Imlay, T.I., Crowley, J.F., Argue A.M., Steiner J.C., Norris D.R., Stutchbury, B.J.M. 2010. Survival, dispersal and early migration movements of captive-bred juvenile eastern loggerhead shrikes (Lanius ludovicianus migrans). Biological Conservation 143: 2578–2582.
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Captive breeding programs are established for species that are threatened or endangered to maintain, increase or restore populations in areas where they exist or had existed in the past. Shrikes, a group of migratory passerine birds, are declining around the world. In Canada, there are less than 35 known breeding pairs, which are listed as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC, 2010). A study carried out by Imlay et al. (2010) looked at captive-bred eastern loggerhead shrikes (Lanius ludovicianus migrans) in Carden Plain, a Nationally Significant Important Bird Area in the township of Kirkfield, Ontario. These birds were a part of a captive breeding and release program established in 1997 by Environment Canada as a National Recovery Strategy for the species.
Imlay et al. examined post-release survival both before and right after migration had begun, and the natural and human causes of mortality using ground and aerial telemetry. The study was carried out in 2007 and 2008. Captive shrikes with radio transmitters attached were released 5-9 weeks after fledgling (beyond the normal age of independence in the wild, which occurs at about 4 weeks). Eighteen were released in 2007 and 20 were released in 2008, usually in batches in order of when they hatched.
On the ground, birds were tracked every 1-2 days. When encountered, carcasses were retrieved to assess causes of mortality: cleanly plucked feathers was assumed to be a result of raptor predation, chewed body and missing feathers from mammalian predation, and relatively intact bodies from starvation. Shrikes were also tracked through aerial searches four times in 2007 and ten times in 2008. Aerial monitoring was done to determine if they had dispersed from the release site, and then migrated; if the birds could not be found in the study area for several days and couldn’t be re-located by ground, they were assumed to have migrated.
The average annual return rate of juvenile shrikes was about 4.6%, which is about four times higher than the average return rate seen in wild shrikes. Overall survival rate of juvenile shrikes was about 75%. Nine juveniles were found dead, and predation was the cause of mortality in five birds, and unknown for the other four birds. Mortality was not significantly correlated to release date. Migration occurred in August and September, with age at migration negatively correlated to hatch date (i.e. those that hatched earlier, tended to migrate later). During the first stages of migration, no cases of mortality were found.
Providing captive breeding programs for migratory passerines can be difficult. Passerines are known to have high natal dispersal (dispersal from the birth site to a new site where the bird will reproduce), making them hard to track, as it is possible they may recruit in distant populations or die en route to the dispersal site. These types of birds also tend to show high mortality during migration. Although positive results have been revealed for the captive breeding program examined by Imlay et al., it is still very important to understand the reason for the decline of the eastern loggerhead shrike population in order to continue successful efforts in reviving the shrike populations.
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