Definitely, especially if you take the hobby seriously. While it’s fine to own just one bird guide if you only want to identify an occasional visitor to your backyard, if you intend to go on bird-watching hikes or vacations, or you just want to familiarize yourself with a wider range of birds than the ones in your immediate area, buying two or more field guides gives you a broader pool of information and different photos or illustrations to compare. Plus, you can buy a small, lightweight guide for traveling or hiking and keep your larger, more comprehensive guide at home.