Thursday, January 15, 2009

the answer to your prayers

No, it's not the flock of ducks resting in the lee of the pier at Tiscornia beach (can you find the 5 species present?)

No it's not this picture of a winter bird in Michigan in actual sunlight...

It's that I may finally have an answer to the great cackling/canada debate!!!  OK, so there wasn't all that much debate, just mostly rambling half-baked ideas, but, in the the North American Birds that was in the mailbox yesterday (volume 62 no 3 - with a swallow-tailed gull on the cover), was an article titled "Distribution and Identification of Cackling Goose Subspecies," by Steve Mlodinow, et al.  It also includes parvipes Lesser Canada Goose for obvious reasons.

It includes this encouraging statement:  "By our estimate (based on current knowledge), the chances for a solid identification of a lone photographed bird may be as low as 10-20%."  WOW.  Put those cameras away and study the bird - there's a novel concept.  Since a lot is based on comparative size as well as the head shape (which varies with every twist, turn, flexion, or extension), one photo can be very misleading.

I summarized the differences the article describes and put my editorial after each comparison:

Head shape:
    hutchinsii - "short steep forehead," with "somewhat flat crown, peaking slightly toward rear
    parvipes - some have "short steep forehead," with "sloping crown, with rounded rear-crown"
That seems at best moderately helpful as that degree of variation could be seen in a single bird changing its posture.

Bill shape:
     hutchinsii - "long and narrow in profile," often with a drooped tip.  "Culmen never convex"
     parvipes - "long and slender," often with a rather pointed tip. Culmen sometimes convex.
The morphology of the bill tip seems pretty subtle, but I definitely get a sense of it in many of my photos (of both definite cacklers, as well as some of my problem birds).  "Pointed" may be a bit too strong a description looking at the pics in the magazine of parvipes, a person gets that sense more from views that show the whole bird rather than super-zoomed in head shots.  I don't appreciate culmen shape differences at all in the pictures.

Underpart coloration:
     hutchinsii - "almost complete overlap with other taxa."  "Great majority white or whitish breasted
     parvipes - "majority of birds quite white-breasted"
So the underpart color is basically worthless though perhaps the noticeably darker birds are still suspect (if someone is worried about runty interior Canada geese).

Cheek patch:
     hutchinsii - many to most "show a step-off narrowing of the cheek patch at the level of the eye uncommon in other taxa, excepting parvipes."
Honestly, I have a lot of photos where large interior Canadas show the except same shape of cheek patch as a hutchinsii cackler so I don't think this is helpful even to separate a cackling from a dwarf Canada and am surprised they italicized it as being all that important.  I don't appreciate the differences between the different subspecies of cacklers in their photos either.

Gular stripe (the narrow black line sometimes present in the midline of the throat bisecting the white patch, best seen with the bird feeding facing away from the observer):
     hutchinsii - about 25% show a gular stripe
     parvipes - less than 1% show a gular stripe (except some dark populations from Alaska)
This is interesting.  I've never paid any attention to this detail, but these data would suggest that while a bird without a gular stripe could be either species, a bird with a gular stripe is at least 25x more likely to be a Cackling goose on that feature alone.  Interestingly at least 2 of my problem birds show at least a partial gular stripe in my photos.  Of course just to be contrary, one of their photos of parvipes has a gular stripe.

Wing covert pattern
      hutchinsii - feathers darken distally with a pale terminal band.  Some with duller pattern.
      parvipes - "similar to hutchinsii"
Well, this would explain a good deal of my problems.  Per these authors, this is not a useful feature.

  hutchinsii - "Neck held down at angle when feeding, with little or no loop"
parvipes - displays a "distinct loop in neck as feeds" due to longer neck
I think this must refer to when the bill is actually in the grass as I have a couple pictures of cackling geese with a nice loop in the neck as they're looking down

SO to summarize, the exact head and bill shape is the most useful.  The color of the under- and upper-parts are essentially worthless.  The presence of a gular stripe makes the bird very likely a hutchinsii Richardson's Cackling (though most won't show this feature).  Of my various problem birds, most I now believe are indeed cackling, though I still have a couple I'm not completely convinced on (though these tend to be some of the lower quality photos).

Lastly, anyone actually staying with this entire post likely had no problem with the 5 ducks in the first photo, but just in case the photo quality didn't allow them to pop out (it was still cloudy then and snowing slightly), common goldeneye and bufflehead should be obvious.  A drake common merganser is visible in the middle of the photo about 1/3 of the way down (and a female about 1/3 of the way down all the way on the right, also one just to the drake's left).  Scaup are scattered here and there (which are almost universally greaters on Lake Michigan in winter) and there are a few red-breasted mergs high in the background as well as on the upper left.  A WW scoter was in the flock but I don't believe is in the photo.


1 comment:

col said...

Latest research on goose speciation now available here:

Hope it helps!