Final answer:
The early 1900s decrease in bird populations is introduced through a discussion on the impact of human activities like overhunting, habitat destruction, and pesticide use which led to significant extinctions and declines.
Step-by-step explanation:
The author introduces the decrease in bird populations in the early 1900s by discussing human activities that led to extinctions and declines. Among these activities, overhunting and habitat destruction were significant factors, as seen with the extinction of the passenger pigeon. The use of pesticides, such as DDT, also contributed to the decline, contributing to the extinction of species like the dusky seaside sparrow. Later studies reinforced the impact of human actions on birds, showing that hundreds of species are at risk today, and it is imperative that humans take steps to protect these vital natural resources.