Distribution, abundance, and habitat characteristics of Coastal Cactus Wrens (Campylorhynchus brunneicapillus) in San Diego County, California—2024 data summary

Abstract

Executive Summary We surveyed for coastal Cactus Wren ( Campylorhynchus brunneicapillus ) in 507 established plots in San Diego County in 2024, encompassing four genetic clusters (Otay, Lake Jennings, Sweetwater/Encanto, and San Pasqual). Of the 507 plots, 376 have been surveyed every year starting in 2020. Two surveys were completed at each plot between March 1 and July 31, 2024. Cactus Wrens were detected in 216 plots (43 percent of plots). Cactus Wrens were detected in 34 percent of the plots that have been consistently surveyed since 2020, indicating a higher plot occupancy rate than in 2023 (26 percent) and a similar plot occupancy rate to 2022 (31 percent), 2021 (34 percent), and 2020 (35 percent). There were 175 Cactus Wren territories detected across all survey plots in 2024. We documented 111 territories in the plots that have been consistently surveyed since 2020, which is an increase from 85 territories in 2023 and from 94 territories in 2022, and similar to the number of territories documented in 2021 (113) and 2020 (109). The number of territories declined from 2023 to 2024 in the San Pasqual genetic cluster but increased in the Otay, Lake Jennings, and Sweetwater/Encanto genetic clusters. At least 86 percent of Cactus Wren territories were occupied by pairs, and 176 fledglings were observed in 2024. We observed six banded Cactus Wrens in 2024, all of which we could identify individually by color band combination. Adults of known age ranged from 5 to 8 years old. All individually identifiable adult Cactus Wrens occupied the same territory in 2024 that they occupied during 2022 or 2023. We detected no movement of banded Cactus Wrens between genetic clusters during 2024. Vegetation at Cactus Wren survey plots was dominated by coastal sage scrub shrubs, such as California sagebrush ( Artemisia californica ), California buckwheat ( Eriogonum fasciculatum ), lemonade berry ( Rhus integrifolia ), San Diego County sunflower ( Bahiopsis laciniata ), and laurel sumac ( Malosma laurina ). Cactus Wrens preferentially occupied plots dominated by California buckwheat in the Otay genetic cluster and avoided plots dominated by lemonade berry (results of this test were marginally significant). Cactus Wrens also appeared to prefer plots dominated by broom baccharis ( Baccharis sarothroides ) in the Sweetwater/Encanto cluster and plots dominated by California buckwheat in the San Pasqual genetic cluster, although these preferences were marginally statistically significant. No definitive signs of fungal pathogens were observed on cactus within and around survey plots. Blue elderberry ( Sambucus mexicana ) was detected at 43 percent of plots, and Cactus Wrens preferentially occupied more plots with elderberry than were available. Very little dead or unhealthy cactus was observed within all survey plots, and Cactus Wrens preferentially occupied plots with lower amounts of dead and unhealthy cactus than were available. Over 80 percent of plots had more than 5 percent of cactus crowded or overtopped by vines and shrubs, and Cactus Wren occupied plots in proportion to the degree of shrub and vine crowding or overtopping. Non-native annual cover was less prevalent in survey plots in 2024 than in 2023, and Cactus Wrens preferentially occupied plots with less non-native cover.