The Occurrence and Effect of Inbreeding in San Clemente Loggerhead Shrikes (Lanius ludovicianus mernsi)

San Clemente Loggerhead Shrikes (Lanius ludovicianus mearnsi) have been bred in captivity since 1991 following their listing as an endangered subspecies in 1977. For 25 years, breeding of this captive population has been managed to avoid inbreeding using protocols based on behavior, demographics, genetics, and ecology. Yet despite the broad application of these comprehensive protocols, inbreeding within this subspecies has never been quantified outside this framework. To evaluate the effects of the population bottleneck and captive management techniques on the population, analysis of overall levels of inbreeding, founder representation, and the division of inbreeding into partial coefficients attributable to founder lines was conducted using the pedigree of the population. Inbreeding depression was then measured by comparing inbreeding levels to measures of survival (egg fate and overwintering survival) and to measures of reproductive success (the numbers of offspring produced). Gene diversity was 93%, and the largest average contribution from a single founder to the inbreeding level was 18% in 2015 (founder SB 235). Moderate inbreeding depression (2B=5.38 and 3.06) was identified in juvenile survival and egg fate, but later life history traits that affect fecundity were not affected by inbreeding. This study supports the continuation of releases and parent-rearing as part of the population’s management. It also provides specific recommendations for breeding that avoid descendants of SB 235, while seeking out descendants of SB 230, who has made negligible contribution to the measured levels of inbreeding. These findings should help this longstanding project maintain a viable and sustainable population of Loggerhead Shrikes on San Clemente Island.