ST. GEORGE — With an unsustainable number of juvenile tortoises dying, the Washington County Habitat Conservation Advisory Committee got the ball rolling on raven mitigation after years of research.
On March 28, the committee, which oversees the Habitat Conservation Plan, or HCP, moved forward with federal partners to create a raven management plan. Various strategies will be implemented to reduce young tortoise deaths, including egg oiling.
Egg oiling involves applying oil to raven eggs, preventing embryos from developing. It is considered a nonlethal and humane method to control bird populations. HCP Administrator Cameron Rognan described the measure as “birth control for ravens” while speaking to the Washington County Commission earlier this month.
Egg oiling increases survival rates of juvenile tortoises, “almost resetting us back to where we need to be in terms of survivorship,” Kerry Holcomb from the U.S. Fish and Wildlife Service’s Desert Tortoise Recovery Office said at Thursday’s HCP Technical Committee meeting.
One reason for this is that the practice can reduce the nest’s energy demand, as adult birds will not have fledglings to nurture. Additionally, when eggs are oiled rather than removed, adults are more likely to remain in the area and defend their territory, excluding other ravens, Holcomb said.
The strategy can also reduce raven populations and prevent ravens from teaching their young to attack small tortoises, St. George News reported.
The raven monitoring program began in 2015 and predation has been observed yearly since. HCP utilized “techno tortoises,” 3D-printed decoys, to track raven-tortoise interactions, Mike Schijf, a Red Cliffs Desert Reserve biologist, previously told St. George News.
The reserve partnered with Holcomb to analyze data collected from decoy stations to measure predation rates, and point count surveys to measure raven population densities, Schijf said.
They found ravens at approximately 39% of observed sites, equally about 2.7 birds per square kilometer, which exceeds the suggested maximum target density of 0.89, he said.
Raven observations were generally higher outside the reserve and highest in the Warner Valley and Sand Mountain areas, Schijf said.
One driver of the raven population increase is people, Schijf said. The birds’ numbers grow as Washington County does, and they benefit from the accompanying “subsidies,” such as landfills, artificial reservoirs and power lines.
Researchers reportedly observed ravens attacking techno tortoises 10 separate times at five of the 23 decoy sites.
Young tortoises are at higher risk of predation because their shells have not hardened yet, allowing ravens to break through and consume their insides.
Depredation rates within the reserve were estimated at a 12% chance annually, exceeding the sustainable conflict threshold of 7.8% for 0 to 10-year-old tortoises, Holcomb said.
Juveniles within 2 kilometers of a raven nest are at risk, Schijf added. Current rates are considered unsustainably high and could put the Upper Virgin River population at risk of functional extinction.
Ravens have a population-level impact on tortoises, Holcomb said. When researchers find a single carcass, it was likely dragged out by a rodent or scavenger seeking calcium or moisture and typically represents dozens of others.
“What we see are reports of survival of (0 to 10-year-old tortoises) that are below sustainable levels and, in some places, completely wiping out any efforts to either head start animals or to monitor juveniles,” he said.
The Habitat Conservation Advisory Committee unanimously approved a motion requesting the Fish and Wildlife Service pursue a raven degradation permit and the associated environmental assessment while asking Schijf and the Technical Committee to gather data and refine the plan.
The Washington County Commission also voted in favor of a resolution approving raven management as recommended by the HCP on April 4.
On Thursday, the Technical Committee determined that mitigation measures should encompass the entire Upper Virgin River Recovery Unit. Egg oiling will be prioritized in the Red Cliffs Desert Reserve with the addition of a 2-kilometer buffer.
Throughout the unit, the committee is considering other strategies to decrease subsidies that benefit ravens, including installing lasers at landfills to drive the birds away, new utility poles without flat surfaces to discourage nesting and a roadkill removal program, Rognan said. Holcomb will seek approval from the Fish and Wildlife Service Migratory Bird Program for laser use.
Additionally, Holcomb suggested increasing public outreach to change community behaviors subsidizing ravens, like having uncovered trash cans or leaving pet food outside.
Habitat Conservation Plan technical and advisory committee members will remain in communication with Holcomb and various partnering agencies throughout the process and continue to refine the plan, Rognan said.
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