According to Rosen, the book blames it on the development of railroads and logging after the Civil War. While America was still largely rural, hunting didn't dent the huge flocks that used to block out the sun. But shortly thereafter, people could find out where flocks were over telegraph wires, travel there to hunt, and sell birds to people in cities, transported by railcar. The invention of refrigerator cars in 1878 didn't help. Widespread logging also took out many of the forests that the birds depended on for seeds. Furthermore, the birds likely thrived on having such enormous populations, finding safety in numbers. Once these populations plummeted, they lost their competitive advantage.