Why did the passenger pigeon become extinct?

The passenger pigeon (Ectopistes migratorius) became extinct primarily due to intense, sustained slaughter by humans as settlers moved westward across North America.12

Although billions of these birds once inhabited eastern North America, they were hunted on a massive scale for their meat. The birds were slaughtered by the millions annually and shipped in railway carloads to be sold in city markets. Hunters frequently targeted their nesting grounds, often annihilating entire colonies within a single breeding season.13

This unthinking exploitation caused a precipitous decline in the population starting around 1870. Despite some unsuccessful efforts to breed the species in captivity, the last known passenger pigeon, named Martha, died at the Cincinnati Zoo on September 1, 1914. The extinction of this species serves as a stark reminder of the consequences of human avarice and later became a significant catalyst for the modern conservation movement.132