Cactus Wren Survey Results Show Species Continues to Struggle

Date: May 10, 2024

A Community Science Cactus Wren Program was established in 2014 by the Conservancy to train volunteers to serve as Cactus Wren Monitors.  Trained volunteers make observations on the nesting, breeding location and other characteristics of this threatened species of Special Concern. Volunteers record their field observations on the mobile app Survey123.  With the help of these dedicated volunteer monitors, we now have a better understanding of such things as how close the wrens stay to the plants near their birth site and how much they explore developing plants in nearby areas.

The coastal cactus wren (Campylorhynchus brunneicapillus) (CACW) lives exclusively in coastal sage scrub habitat areas including on the Palos Verdes Peninsula and requires at least one acre in size with 30% prickly pear cactus (Opuntia spp.) and large coastal cholla (Cylindropuntia prolifera).  The wrens spend ninety percent of their foraging time on the ground, feeding on insects year-round, and feeding on fruit and plants during cooler months. Adult birds are highly sedentary and tend to return to the same breeding territory each year. They infrequently explore additional habitat, only doing so if they need to expand their forage area.  The wren’s natural tendency to stay close to its natal territory and not move great distances underscores the importance of having quality habitat throughout the preserves to help the species survive. 

The Survey area for the CACW was within eight reserves (Abalone Cove, Alta Vicente, Filiorum, Forrestal, Ocean Trails, Portuguese Bend, San Ramon, and Three Sisters) of the Palos Verdes Nature Preserve located in the city of Rancho Palos Verdes, CA. Volunteers conducted at least two surveys for each month of the survey period which began in March and ended in August. Volunteers walked their predetermined trail route documenting visual or audial observations of CACW.  Additionally, weather and wind observations were included because the birds’ presence is impacted by weather.

Over the last decade, the cactus wren population of the Palos Verdes Nature Preserve has been experiencing a decline in observed territorial breeding behavior with similar declines expected in their total population size.  Several causes of cactus wren decline have been identified as potential and likely drivers of declining regional presence and nesting success of cactus wren. These include: invasion by non-native plant species, heightened predation pressure in urban areas, unfavorable weather conditions (drought, seasonal shifts in rainfall, and cool early spring temperatures), and human disturbance. This program has found evidence to support each of these factors as present in the Preserve. It is expected that these issues are working synergistically to create a complex set of overlapping challenges.

The cactus wren was exclusively found in reserves providing the highest quality habitat with large expanses of cactus and specifically mature cactus plants. These locations, Alta Vicente, Filiorum, Three Sisters and Ocean Trails are considered “core habitat” or locations of central importance to cactus wren breeding in previous years. In 2022 the “core” areas of Alta Vicente, Three Sisters/Filiorum, and Ocean Trails were still occupied by CACW. Both Alta Vicente and Three Sisters/Filiorum Reserves had confirmed nesting sites. Nests were present in Alta Vicente, Three Sisters, Filiorum, and Ocean Trails Reserves. Juvenile CACW were observed at both Ocean Trails, Three Sisters, and Alta Vicente Reserves, a good sign of nesting success. In 2023, cactus wren were observed in new locations, including White Point Nature Preserve and Abalone Cove.  Our Stewardship team is working to figure out why this might be with help from data collected by cactus wren monitor volunteers.  Continued nesting success during the 2023 breeding season in Alta Vicente and Ocean Trails Reserves gives us hope for their future.

With new phases of the restoration at Abalone Cove, planting coastal sage scrub and cactus scrub habitat and exposing overtopped mature cactus, we expect to observe more cactus wren activity in future years. Additionally, the acquisition of the new 96-acre Wildlife Corridor north of Abalone Cove may provide cactus wren easier access to the reserve through the habitat corridor linking Filiorum, Three Sisters, Portuguese Bend, Forrestal, and the new Glass and Lay Reserves.

To meet or mitigate challenges faced by cactus wren in the preserve, the Land Conservancy has defined several management activities to improve the viability of the Palos Verdes cactus wren population.

Recommended activities include:

  • Continued removal of invasive non-native plants from cactus rich areas
  • Continued installation of new cactus plantings
  • Continued creation of foraging habitat (bare ground) surrounding cactus patches
  • Possible implementation of nesting boxes
  • Transplanting adult CACW individuals and swapping eggs from nearby CACW populations for genetic longevity purposes

If you are interested in finding out more about becoming a community science monitor, please visit: insert box:  recent wildlife camera/audio action/Trainings