A quick read of some of the literature concerning the cause or causes of the extinction of the Passenger Pigeon provides more questions than answers. But it appears far too simplistic to confidently say that the cause was either over-hunting or habitat loss.
Some sources say that the massive population of billions of birds evolved slowly and that the survival of the species relied on this huge population density. Yet other sources say that the population numbers rose and fell dramatically numerous time during their time on this earth. In some articles, both points are made. They say that the species' survival depended upon huge numbers over the entire Eastern U.S., but then went on to say that those numbers crashed and rose numerous times since the last Ice Age. How can both statements be true?
Other sources say there was surprisingly little genetic diversity in such a huge population, yet over-hunting is blamed for the crash and extinction rather than other factors such as some unrecognized malady or disease. The last surviving bird lived something like 29 years and never produced a single egg. Did hunting have anything to do with that? It sounds as if a hell of a lot of them stopped producing eggs, and we can't blame DDT or Round-Up.
Some of these papers state that single flocks that numbered in the hundreds of millions laid waste to their own nesting and roosting areas due to eating all available food and contaminating the forest with their droppings. Yet hunting is again blamed when hunting would have thinned the population to a point where they weren't consuming all of the food and poisoning the forest with excessive droppings. It's interesting to read these theories, but you have to remember that none of them are able to offer a proven and definite cause for the extinction. And the more you read, the more you come to realize that a lot of what passes for good science is little more than unfounded speculation.
