Rothschild's motivation to expand on a lecture he previously presented at the Ornithological Congress in 1905, which sparked significant interest in extinct avian species. He expresses gratitude to those who contributed to his research and mentions that the book would differ from the original lecture due to the extensive additional research undertaken. Rothschild sets the stage for a detailed exploration of extinct and endangered birds, systematically organizing the species into two categories: those with both skeletons and some knowledge about their external features, and those known only from bones or ancient descriptions. He acknowledges the complex causes of extinction, emphasizing the detrimental impact of human activity and the alteration of natural habitats. (This is an automatically generated summary.)