... and the big bird that didn't.

Its varied diet may have permitted the California condor, one of today's largest and rarest birds, to survive the mass extinctions at the end of the last ice age, according to a new study. Many species of large land mammals died off about 12,000 years ago. Some scientists blame those extinctions on changes in climate, hunting by humans, virulent diseases, or a combination of those factors. Whatever the cause of the die-offs, they removed a considerable source of carcasses for scavengers. As a result, many scavengers suffered population crashes as well, says Kena Fox-Dobbs of the University of California, Santa Cruz.

California's La Brea tar pits became a tomb for many predators and scavengers, and scientists there have examined the creatures' remains to glean hints about ancient diets. Avian fossils excavated include California condors and representatives of other large extinct species such as the western black vulture (Coragyps occidentalis) and the teratorn (Teratornis merriami). The three types of scavengers seem to have been common in the region during the last ice age, but only the condors survived the post-ice age extinctions.

Nitrogen isotopes in collagen extracted from the fossil bones of the species provide important details about those birds' diets, says Fox-Dobbs. In particular, the ratio of nitrogen-15 and nitrogen-14 in bone collagen distinguishes predators and scavengers that consume grass-grazing herbivores from those that consume leaf-eating browsers. The highest proportions of nitrogen-15 isotopes are found in animals that eat sea lions, whales, and other marine mammals, she notes.

Marine mammals didn't go extinct in large numbers at the end of the last ice age, so that particular food supply--one that modern-day condors have been known to exploit--remained relatively steady, says Fox-Dobbs.

Analyses of the bone collagen extracted from the teratorns and western black vultures preserved in the La Brea tar pits between 35,000 and 10,000 years ago suggest that those birds consumed only the remains of browsers such as ground sloths and grazers such as bison. However, some of the condors had proportions of nitrogen-15 isotopes that can be explained only by various amounts of marine mammals in their diet, says Fox-Dobbs. The chemical analyses suggest that the birds' diets didn't change significantly during the 25,000-year-period in question.

Nitrogen-isotope data from condors that lived in other regions of North America during the same era bolster the notion that the birds survived by eating sea mammals. Chemical analyses of bone collagen from condor fossils found in Texas, New Mexico, and northern Florida, however, suggest that those scavengers fed only on grazers.

The condors that lived in those areas died out at the end of the ice age, which left only those in coastal California to perpetuate the species.--S.P.