More problematic are the "Gray-headed" group which combines the gray head and sides of the slate-colored group and the brownish back of an Oregon type and the taxonomically suspect "Pink-sided" group which was lumped with Oregon Junco long before 1973.  The characters that are supposed to distinguish "pink-sided" from Oregon forms are largely subjective.  Broad differences can be seen when large numbers of individuals are examined over the range of the complex, but overlap in characters between alleged Pink-sided individuals and pale female Oregon Juncos are so overwhelming that any effort at a confident diagnosis for a given individual gets lost in the statistical noise of normal variation. 

It's very likely that the average junco does not make the distinction either.  When the ice sheet covering the continent melted at the end of the last ice age, the geographic barrier to reproduction was gone and juncos started mixing it up once again.  There was not enough "alone time" for these populations to develop strong morphological or behavioral barriers to reproduction.  Speciation takes time and animals with wings need more time than animals stuck on the ground.

Recognizable forms within a species complex add richness to the ecological story of life on Earth.  The sometimes subtle differences we see in morphology and behavior connect the organism to its local habitat.

And the ubiquitous junco is an obvious example of how many things we still don't understand about the boundaries between diagnosable forms and species, and of the difficulty we have in describing what ecological diversity really means.

References

Beadle, D and J. Rising. 2002. Sparrows of the United States and Canada: the photographic guide. Academic Press, San Diego.

Nolan, Jr., V., E. D. Ketterson, D. A. Cristol, C. M. Rogers, E. D. Clotfelter, R. C. Titus, S. J. Schoech and E. Snajdr. 2002. Dark-eyed Junco (Junco hyemalis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/716doi:10.2173/bna.716

Pyle, P. 1997. Identification Guide to North American Birds. Slate Creek Press, Bolinas CA.

As I have said before, we live in the Age of Field Guides.  And no one should be surprised that as the market swells, the list of critters that get the field guide treatment has also expanded.  I thought I might mention a few new field guides I've recently acquired and when I say "new", I mean new to me.  Some of them have been out for a while.

I'll start with the book that had all the larophiles a-twitter: Gulls of the Americas (Howell and Dunn 2007).  This is a book only a gull-watcher could love.  If you are into gulls, you already own this book and will probably disagree with anything negative I say.  If you're not into gulls, you won't solve any of your anxieties by getting this guide.  It's over-sized which means it's not something one would be taking into the field routinely.  It is laid out with an extensive section of photos in the front and separate species accounts in the back and is very difficult to navigate through.  There is simply no graceful way to find the photos of any particular species.  Most of the photos are excellent, but I say this as someone who already has a good idea of what he's looking at.  This is not a book you buy to become a gull watcher.  It's a book you get after you pretty much know what you're doing.

Identification Guide to North American Birds, part II (Pyle 2008) - This is another long awaited guide and another book that is pretty much only useful to specialists.  It picks up where part I left off, detailing the measurements, aging and sexual differences in ducks, seabirds and raptors (among other species).  If you're a bird bander, work at the rehab hospital or you're someone who spends time looking at dead stuff on the beach, this is the book for you.  Others may find it less compelling.

Land Snails of British Columbia (Forsyth 2004) – At last, a decent field guide on snails and slugs.  More color photos would have been useful and I'm sure it's missing a few species from more southern latitudes, but this is a must for would-be gastropodists.

Mushrooms of the Pacific Northwest (Trudell and Ammirati 2009) – This guide follows the successful formula of recent Pacific Northwest photoguides produced by Timber Press.  I am not a mushroom guy.  What I need is a guide with lots of photos so I can get close to the ID of the mushroom I'm looking at, but have no intention of putting in my mouth.  The guide is divided based on morphological types (eg. gilled, boletes, corals, etc.).  The photos are well done and there are several species per page.  I'm sure that real mushroom aficionados have books they will tout as being better, but as a straight identification guide, I feel far more comfortable with the layout of this book than others I own.

Dragonflies and Damselflies of the West (Paulson 2009) – This is another specialist's guide.  It is filled with detailed species accounts for the region which include the most recent information on range (with maps).  There are good illustrations designed to help sort out species that require the close inspection of reproductive parts to identify.  The photographs are often disappointing, however, and not always helpful as identification tools, but I suspect this is partly an artifact of the d-flier discipline which has only recently opened itself up to non-capture observational identification as a means of interacting with odonata.

Field Guide to Beetles of California (Evans and Hogue 2006) – This is part of the California Natural History Guides series, but unlike many of those which were illustrated guides, this is a photoguide.  As suggested by the possibly apocryphal quote attributed to J. B. S. Haldane, the creator would seem to have had "an inordinate fondness for beetles."  There are a lot of beetles and as a result, there is simply no way any field guide, even a regional one, can treat the subject of beetles completely.  This guide gives it a go, however, with a full 25% of the book given over to very good photos that will, in most cases get the field coleopterist to family if not genus.  Written treatments are primarily at the family level.  Representative species are discussed, though obviously many have been left out.  But in a world where beetle guides are few and far between, this is a real find.