Like most people who like the outdoors, I notice birds. The reason that many non-birders own bird books is that we want to identify and have some understanding of what we are seeing. While I am familiar with some bird sounds- the cranky screech of a disturbed great blue heron and the high pitched call of a hawk (inappropriate, I always felt, for such a fierce bird), I recently realized that I can't really identify any song birds by their calls.
It was Lyanda Lynn Haupt's The Urban Bestiary which called my attention to this deficiency in my nature appreciation. Her book contains tons of information about a variety of animals which may be found in cities and close-in and far-out suburbs, and she writes with wit and eloquence. Chapters are devoted to coyotes, moles, squirrels, bears and others, but her real love seems to be birds. She wants others to share this love by tuning in to all aspects these creatures and advises the useful beginning point of learning the robin's song. "If you do this", she says, "you will be more familiar with birds than 98% of Americans."
Many of the song birds (or perching birds or passerines) have left the area for the season, but some will remain and I'm developing the habit of listening more closely while outdoors. Using all our senses is a way of being in greater touch with the earth, a way of being more alive.