At first glance, why anyone would want to save California condors is not entirely clear. Unlike the closely related Andean condors with their white neck fluff or king vultures with their brilliant black-and-white colouring, California condors are not much to see. Their dull black colour when contrasted with white underwings — featherless head and neck, oversized feet and blunt talons are hardly signs of beauty or strength. Their appeal begins to become evident when they take flights. California condors can soar almost effortlessly for hours, often covering hundreds of miles a day — far more than other creatures of the air. Only occasionally do they need to flap their wings — to take off, change direction or find a band of warm air known as thermal to carry them higher. When it was discovered that the condor population was becoming dangerously small, scientists and zookeepers sought to increase condor numbers quickly to preserve as much of the species' genetic diversity as possible. From studying wild condors, they already knew that if a pair lost an egg, the birds would often produce another. So the first and sometimes second eggs laid by each female in captivity were removed, artificially incubated, and the chicks raised using hand-held puppets made to look like adult condors. Such techniques quickly proved effective. Despite these successes, the effort to save California condors continues to have problems, evoke criticisms and generate controversy. Captive-hatched condors released to the wild have died at what to some people are alarmingly high rates. Others have had to be recaptured after they acted foolishly or became ill. As a result, the scientists, zookeepers and conservationists who are concerned about condors have bickered among themselves over the best ways to rear and release the birds. Some of the odd behavior. on the part of these re-released birds is hard to explain. At times they landed on people's houses and garages, walked across roads and airport runways, sauntered into park visitor centers and fast food restaurants, and took food offered by picnickers and fishermen. None are known to have died by doing so, though. Most recently, some of the first chicks hatched in the wild died after their parents fed them bottle caps, glass shards, pieces of plastic and other man-made objects that fatally perforated or blocked their intestines. These deaths may be due to the chicks' parents mistaking man-made objects for bone chips eaten for their calcium content. Mike Wallace, a wildlife specialist at the San Diego Zoo, has suggested that some of the condors' problems represent natural behavior. that helps them survive as carrion eaters. The real key to successful condor reintroduction, he believes, lies in properly socializing young condors as members of a group that follow and learn from older, preferably adult birds. That, he argues, was missing from earlier condor releases to the wild. Typically, condors hatched in the spring were released to the wild that autumn or winter, when they were still less than a year old. Now, condor chicks at several zoos are raised in cave-like nest boxes. The chicks can see older condors in a large flight pen outside their box but cannot interact with them until they are about five months old. Then the chicks are gradually released into the pen and the company of the social group. The group includes adult and older juvenile condors that act as mentors for younger ones. According to the passage, the most impressive feature of the California condor is ______.