From page 18 of Better Birding:
Songs differ from “calls” in that songs are longer, often more complex vocalizations. In most birds (though not all) songs are given by males, usually on the breeding territory, and serve the dual purpose of attracting females and driving away rival males. In some species both males and females sing. When females sing, it is less often about attracting males (though it probably does help strengthen pair bonds) and usually more about territorial defense against neighboring pairs. Often, the species in which females sing are less migratory or resident, so territories require year-round defense from rivals.
Better Birding reveals the techniques expert birders use to identify a wide array of bird species in the field—quickly and easily. Featuring hundreds of stunning photos and composite plates throughout, this book simplifies identification by organizing the birds you see into groupings and offering strategies specifically tailored to each group. Skill building focuses not just on traditional elements such as plumage, but also on creating a context around each bird, including habitat, behavior, and taxonomy—parts so integral to every bird’s identity but often glossed over by typical field guides. Critical background information is provided for each group, enabling you to approach bird identification with a wide-angle view, using your eyes, brain, and binoculars more strategically, resulting in a more organized approach to learning birds.