Many birders keep field notebooks in order to study birds in greater detail and to better understand patterns in a species' occurrence. I kept a field notebook regularly during the past ABA Young Birder of the Year Contests, but I've actually deviated from this as of late. I do, however, keep a sketchbook filled with numerous notes for each species. I like to call it "my own, personal field guide".
The basic idea is that I write down the species' name, and then I record every little detail that I have observed with that species. I do not record birds that I have never seen or heard, nor do I note field marks I have never personally observed, even if I have seen that species before. Here's my first example.
I am more confident in my writing abilities than in my illustrating abilities, so there are very few drawings in my book. Of the few that I have, most are in black and white. Some don't even show the whole bird. (Below, I used the flash for these photos, so the black colored pencil shines. This isn't noticeable when the book is in your hand.)
My favorite things to document are bird vocalizations. I have learned a lot about the meaning of a bird's call or song just in our backyard.
Lastly, I have a few pages about general bird behavior and information, such as how birds act during especially windy conditions, how they behave during storms, or information about gulls.
The great thing about keeping this "field guide" is that I can add to it over time. And I know that the more I study birds, the better birder I'll become.