Most of what I know about this issue I learned from the newspapers. As I understand it, most recent condor nests in southern California have contained varying amounts of trash - pull tabs off beer and pop cans, pieces of glass, bottle caps, screws, metal washers, etc. At least four dead condor nestlings from these nests had significant quantities of junk in them, and the cause of death of two of them was definitely attributed to the trash. Pieces of glass had perforated one chick's stomach, and metallic poisons from the trash may have contributed to other deaths. The trash is apparently being picked up in the oil fields near the Sespe Condor Sanctuary and at the edges of roads in the Angeles and Los Padres National Forests.
The presence of trash in condor nests is not an entirely new phenomenon. Foreign objects were occasionally found in historic condor nests. These items might have been picked up by adult condors incidental to their feeding, or might have been ingested while "fiddling around." (Both adult and immature condors have been seen to play with a variety of objects - sticks, stones, bones - and might inadvertently swallow them while tossing them around.) However, the amount of trash in recent nests far exceeds anything ever found in pre-zoo program birds.
I've heard two suggestions advanced as to why zoo condors eat trash. One is that they are actively seeking bones (from which to obtain calcium), and are mistaking trash for bones. The other is that the birds are bored because provision of supplemental feed for them means they have a lot of free time that would otherwise have been spent foraging for carcasses. I have trouble with either explanation.
Calcium shortages have been identified in African vultures, when nestlings were found with malformed bones. Bone fragments and chips are being provided at some Old World vulture feeding stations, and may be doing some good. However, we never saw anything in the wild California condor population that suggested a calcium shortage, and we never saw condors that seemed to be specifically seeking bones. (I haven't heard of the zoo birds exhibiting calcium deficiencies, but I might not be up to date on that score.) Whether they might or might not need to search out supplemental sources of calcium, it seems pretty far-fetched to be that condors would mistake metallic, glass and plastic debris for bones, or that they would expect to find bones in such situations.
Condor boredom arising from supplemental feeding leaving the condors too much time on their hands (wings) seems equally unlikely to me. When we were providing supplemental food in the 1970s, we couldn't keep the condors from foraging for food to supplement the supplemental food. Lately, one of the concerns I've heard expressed about supplemental feeding the zoo releases is that the birds more and more eschew the food provided in favor of more natural foraging patterns. (This is frustrating the attempts to keep condors on lead-free food.)
So far, the trash problem appears limited to the birds using the area around the Sespe Condor Sanctuary. There are some obvious trash sources available to this population, but broken glass, bottle caps and other debris are ubiquitous to road edges in the forest and range areas of central and southern California. Also, the oil fields and road edges near the Sespe Sanctuary were just as trashy (maybe more so) in the 1960s and 1970s as they are now, and we never saw condors rooting around in those areas.
My guess is that trash feeding is a neurotic behavior related to care and conditioning of the birds while at the zoo. If the Sespe birds are coming from one particular zoo, and the populations without apparent trash problems are originating mostly from other zoos, the problem may result from the program at one particular facility. If so, it might not be too hard to solve. An earlier problem of zoo condors being electrocuted on power poles was apparently solved fairly easily by avoidance training before the condors were released.