In a quite useful five-part series, Steve N.G. Howell explains how field notes work and how and what you might want to record, either in the field or in the motel at the end of the day. He saves the best advice for the last installment:
In conclusion, your notes are your notes. Write what you want, but in later years you’ll only have yourself to blame if your old notes don’t contain the information you find you want. If you have time, write it all down. If you don’t, pick and choose. But whatever you do, or don’t do, the main thing is to enjoy birding.