|MadSci Network: Evolution|
This is actually a more complex question then it at first appears. My source for this information is the book "The Birder's Handbook", written in 1988 by Paul Ehrlich, David Dobkin, and Darryl Wheye. If you are interested in birds this is an excellent second or third book to get (after a good US field guide and maybe a good local guide). The essay about the passenger pigeon brings up an interesting point. Apparently, once the number of passenger pigeons got small enough that hunters couldn't make a profit hunting the birds they were pretty much left alone. This left tens of thousands of birds still alive. These birds also still had what seemed to be good forest habitats left for feeding and breeding. But for some reason the population of passenger pigeons dwindled until none were left. There were even some attempts to maintain captive flocks but this didn't work either.
One possible explanation for the continued decline and extinction of the passenger pigeon was that, once flocks fell below a certain number (which would have been a very large number) the birds could not produce enough offspring to make up for losses to predators. One of the ways that people harvested young pigeons was to just walk under the nesting trees and pick up fallen nestlings. This makes it seem like passenger pigeons weren't the best nest builders or maybe the most attentive parents. This would fit in with the idea that there had to be large numbers of pigeons to make up for predation.
Another factor we need to consider to answer your question is, if humans hadn't hunted passenger pigeons down to extinction, could other effects have lead to their extinction? Passenger pigeons lived in large flocks that used large forest tracts for nesting and food. One of the effects of human activity in America was the chooping down of large forests, especially in the Eastern US. This is known as fragmentation, where continuous stretches of forest are reduced to smaller and unconnected remnants. It is possible that, if we hadn't reduced passenger pigeon populations by hunting we might have driven them to extinction through the removal of the habitat they needed for food and nesting.
So, the short answer to your question is that passenger pigeons may not have been able to survive even without the massive hunting that took place. Probably the main reason that they were able to coexist with the Native Americans was the lower population density and environmentally friendly lifestyle of these people.
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