Loggerhead Shrike > Shrike Recovery Program > Research > Release Program > Predator Research and Management

Predation is considered an important factor limiting passerine populations in many habitats. Predators limit population growth by reducing nest productivity and increasing adult and juvenile mortality. For imperiled wildlife, such as the San Clemente Loggerhead Shrike (Lanius ludovicianus mearnsi), predation levels may not be the ultimate cause of extinction but may constrain the population, rendering it vulnerable to stochastic and demographic threats. Predation threats may be especially severe on oceanic islands where non-native predators have become established. In these cases, native wildlife is mal-adapted for the behavior and abilities of non-native predators that are typically fast colonizers.
San Clemente Island is host to a diverse suite of both native and non-native predators. Non-native predators include feral cats (Felis catus) and black rats (Rattus rattus) whereas native predators include Red-tailed Hawk (Buteo jamaicensis), Common Raven (Corvus corax), San Clemente Island Fox (Urocyon littoralis clementae) and the San Clemente Deer Mouse (Peromyscus maniculatus clementae).

In 1998, we became involved with the shrike recovery efforts and accepted the challenge of finding innovative methods to provide protection to nesting Loggerhead Shrikes. Due to the sensitive nature of Island fox populations, non-lethal control is desired. In 1998, we experimented with the use of a shock collar system to deter foxes from shrike nests. This system was designed to repel foxes from nest trees using a shock antenna system similar to the commercially available “invisible fence” system used by pet owners. Foxes are trapped and fitted with collars, which administer an electronic shock upon close approach to an antenna. During its first season of use, the shrike population experienced the most successful breeding season recorded in recent history

We also began monitoring raptor populations in 1999. The goal of this work was to gain a better understand the island’s native avian predators and how they interact with, and potentially affect shrikes. Primary aspects of the study include:

• Standardized hiking and driving surveys to determine density and abundance of diurnal raptor and corvid populations

• Monthly nocturnal owl surveys to determine species presence/absence, seasonal distribution, and density and abundance

• Island-wide nest finding and monitoring of red-tailed hawks, American kestrels, and common ravens

• Captive-rearing and release of nestlings removed from raptor nests located within the protective buffer of active SCLS territories

• Maintenance of a GIS database comprising nest locations of all three species

• Determining provisioning rates and prey composition using wall-mounted video cameras

• Patagial-tagging of nestling and adult red-tailed hawks and common ravens to allow individual identification at distance. Data will be used to generate heretofore-unknown general life history information on the island’s raptors (e.g., home range, natal dispersal, fecundity, etc.)

• Determining seasonal distribution of migrant and wintering raptors on SCI

Armed with a greater understanding of the island’s raptors, we hope to minimize potentially negative shrike/raptor interactions. The ultimate goal of the project is to manage raptors for the shrike recovery effort, while continuing to monitor their populations, to adapt our management strategies to their needs, and to identify and maintain a healthy insular island raptor population and ecosystem.

In addition to the fox and raptor work, we use other means to protect shrike nests during the breeding season. One method is to install sheet metal flashing on tree trunks to prevent climbing predators access to nests. This technique likely reduces predation from rats, cats and foxes, as these predators are not able to climb up the tree to access the nest.