Texas April 2007, courtesy Roger Foster
The use of Latin names for species provides birders with a universal language that ensures no matter where you are in the world two observers should be able to establish a name for the bird you are looking at regardless of the other language barriers they have.
Despite this there are still areas where terms can be used to describe differences amongst the same species that can create confusion.
Let's take Snow Goose Anser caerulescens most birders know it occurs in three forms, phases morphs and they are white snow and dark blue and (phew) intermediate. Clear, mmm.
If we look at the history in recent birding literature we might get somewhere. In the old days before I was even a twinkle in an eye the non-white version was often referred to as "Blue Goose", Peterson, Mountfort & Hollom in their 'Field Guide to the Birds of Britain' referred to it thus but stated it was a 'colour phase of this species'. Moving on in time a little to 1977 and Cramp & Simmons in 'Birds of the Western Palearctic' used the terms 'white morph' and 'dark morph' throughout their definitive text, with two single bracketed references to 'Blue Snow Goose' also mentioned. By 1988 Madge & Burn in 'Wildfowl An Identification Guide..' had reverted to using the term phase and wrote 'occurs in two distinct colour-phases, a white or snow phase and a dark or blue phase' they then adopted the terms 'snow phase and blue phase' in the remainder of their text. Fast forward to 1999 and Svensson & Grant's treatment in 'Collins Bird Guide' probably the most widely used field guide in Britain and Europe since, refers to 'two morphs' a 'white morph' and a 'dark morph' For once however they shed some light on the term 'morph' in the terminology section defining it as 'variant of certain appearance within a species, not geographically defined (cf.subspecies). The same as formerly often used phase'
So why did 'morph' replace 'phase' and why do many in the UK now insist on using 'blue' rather than the more accurate 'dark'?
Phase has multiple meanings, one of which implies that the condition referred to is temporary and will move to another 'phase'. Other observers have speculated that during the 20th century as more amateurs came into ornithology (and it became birding) that these meanings were confused and that a new term was sought that didn't imply a time related change, hence the introduction of morph. So morph whilst the new kid on the block relatively speaking being the term used in BWP and by Svensson & Grant would seem to be the way forward.
Which leaves us with 'blue' or 'dark'. Blue in conjunction with morph appears to have been adopted fairly recently in the UK, I can find no reference in BB so it is probably the final legacy of the old 'Blue Goose' name. There are not many sources where I can find extensive use of the term 'blue morph'. One is in the, otherwise above criticism, electronic BWPi where the images have been labelled 'blue morph' in contradiction to the Handbook text that fails to use this term even once. Taking a glance over the pond where Snow Goose is actually a wild breeding and resident species the definitive 'Birds of North America Online' prepared jointly by Cornell Lab of Ornithology and American Ornithologists' Union make reference to both 'dark' and 'blue' morph and current US birding guru David Sibley uses the term 'dark' morph in his Field Guide to Birds of North America, simply including a passing reference to the old Blue Goose name. There are many other authors who use multiple terms within the same text without comment (Lewington, Alstrom et al) suggesting a degree of comfort with the interchangeability of the terms.
If we are content to use the new term 'morph' rather than the older 'phase', then is there any reason not to adopt the term 'dark' in preference to 'blue' it would at least ensure that we were in synch with the major texts currently available on the species. The more technically accurate description might be 'dark bodied morph' though this is unlikely to catch on in the sound byte era as it's less punchy.
You can wake up now, I blame the blue dark nights.