and provincial conservation authorities, to devise a strategy to protect the remaining dwarf tortoise populations.

Two decades of research in the Mojave Desert, aimed at understanding the raven-tortoise conflict, and to come up with innovative solutions, including green lasers and 3D-printed decoy shells, is helping inform the way forward in South Africa. The US, however, has a 20-year head start, Schuman noted. 

“What is clear though, is that time is running out fast and we cannot afford to be timid in our approach. Tough decisions are going to need to be made when it comes to protecting tortoises from crow predation and any further loss of habitat.”

A range of tools is needed to tackle the problem, she added. “We are talking to a lot of experts from around the world … to try and learn from the lessons elsewhere and then to try to apply some of that here …

“We’re having to really think about what one does and how one does it. There’s lots of different options that we’re looking into, but we’re going to have to try to exclude the crows from core areas — not the entire Karoo — but you might have a series of rocky outcrops where tortoises are.”

Any recovery will be difficult to measure as there won’t be an increase in the tortoise population overnight. “It will be a slow recovery, if in fact at the low densities they’re at now, they’ll be able to recover.”