"The Sibley Guide to Birds" mentions that under exceptional field conditions, one might be able to see the more graduated shape of the tail feathers in Black-capped Gnatcatchers (BCGN) as viewed from below. By comparison, Blue-gray Gnatcatchers (BGGN) show much less difference between the lengths of each individual tail feather. Specifically, he illustrates this with closed-position undertails. But what about uppertails? This is far too much detail to expect from a standard field guide, so I dug out more detailed references when I got curious about a recent photo I took.
What's even better than studying tail shape under brief but exceptional field conditions? A photo! While not the sharpest image in the world, I was lucky enough to catch a Black-capped Gnatcatcher in mid tail-swing as it was moving low through some mesquite (see the entry for Patagonia from 1/25/2009). I borrowed an image from Peter Pyle's "Identification Guide to North American Birds, Part I" (Slate Creek Press, 1997) to demonstrate, since illustrator Steve N.G. Howell drew such an excellent detailed comparison of gnatcatcher tail morphology and patterns. It's a bit pricey, but every advanced birder and serious ornithologist should own a copy! It's worth every penny if you want to scrutinize obscure traits that are probably not visible in the field...
Graduated tail shape tends to be most easily observed from below, since the outermost tail feathers fold underneath of the innermost tail feathers. An evaluation of the tail shape from above, it stands to reason, would be just as valid so long as the tail is spread so you can see the length of each feather compared to the next. No, you probably can't use this in the field at a quick glance, but catch a photo at exactly the right split second and suddenly it becomes useful.
Please excuse the following images from the Pyle guide being flipped; I wanted to compare the patterns of coloration on the tail as well, and my picture is most in-focus on the right hand side (if you ignore those branches).
BGGN tail morphology and pattern, page 378 in above-referenced book (Pyle 1997).
BCGN near Patagonia, 1/25/2009 (Yerger) compared with BCGN tail morphology and pattern, page 384 in above-referenced book (Pyle 1997).
The outermost tail feather (r6) in the photographed bird is clearly much shorter than the second feather from the outside (r5). I feel that you can also draw some very clear conclusions about the bird's ID by comparing the pattern of white in the outermost tail feathers, which is why I have the images flipped. But Pyle's Guide doesn't specifically mention this and I can't find all of the references I need, so I won't speculate about how useful it is!
Hope some folks find that interesting,