From page 14 of Better Birding:
Birds, like all animals, have evolved to take the best advantage of their environment. For example, the Northern Harrier glide and swoops low over fields and marshes, periodically flapping and hovering because that enables it to see the small rodents it preys on. Birds like wrens and rails are dark in plumage because they are most often found in dense habitats, the better to blend in with shadows. Species that make their home in the desert are often paler. The intuitive birder keeps these things in mind when looking for a specific species of bird out in the field.
Better Birding: Tips, Tools & Concepts for the Field
George L. Armistead and Brian L. Sullivan
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.