The drastic population decline during the 20th century is principally attributed to persecution and accidental ingestion of fragments from lead bullets from carcasses, resulting in lead poisoning. Lead poisoning remains a threat for released birds and has caused many fatalities and resulted in the treatment of many more birds7,17; 9 of 13 birds released at the Pinnacles National Monument in California had to be recaptured and tested for lead poisoning after feasting on a field of squirrel carcasses shot by hunters using lead-shot in 2006. It is particularly prone to the threat of lead-poisoning owing to its longevity and delayed onset breeding strategy, and given the distances it travels to forage, meaning lead can build up in the blood to dangerous levels over many years having been ingested over a broad area18. Shooting and accidental poisoning continue to be the principal threats to condors and at current levels threaten the long-term sustainability of reintroduced populations22, but lead ammunition is being banned within the species's range in California and there are increasing indications that the federal government will gradually phase out the use of lead across the U.S. Despite efforts to reduce the threat of lead-poisoning, it is reported that over 90% of condors released in Arizona still test positive for lead31 and in January 2010 three birds were found to have died from lead-poisoning in northern Arizona32. A study conducted recently in California, using samples collected in 2004-2009, suggests that around one third of condors there are experiencing toxicological effects from lead ammunition33. Publicity and awareness raising campaigns appear to have successfully reduced persecution. Ingested anthropogenic material was recently responsible for the deaths of two nestlings and strongly implicated in a number of other deaths23. The dead condors were found to have swallowed glass fragments, wire, plastic cartridge cases etc23. Two birds were shot in California in 2009. Both were alive as of April 2009, both being treated for lead poisoning28. Puppet-reared birds may be more prone to exhibit problematic human-oriented behaviour such as tameness and vandalising property than parent-reared birds8. However, there is no apparent difference in mortality between released birds that were puppet-reared and those which were parent-reared26. In the early 1990s a number of captive-reared birds were lost owing to collisions with power-lines, but this behavioural problem has been addressed using a conditioning programme with fake power poles14. The spread of West Nile virus is not anticipated to be a problem for the species as most birds are vaccinated14. Overall survival of released birds has been high4, although without the capture, treatment and re-release of lead contaminated birds it is like that rates of mortality in the wild still exceed sustainable levels25.
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