White Goose on the ice

Another one from Colorado (I'm going to get back out tomorrow!). This is perhaps the best Ross's Goose shot I've ever managed:

Ross's Goose (Chen rossii), Memorial Park, Colorado Springs, El Paso Co, CO  1/1/2014

 

 These guys are close cousins to the larger Snow Goose (C. caerulescens). Cerulean means blue -- an odd name for a white goose! Or it would be, if all Snow Geese were white -- in fact, a significant number of them are a combination of blue-gray on the body and white on the head, a plumage variation often referred to as Blue Goose. Interestingly, Ross's Goose has a similar variation, although blue Ross's Geese are darker with less white on the neck.

 

Both of these species are Arctic breeders, and both of them have been increasing dramatically in recent decades, after declining throughout the 19th century, largely as a result of market hunting. As their populations have waxed and waned, their breeding ranges have repeatedly come into contact and then separated. In recent years, birders have reported increasing numbers of apparent hybrids between the species. Although this could be in part because we're better able to spot the hybrids, it is likely largely a result of their increasing populations coming back into contact. It's even been suggested that the clinal pattern (slowly changing across a species range, in this case from west to east) in proportions of Blue Geese to Snow Geese is the result of two phenotypically distinct populations coming back together and mixing, with no apparent barriers to reproduction. (1)

 

In 1992, Quinn found that mitochondrial DNA patterns in Lesser Snow Geese (C. c. caerulescens) was best explained by two formerly allopatric (non-overlapping) populations merging (2), and in 2002 Weckstein reanalyzed that data, and found evidence for two hybridization events with Ross's Geese as well (3).

 

So, it appears on further examination, that there used to be three distinct populations, Ross's, Blue, and Snow Geese. Two of them (Blue and Snow) clearly didn't differentiate sufficiently to call good species, as they now show no evidence of reproductive barriers. Ross's appear to have interbred with Snows at some point in their previous history (perhaps an explanation for the blue morph in Ross's?) but until now have managed to maintain a separate status. The increasing number of hybrids today, though, hints that this reproductive isolation may be largely a geographic accident -- testing this idea will probably ultimately involve work on the tundra, but in the meantime it's a place where birders can make an interesting contribution to the science, simply by keeping an eye on those white geese. (And thoroughly documenting any suspected hybrids, of course.)

 

(1) Cooke, F., D. T. Parkin, and R. F. Rockwell. 1988. Evidence of former allopatry of the two color phases of Lesser Snow Geese (Chen caerulescens caerulescens). Auk 105:467–479. 

 

(2) Quinn, T.W. (1992), The genetic legacy of Mother Goose– phylogeographic patterns of lesser snow goose Chen caerulescens caerulescens maternal lineages. Molecular Ecology, 1: 105–117. doi: 10.1111/j.1365-294X.1992.tb00162.x

 

(3) Weckstein, Jason D., et al. (2002). "Hybridization and population subdivision within and between Ross's Geese and Lesser Snow Geese: a molecular perspective." The Condor 104 (2): 432-436.