Determining mortality causation and having accurate estimates of survival rates for wild populations is challenging; however, this knowledge can be crucial for the success of endangered species recovery projects. The San Clemente loggerhead shrike (Lanius ludovicianus mearnsi) is a federally endangered subspecies endemic to San Clemente Island (SCI), California. Despite an intensive >20 year recovery program that includes non-native predator management and captive breeding and release, the population has not recovered. Nest camera studies have identified key nest predators and failure rates; however, a recent conservation planning exercise highlighted remaining data gaps regarding sources of mortality for juveniles and adults. To begin filling these gaps, we initiated a known-fate telemetry study in 2017. Half (10/20) of the captive-hatched juveniles released in 2017 were fitted with transmitters and located daily until transmitter battery failure (nominal life 116 days) or a mortality was recovered. We estimated daily survival rates using a known-fate model in Program MARK.
Results/Conclusions
We followed 10 shrikes for a total of 729 exposure days (range 7–134). Seven shrikes survived until the transmitter batteries expired (78–133 days post-release). The remaining three were predated; one by an island fox (Urocyon littoralis clementae), one by a feral cat (Felis catus), and one by a black rat (Rattus rattus). The most highly supported models (using a weighted average) yielded a daily survival rate of 0.996 and a cumulative survival rate (probability of surviving the 134 day study period) of 0.57 (95% CI = 0.23–0.85). This study was our first attempt to identify cause-specific mortality and calculate fine-scale survival rates of juvenile shrikes on SCI. Island foxes and black rats were previously known nest predators, yet their impacts on juvenile survival were unknown. Feral cats were included in endangered species listing as a primary threat, though prior to this study there was little empirical evidence they preyed on shrikes. Knowing direct sources of mortality will help inform strategies and highlight priorities within our predator management program. The survival estimates from this research will help parameterize population viability models. Overall, these data will improve our conservation management plan and recovery goals for the species.