In 1999, the Institute for Wildlife Studies (IWS) was contracted to develop and implement new approaches to release captive-reared San Clemente Loggerhead Shrikes (SCLS) onto San Clemente Island (SCI) in an attempt to bolster the dwindling wild population. This release work was conducted in conjunction with (1) a captive breeding program (Zoological Society of San Diego), (2) research regarding factors that might be contributing to the decline of the SCLS (IWS), (3) intensive monitoring of the wild population (Point Reyes Bird Observatory), (4) predator control measures  to reduce the vulnerability of SCLS to native and non-native predators (IWS), and (5) island habitat restoration (Soil and Ecology Restoration Group).
We place captive-reared shrikes in field cages (Figure 1) located in suitable habitat. The majority of suitable SCLS habitat is located in deep canyons located throughout SCI (Figure 2). After a period of acclimation, shrikes are released from their cages and monitored daily. To ease the transition of shrikes from captivity to the wild and potentially increase survivorship, we provide supplemental food post-release. From 1999–2004, we have used four different soft-release techniques to encourage high survivorship and recruit released shrikes into the wild breeding population.
Single Adult Releases - The goal of the single adult release is to take advantage of solitary adults already holding a suitable breeding territory. We hope that in situations where there are an insufficient number of adults in the wild, releasing a captive-reared adult to the solitary territory holder will result in a successful pairing and a subsequent breeding attempt. We have used this technique in 1999–2002 and in 2004.
Bonded-Pair Releases – Bonded releases attempt to establish breeding pairs (pre-paired in captivity) in vacant but suitable breeding habitat. This technique allows the manipulation of breeding pairs to maximize the potential genetic contribution of offspring to the wild population. This technique benefits the wild population through increased juvenile production as well as additional breeding production in subsequent years if adults survive. We have utilized this technique from 2000–2004.
Family Releases – To release shrike families, we place captive-reared shrike pairs in large cages sufficient for them to build a nest and rear young. These cages are within vacant but suitable canyon habitats throughout SCI. After the young reach an age of ~ 30 days, the entire family is simultaneously released. The family release technique allows for artificial double clutching as family adults can renest after release from the cage, increasing the annual productivity of the population. Preliminary analyses also suggest juveniles produced in family cages may experience higher survival than wild juveniles due to the protection afforded by the cage and the extra parental care provided during this vulnerable stage. These factors enhance the likelihood of recruitment into the breeding population in future years. We released families from 1999–2001, and are planning family releases in 2005.
Juvenile Releases – From 1999–2004, we released captive-reared juveniles between the age of 53 and 84 days old after a week-long acclimation period in field cages. Juvenile cages are located in areas with suitable escape cover, and most juveniles generally disperse from the release site within 20 days. This technique offers the possibility of rapidly increasing the wild population by releasing groups of 3–7 juveniles simultaneously. High post-release survivorship to the next breeding season can substantially increase the number of potential breeders from year to year.
 

PRELIMINARY RELEASE RESULTS 1999–2004

From 1999–2004, we have released 210 captive-bred San Clemente Loggerhead Shrikes into the wild on SCI (Table 1). The survival of shrikes through the first month post-release for all years combined is 66%. For the 190 SCLS released in 1999–2003, the survival rate for the first year post-release is 35%. Juveniles survived at a higher first year survival rate (40%) than shrikes released as adults (23%) during this interval. Furthermore, 26 (76%) of the 34 birds that have survived > 2 years in the wild were released as juveniles. These data indicate that the release of juvenile shrikes is a successful technique for rapidly increasing the population and providing future breeders to the population. Although shrikes released as adults do not survive as well as juveniles to one-year post-release, they do provide immediate benefits to the population through post-release breeding. Of the 61 shrikes released as adults from 1999–2003, 29 (48%) bred (defined as laying at least one egg) in the wild either during their year of release or in subsequent years. Of the 52 juveniles released from 1999–2003 that reached breeding age (survived to February 1^st of subsequent year), 41 (79%) have bred in the wild.

These data suggest that the techniques employed to condition and release shrikes are adequate to assimilate birds into the natural environment. Our release program has accomplished high rates of survival for the first month post-release and the breeding productivity of shrikes released as both adults and juveniles has been substantial. From 1999–2004, the minimum number of breeding shrikes has increased from 19 to 80 individuals, of which 35% are of captive origin (Figure 3). Furthermore, 60 (32%) of the 190 SCLS released between 1999–2003 have successfully fledged 250 young, 184 (74%) of which have survived to independence (age 40 days). It is apparent that the release program efforts are helping to pull this imperiled songbird back from the brink of extinction.

Figure 1. Large, steel release cage used for family releases of SCLS on SCI. Cage dimensions are 20 ft x 20ft x 10 ft. Back to text...

Figure 2. Family release cage (right) and attendant shelter located within a canyon on SCI. Surrounding woody vegetation provides escape cover for released SCLS as well as nesting habitat for adults. Post-release, SCLS are supplementally fed crickets, mealworms, and mice. Back to text...

Figure 3. The minimum number of breeding individuals in the wild population of San Clemente Loggerhead shrikes on San Clemente Island, California, separated by origin (wild-born vs. captive-reared). Note that the number of individuals is not divisible by two so as to equal the number of breeding pairs, as some individuals were part of multiple pairs within the same year. Back to text...

Table 1. Number of San Clemente Loggerhead Shrikes released by IWS since 1999, including those that have bred and the number still surviving (as of 1 June 2004) by each release type.

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