The average person can name more bird species than they think, but do we really know what a bird "species" is? This open access book takes up several fascinating aspects of bird life to elucidate this basic concept in biology. From genetic and physiological basics to the phenomena of bird song and bird migration, it analyzes various interactions of birds - with their environment and other birds. Lastly, it shows imminent threats to birds in the Anthropocene, the era of global human impact.