There’s no such thing as a Red-breasted Sparrow. There is, however, a female Eastern Towhee, which when you see it for the first time looks like nothing so much as a Red-Breasted Sparrow. However, if you look in your field guide for “Red-Breasted Sparrow” you won’t find it. If you keep looking a few pages past the sparrows, you’ll normally find the towhees though. But even that may not be enough.
To make matters even more confusing for the novice, the female Eastern Towhee does not in fact have a red breast. It has red flanks. However when you see it in the field, it’s often foraging on the ground so the breast is hidden. Consequently it ends up looking like a cross between an American Robin and a sparrow. Furthermore, the male Eastern Towhee doesn’t really look like a red-breasted sparrow at all, and some of the more basic field guides only show the male. So if you didn’t find Eastern Towhee in your field guide, or didn’t make the connection between the one in the book and the one in the field, you can now Google for “red-breasted sparrow” and find this page. How cool is that?