This book was a disappointment. I'm very interested in the topic but much of the book reads like a scientific paper. There is a lot of good, interesting information but, unless you're particularly interested in Carolina parakeets, as I am, suffering through the writing is probably not worth it.
The crux of the book is his postulation that the parakeets diet of cockleburs made them toxic to most predators thus their bright feathers, gregariousness and ability to "sleep" at night. These abilities were no match against humans who killed them with ease. One shot took out droves and then the survivors would gather around the fallen, making shooting the rest even easier.
Because cockleburs grow around human dwellings the parakeet was drawn to areas where they came in contact with livestock and other sources of exotic diseases, conceivably nail in the coffin for the parakeet.