The San Clemente loggerhead shrike is an endangered, medium-size, predatory songbird endemic to San Clemente Island, a Navy installation and vital training ground for the military. In the 1990s, the shrike population dropped to as few as 14 individual birds due to historic landscape-level changes to the island, as it was used for ranching prior to its transfer to the Department of Defense in 1934—sustaining over 100 years of overgrazing by livestock and feral ungulates, the latter of which were removed by the Navy. In response, the Navy initiated the shrike recovery program and established a shrike working group to protect the population against the threat of extinction and restore it to historical levels. This working group continues to this day and includes SDZWA.
Within the working group, SDZWA’s San Clemente Loggerhead Shrike Conservation Breeding Program has three main functions: provide juveniles and adults for release to help bolster the native population; maintain an assurance population to act as a genetic reservoir for the entire population on the island; and provide care for salvaged or injured eggs or birds. Given the long history of the project, this purpose is accomplished by on-island program staff with the aid of other departments throughout SDZWA. With 34 years of experience, SDZWA and our partners have worked hard to create a robust and dynamic toolkit of conservation techniques to aid in the shrike’s recovery.
Getting Established on the Island
In the early history of the program, the focus was on artificial incubation of eggs, hand-rearing of chicks, and figuring out the basics of shrike husbandry and breeding. But once those protocols were established, the program quickly transitioned to establishing techniques that would maximize both the quantity and quality of shrikes available for release.
In the early 2000s the program switched its focus to parent rearing, and that remains a key factor to the success of the program and improved survival of individuals post-release. The conservation breeding flock is located on San Clemente Island, which ensures all birds are acclimated to their native environment. All birds in the flock are fed live prey, which encourages natural behaviors, prepares individuals for release, and improves the health and the welfare of the birds. Native prey, including insects, mice, and lizards, are also provided by staff to guarantee the birds can recognize and handle food items available in the native range. Native shrubs are used for perching, cache sites, and potential nesting sites, and native plants are collected and provided for nesting material at the start of each breeding season.
Every effort is made by the conservation breeding program to mimic the shrike’s natural history and conditions found in its native range, including syncing the phenology (cyclical and seasonal natural phenomena) and behaviors of the shrikes under our care to promote natural breeding behaviors. In many shrike species, the males will larder, or store, their food items by caching them on branches as a signal to the females of their suitability as mates and parents—and our males are no different. While the timing of breeding varies annually, it usually occurs between March and July. Program staff manage the breeding of the flock to maximize genetic diversity in the entire population. In the conservation breeding flock, care staff conduct behavioral observations of the birds to assess behavioral compatibility of potential mates; oversee the progress of breeding activities, like nest building; and assess the foraging and flight skills of weaned juveniles destined for release.
With the right conditions established in the aviaries, and under the watchful eyes of care staff, parent shrikes are able to raise their young to independence without the need for artificial incubation and hand-rearing techniques. Though release and breeding practices can vary from year to year, we usually release between 25 and 35 juveniles annually.
Island Forces
Over its history, the recovery program has had many successes using the tools available to us. In 2013, there were 70 breeding pairs in their native habitat, and every native shrike had some portion of their ancestry that could be traced back to the conservation breeding flock. Despite the efforts of the working group partners and these successes, the shrike population has dipped in recent years to as few as 22 pairs. This decline is suspected to be largely tied to the El Niño and La Niña weather cycle; drought years cause a decline in prey abundance. Throughout the long history of the shrike recovery program, the working group has continued to place an emphasis on adaptability and innovation when it comes to our conservation practices—dynamic threats necessitate dynamic responses. We are currently focusing on creating new tools and adaptive response plans to bolster the native flock and continue to improve the quantity and quality of release shrikes.
For example, staff can artificially incubate, hatch, and hand-rear individuals when needed, but have also pioneered fostering techniques using active breeding pairs that can raise those individuals instead. These pairs have been very successful when presented with salvaged eggs or chicks that need continuous care. Post-release survival of hand-reared shrikes is 20 percent less than that of native range-born individuals, whereas survival of parent-reared shrikes is comparable to their native counterparts, making fostering another key strategy in the toolbox. Care staff create behavioral profiles of our breeding pairs so we know who is or who will make good foster parents, and we are able to augment clutches, exchange clutches, and even swap eggs for chicks with a 100-percent success rate. These past few years, we have also been working with the Wildlife Welfare and Wildlife Health teams to get genetically valuable birds that have never bred to contribute to the population. With additional calcium-rich supplements and eliciting improved nest building behaviors by varying the presentation of nesting materials in aviaries, we have more individual shrikes contributing to the breeding effort.
Insular Isn’t Isolated
In 2022, the partners within the shrike working group and representatives from many SDZWA departments met to brainstorm additional measures to help the recovery of these birds. The 2023 and 2024 breeding seasons were our flagship years for implementing these new ideas. We’ve started planting native plants around and within the aviaries that are specifically meant to attract insects that are a significant source of food for the native shrike population, especially during drought years. With the recovery of other species on the island, including peregrine falcons that will predate shrikes, we’ve also begun implementing predator avoidance training, making sure that all juvenile shrikes destined for release are exposed to a falcon replica before they reach independence to allow the parent to model the appropriate duck-into-cover technique. Last year, with a wet winter and high prey abundance forecasted, care staff were also able to double the number of shrikes available for release with a total of 55 birds raised to independence, the most ever for this program. We achieved these numbers by reintroducing pairs and allowing them to have a second clutch after the first set reached independence. Twice as many chicks meant twice as much work, but our team and our partners rose to the challenge and are planning on using the same technique again this year. It is impossible to predict what the 2024 breeding season will be like for the flock in the native range, but we can be sure that the conservation breeding team is ready for another banner year for the conservation breeding flock, which can only be a boon for the entire population.