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.