To identify this bird, we first need to take a good look. The overall color is gray-brown with a bit of rust on the sides and top of the head. The pinkish-orange beak is short and triangle-shaped; it’s a seed-eater. We can only see a bit of shin of one leg, and it’s peachy-buff. The eye is dark, with a dark line at the rear. There’s a faint, incomplete white eye-ring. Finally, we can compare the size of the bird to the Russian olive foliage around it and see that it’s quite a bit smaller than a robin.
Clearly this is a “perching bird” or “songbird”—a Passerine. Those are the ones in the latter half of the field guide. While there are a few outliers, a small, brown, seed-eating bird is likely to be a sparrow in the family Emberizidae. We can look there to begin with. If nothing pans out, then it’s time to search elsewhere.
There are 49 species of North American sparrows, in 17 genera. We can quickly eliminate many of them—towhees are larger, many are distinctively striped or have a dark spot in the middle of their breast. Many field guides have an introductory page at the beginning of each family of birds, making it easy to scan the possible species. (Just remember, birds come in two genders, and these pages often only show the females.) Looking at the page of sparrows in my Sibley’s guide, I can easily list those with plain gray-tan breasts: Olive Sparrow, those in the genus Aimophila (Rufous-crowned, Cassin’s, Botteri’s, Bachman’s, and Rufous-winged), Field, Brewer’s, Clay-colored, Chipping, and White-crowned. That’s still a lot. How can we pare down the list?
While vagrants are always a possibility, some of these birds are rare or absent in Colorado. Olive Sparrows barely cross into the U.S. in south Texas, while Botteri’s is only seen there and in far-southern Arizona and New Mexico. Bachman’s lives in the southeastern U.S. and the Rufous-winged is another southern Arizona species. Rufous-crowned Sparrows are more likely in Arizona and New Mexico as well, although possible. Field Sparrows are eastern birds, but found in far-eastern Colorado (and I didn’t tell you where in the state I saw the bird). The rest are more likely.
Now let’s look at the beak. Beaks are usually relatively easy to spot, and the bright yellow-orange of this one is significant. Going down the list of remaining “suspects,” we’re left with four species. Cassin’s and Rufous-winged Sparrows have dull yellowish beaks, not really the bright color we see in the photo. Field and some White-crowned Sparrows have beaks that are just the right color. (Sibley calls this color pink, but pink and orange are hard to distinguish under outdoor lighting, especially at the beginning or end of the day.)
So, is this a “gray adult” Field Sparrow? The coloring is right on both the breast and the head. There’s a white eye-ring, but it’s pretty distinct—a bright white that really stands out. Also, Field Sparrows lack a distinctive brown stripe behind the eye. Hmmm. How about the White-crowned Sparrow?
Clearly, this bird doesn’t have a white crown, so why are we even considering this possibility? Well, not all White-crowned Sparrows have white stripes on their heads. Juvenile (first winter) birds have rusty brown stripes. It’s so easy to forget that not every bird is an adult male! The “first winter, west taiga” individuals have an indistinct, incomplete eye ring, with a brown line extending from the rear of the eye. It’s a better match to our bird.
Identifying immature birds requires a field guide containing that kind of information. Many smaller, incomplete guides, usually those covering a specific state or region, lack pictures of female or young birds. This is why I don’t recommend getting a guide just to Colorado birds, for example. A complete North American field guide is a far better investment. There are a number of excellent guides from which to choose.