I have a composition notebook (you know, the old hardback $1.00 kind from high school). The first few pages have places to list all my poultry (date acquired or hatched, identifiers, bands, names, disposition, etc.). After that I just treat it like a journal and make an entry for every day including eggs laid, eggs sold, any unusual occurrences, any treatments or special care one of them might need, and usually weather and any interesting family stuff or activities. At the end of each page I total the number of eggs added to the number of eggs from the previous page as well as the amount of eggs I may have sold or given away. It's not very scientific but it lets me keep up with what's going on.