Sunday, December 20, 2009

more than chickadees on the CBC

Having your count area include Lake Michigan waterfront always makes for a more entertaining day.

The following is a photographic rendition of what the purple sandipiper that flew past us and landed for less than a minute each on Tiscornia Beach, and the North and South Pier, might have looked like if it had rested on say, an algae covered rock, instead of a concrete pier, and sat for an hour, instead of running around constantly, in nice morning light, rather than heavy overcast. Tim managed a few IDable frames with the SLR, but my digiscoping efforts were futile. The pic is from New Buffalo about 3 years ago.
Next is a Ross's type goose that was in the rivermouth. It would appear to be the same bird as Tim found at LMC about a week ago.

The bird's small size (check out the HEGU behind it), short bill, bluish bill base, and utter lack of a grin patch are apparent. The bird's entirely white plumage in winter (per Pyle II) indicate that the bird is an adult. Is it a Ross's goose though? If so, based on the Audubon Society homepage, it would be the fourth for a Michigan CBC in the last 50 years.
The bill is accepted as the best way to discriminate between Ross's, Snow, and hybrid geese. In the positive column, I think it has as little black on the bill as any Ross's-type I've ever seen in Michigan (admittedly the only place I've seen Ross's geese). On the other hand I've seen Michigan birds that have a lot more actual wartiness to the bill base, though this increases gradually as the birds age. Pyle II gives the culmen (upper bill edge) length of Ross's goose as 35-47mm (41mm on average). Olsen and Larsson give the bill length of Herring gull to be 44-62mm (53mm avg). When I take those measurements on the top photo, the goose's culmen is about 80% of the length of the Herring Gull's bill giving the goose's culmen an estimate of 35-50mm (~43mm), somewhat closer to the measurement Pyle gives for pure Ross's (41mm) than for hybrids (47mm). However, even if those rough measurements could be reliable, other sources (Roberson, 1993 and Trauger et al, 1971) indicate that F1 hybrids between Ross's and the smaller (Lesser) Snow Goose subspecies tend to be small birds, many of whose measurements overlap those of Ross's, and are outside the limits of most Lesser Snow Geese. The Trauger paper is an interesting one, accessible through SORA reporting measurements made on 24 "intermediate" white geese and comparing them with about 150 Ross's and about 130 Snow Geese. Culmen length, tarsus (leg) length, total body length, and flattened wing length in adults all overlapped significantly between (presumed) F1 hybrids and presumed pure Ross's geese and were outside the ranges of Lesser Snow Goose. He found weight to be more intermediate. Sibley describes hybrids as intermediate in size. Pyle's average culmen lengths for Ross's, Snow, and hybrid geese are generally within a millimeter of what Trauger came up with but calculated 95% confidence intervals where the culmen length of hybrids overlapped both Lesser Snow and Ross's.
Which brings us to the all-important, unmeasureable, and subject-to-position-and-angle bill interface. Pyle describes this as "malar feathering extending distally to the forehead feathers," in Snow and "malar feathering not extending distally to the forehead feathers in Ross's." Now we get to parse words Bill Clinton style and try to define "malar." Pyle claims to follow Sibley's definition of "malar" even though Sibley in one of the articles referenced by Pyle (Sibley, 2001) explains why Pyle doesn't follow Sibley's definition of "malar." Pyle defines "malar" as "pertaining to the feather group at the posterior end of the gape and extending back to the neck" whereas Sibley defines it as "originating at the base of the lower mandible below the gape and extending back along the sides of the lower jaw." If Pyle follows Sibley's definition then neither bird has malar feathers extending distally to the forehead, for a photo, here's one I posted in March of a bunch of Snows with a Ross's-type. The forehead feathers are more distal to the malar in every bird including the Snows. Pyle must mean the cheek feathering extends forward in Snow and not in Ross's. Here's where a picture is worth a thousand words. Unfortunately Sibley's picture of a hybrid is of a small-billed bird (admittedly with a much more prominent grinpatch than the absent one on this bird) with the upper base of the bill hooking back a little towards the eye, essentially what this bird's does. On the other hand, so do a decent number of the birds on Robert Royse's page photographed in New Mexico.

Interestingly, Trauger concluded in 1971 that intermediate birds comprised about 5% of the Ross's population and felt that it was possible that "pure" Ross's may in the future cease to exist as the population becomes swamped with Lesser Snow Goose genes. As an example, he reported data suggesting that Black Ducks were outnumbered by Mallards by about 6:1 in North America, whereas Ross's goose was outnumbered by about 27:1 making hybrid pairings even more likely. I'm not sure that these Arctic geese aren't any different from the Arctic gulls; who knows where Kumlien's stops and Thayers begins in many individuals, the same may be becoming true for these.
At least in a week they won't need to be reviewed in Michigan any more, right? Right? Hmmmm...
References
Sibley, DA. "What is the Malar?" Birding, 32: 448-451. Oct 2000.
Trauger DL, A Dzubin, and JP Ryder. "White Geese Intermediate between Ross's Geese and Lesser Snow Geese." Auk, 88: 856-875. Oct 1971

2 comments:

Cathy Carroll said...

Amazing analysis, Matt. Again, thanks for taking the time to do it.

Matthew said...

BirdLife Malta – Spring Watch
11th – 30th April 2010

Spring is almost upon us, and once again birds returning to breed in Europe passing over Malta will risk persecution from illegal hunters and trappers.

BirdLife Malta will this year be running the first ever 3 week long Spring Watch Camp. The camp will monitor bird migration, maintain a strong presence in the countryside to deter the illegal killing and trapping of wild birds, and report illegalities to the police.

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If the government opens a spring hunting season, this year’s Spring Watch camp will be vital in documenting and revealing the extent and impact of hunting in spring.

Find out how you can help - please click here www.birdlifemalta.org/conservation/springwatch2010 or contact springwatch@birdlifemalta.org

If you know of anyone else who may be interested in helping fight this worthy cause please feel free to forward them these details.

Kind regards,

Nix Rust
Intern
EVS / LIFE Yelkouan Shearwater Project

BirdLife Malta
57/28 Triq Abate Rigord, Ta' Xbiex XBX1120
Tel: +356 21347646
Fax: +356 21343239
www.birdlifemalta.org