April 14, 2023
Biologists are deploying 3D-printed replicas of hatchlings, lasers and drones to curb predation.
On a crisp spring morning in 2016, biologists Tim Shields and Bill Boarman hiked into a remote area of California’s Mojave Desert to put an idea they had to the test. The arid Mojave touches four states, although the bulk of it lies in California. It boasts the world-record high temperature of 134 degrees Fahrenheit, and much of the sparse terrain is dotted with brush, cacti and, in some areas, Joshua trees. Bighorn sheep, coyotes, jack rabbits, rattlesnakes and scorpions traverse the landscape. Water is scarce, especially at lower elevations, and many animals have adapted to survive on less than four inches of average annual rainfall. The desert tortoise, for example, can survive up to a year without fresh water just by eating plants and reusing water stored in its bladder.
In this extreme landscape, Shields and Boarman set up lifelike, 3D-printed replicas of juvenile desert tortoises, dubbed Techno-Torts, and aimed motion-activated cameras at them. The researchers hoped the fake reptiles would lure ravens, which feed on baby tortoises. The scientists left, and when they returned to check the cameras, they found the birds had swooped in for the prey but left disappointed. With the footage they captured, the two researchers gathered valuable information on how ravens approach and attack desert tortoises.
Shields, who has over 35 years of fieldwork under his belt, formed a company called Hardshell Labs just a year earlier as a way to develop and deploy new wildlife conservation technologies that would help save the threatened desert tortoise. Since then, Shields and his associates have collaborated with the U.S. Fish and Wildlife Service to collect data using hundreds of Techno-Torts in key habitat areas.
The result was an astonishing new study on the impact of raven predation on tortoises. Conducted by Fish and Wildlife biologist Kerry Holcomb, it was published November 2021 and made available online a year later in Human-Wildlife Interactions. According to Holcomb, between birth and age 10, raven predation diminishes the number of juvenile tortoises by approximately 42 percent each year if any part of their core use area is within roughly 1,600 feet of an active raven nest and the local raven population consists of about six birds per square mile, a density that is frequently observed in the western Mojave. That bird count is vastly higher than the number of ravens the land could normally support. The animals are bolstered by human food and water supplies, and nesting in such structures as electrical utility towers and billboards. About another 15 percent of juvenile tortoises die from other causes.
Holcomb explains that, under these conditions, if 4,000 tortoises were born in a given year, after ten years only one of the hatchlings would have survived. According to Holcomb, multiyear studies have established that without ravens, 429 hatchlings out of 4,000 would have survived in that same time period. “Tortoises start to reproduce at about age 13,” says Shields. “Significant losses at an early age due to raven predation are taking a heavy toll on the overall population.”
“When raven eggs hatch, the adults go into overdrive because they have babies to raise,” says Shields. “You have these predatory vacuum cleaners scooping up everything, and it’s not just baby tortoises—its lizards, mammals, snakes, insects and the young of other birds. They just go out and start killing everything, and if baby tortoises are around, they’re really easy to nail.”