Successful dispersal from a natal site to a breeding territory is an essential aspect of population ecology. For endangered species reintroduction and recovery efforts, success of captive breeding and release strategies may hinge on dispersal dynamics; however, dispersal distance often goes unmeasured and may be underestimated in small study areas because long distance dispersal events are not recorded. We explored natal dispersal patterns in the San Clemente loggerhead shrike, Lanius ludovicianus mearnsi, a federally endangered avian subspecies endemic to San Clemente Island, California, for both wild-hatched and captive-hatched/released juveniles. As breeding habitat on the island has recovered following feral goat removal, this intensively monitored population has increased from 14 birds in 1998 to a maximum of 179 in 2009. We followed 260 color-banded shrikes (136 females, 124 males) hatched 2000–2015 and breeding 2001–2016, and measured the distance between natal sites and first breeding territories. We compared male vs. female and wild- vs. captive-hatched and investigated the relationship between dispersal distance and population size.
Results/Conclusions
Mean dispersal distance was 3.21 km. Females dispersed further than males (4.03 km vs. 2.32 km, P < 0.0001). Male, but not female, dispersal distance showed a negative density dependence effect related to adult population size in the year following hatch (for males, r = -0.28; P = 0.001). Captive-hatched juveniles of both sexes exhibited similar dispersal patterns as wild-hatched juveniles. The most frequently used release sites for captive juveniles, located on the periphery of wild breeding habitat, were determined to be within mean dispersal distance (3.21 km) of less than one-third of known suitable breeding territories. As a result, we have begun exploring release site options closer to, and within, suitable breeding habitat. Furthermore, as vegetation on the island continues to recover, shrub growth between historically isolated woodland breeding sites may enable shrikes to find suitable nesting habitat without dispersing as far as they did in early stages of habitat recovery. Finally, our findings can be used to inform future occupancy modeling efforts, habitat enhancement planning, and non-native predator control efforts.