In my various presentations on California condors, I talk about the factors that caused their decline and near extinction. Sometimes we discuss poisons or shooting or collecting for museums or any of a number of other impacts. Taken factor by factor, or time period by time period, the significance of various impacts is hard to quantify. But there is a pretty basic way to talk about the subject. What happened to the condors? We happened.
Condors have a very low reproductive potential. Their ability to replace themselves is greatly limited. They have to stay alive at least six years before they breed for the first time - not long in human terms, but pretty long for a bird. For most condors, six years is likely the minimum age of first successful breeding, assuming it may take another year or so to find a mate and get good at all the techniques. Once nesting starts, the annual clutch consists of only one egg. If the egg successfully hatches, the young bird is normally in the care of the adults well into the year after it hatches, which means the parents have the potential to produce only one young bird every two years. Just to keep the population stable - for each pair to produce the two condors needed to eventually replace them - condors have to stay alive for at least 10 years.
The good news through the 18th Century was that most condors probably did live at least ten years. They are a long-lived bird. We don't know the longevity of wild birds, but in captivity condors have regularly lived 10, 20, 30, even occasionally over 40 years. Because condors have no natural enemies, and most condors died of old age or accidents, the population did have the ability to at least remain stable over long periods of time.
The bad news to this low reproduction but long life stability is that it is easily upset. Ducks and quail live only a few years, but their reproductive potential is so high that their populations can usually maintain themselves through disastrous weather, major changes in predation, greatly increased hunting, or any number of other perturbations. The condor population, on the other hand, was unable to adapt to even the smallest changes in the natality:mortality balance. The first people to come in contact with condors started to shift the balance. Even the occasional condor taken for a Native American ritual, or the occasional condor shot by a curious 18th Century Spaniard, was enough to begin destabilizing the population. Perhaps the only time after that first human encounter that the condor population actually increased was between 1830 and 1848, when the trade in cattle hides left hundreds of thousands of livestock carcasses on the California range, and created a veritable scavenger heaven.
Any benefit that might have come from the livestock boom was quickly overshadowed by the effects of the California "Gold Rush" (lots of guns and lots of bored young men to use them), the subsequent settling of the State (more gunfire, and maybe poisons), and collecting for museums. Between 1792 and 1976, a minimum of 300 condors are known to have been removed from the population, almost all of them by direct human intervention, and with the vast majority of them lost between 1840 and 1925. Because of timing and location, random shooting and museum collecting took the biggest toll on the condors, added to by a variety of other human-caused losses. Consider the effects of this degree of mortality on a population previously dying mainly of old age and accidents. (And remember, these are the documented losses; we'll never know how many others succumbed to shooting, poisons, and other human acts.) I think that, by the 1920s, the condor population was in a decline from which it never recovered. The only reason there were still California condors in the early 1980s was because the long life of individual birds slowed the inevitable decline to (if they hadn't been rescued by the captive breeding program) extinction.
[NOTE: The effects of individual mortality factors are covered in detail in Chapters 3, 4 and 25 of my book "Condor Tales."]