Size
Once you have assigned a bird to its correct group, size can be a clue to its actual species. Be aware, though, that size can be difficult to determine in the field, especially under poor lighting conditions or at a distance. Size comparisons are most useful when the unknown bird is seen side-by-side with a familiar species. In the absence of that, you can use the sizes of well-known birds, such as the House Sparrow, American Robin, and American Crow, as references when trying to identify an unfamiliar bird.
Woodpeckers:
A crow-sized woodpecker would be a Pileated, but one the size of a sparrow might be a Downy Woodpecker (or a Ladder-backed Woodpecker in the Southwest).
Confusing Coloration: A yellow-and-black finch smaller than a sparrow
is probably an American Goldfinch. Evening Grosbeaks have similar
colors and patterns, but are almost robin-sized.
In-between Sizes:
Sometimes you need two reference birds for comparison. A Cedar Waxwing is bigger than a sparrow but smaller than a robin. A Blue Jay is larger than a robin but smaller than a crow.