Bird of the Day: Nelson's Sharp-tailed Sparrow (again)

Some questions have popped up on cayugabirds-l recently regarding the Nelson's Sharp-tailed Sparrows seen in the Cayuga Lake Basin (now departed). I thought I'd add the constructive examination of subspecies to my original post on Nelson's.

It started when Dave Nutter believed he had identified an Atlantic race individual (Ammodramus nelsoni subvirgatus):

During my brief in-focus at-rest partial view, the NELSON'S SHARP-TAILED SPARROW showed the blurry gray streaks on the buff-orange breast indicating an individual of the Atlantic race.

To which Ken Rosenburg responded:

There are actually 3 races of the Nelson's Sharp-tailed Sparrow, and the dullest Atlantic race is not known as a migrant inland. Of the two other races, altera, which breeds only in marshes along James Bay in northern Ontario and Quebec, is duller and intermediate between the Atlantic and the brightest northern prairies race, nelsoni. Based on a fairly nice series of specimens from the old Renwick marshes (of which Hogs Hole is the remaining remnant) from the early 1900s in the Cornell Vertebrates collection, both the inland races should occur here in roughly equal frequency. I believe I have seen birds of both races there in past years, although certain identification to subspecies is never certain. Your dull streaky bird was probably altera - Note that Sibley does not specifically picture this bird, although he shows the 3 disjunct range areas, and it is not clear whether his "interior" bird is a true "nelsoni" or some artist's conglomerate of what interior birds look like.

Ken is quite right, of course. The three subspecies are:

A. n. nelsoni (Nelson's Sharp-tail) - breeding in the Canadian prairie
A. n. alterus (James Bay Sharp-tail) - breeding along the shores of James Bay
A. n. subvirgatus (Acadian Shap-tail) - breeding on the Atlantic coast south to Maine

Neither Rising's A Guide to the Identification and Natural History of the Sparrows of the United States and Canada, nor The Sibley Guide to Birds illustrates all three races. Sibley illustrates an 'interior' bird and an Atlantic bird (subvirgatus), while Beadle (illustrator of Rising's book) paints a nelsoni and a subvirgatus.

Click to view large. (Source: David Sibley. The Sibley Guide to Birds. 2000)

(Source: James D. Rising. A Guide to the Identification and Natural History of the Sparrows of the United States and Canada. 1996)

Sibley does not make mention of the two interior subspecies at all, lumping them in description. Rising calls nelsoni and alterus 'virtually identical', stating that there is 'much confusion' about their identification and, basically, that alterus 'cannot be told from A. n. nelsoni in the field'.

Tom Johnson enters the discussion of the Ithaca birds:

I think it is very unlikely that a Nelson's Sharp-tailed Sparrow (Ammodramus nelsoni) in Ithaca would be an Atlantic bird. Atlantic Nelson's Sharp-tails (subspecies subvirgatus) are fairly strictly coastal beasts. It seems based on a combination of field observations, photos, and the (modest) Cornell University Museum of Vertebrates series of sharp-tailed sparrow specimens, that we see mostly Interior Nelson's Sharp-tailed Sparrows, comprised of two subspecies - Prairie (nelsoni) and James Bay (alterus). Most of the birds that I've seen and that have been collected in Ithaca over the years appear to be alterus. The published information on the subspecific identification of Nelson's Sharp-tailed Sparrows is rather unclear and conflicting, but it seems that the greatest difference lies between A. n. nelsoni and A. n. alterus especially in terms of dorsum coloration/ contrast (A. n. nelsoni generally having considerably more contrast.) Then comes the problem of separating A. n. alterus from A. n. subvirgatus. This appears to be a bit trickier matter, as the two subspecies are fairly similar in many regards, but probably something we don't need to worry much about in the Finger Lakes region.

I've compiled a series of captioned photos of the CUMV skins and some recent photos of Nelson's Sharp-tailed Sparrows from the field here: http://picasaweb.google.com/jaegermaster/comparisonofsharptailedsparrows

It should be noted that the CUMV collection appears to be highly biased towards A. n. alterus, with only a very few apparent A. n. nelsoni present in the Nelson's series. Hopefully I'll be adding some photos of a larger number of A. n. nelsoni specimens in the near future.

Dave replies (I particularly like his closing comment):

Based on your comments about possible inutility of breast plumage for subspecific ID, photos 3 & 4 in the series, and my limited view, I could not identify the Nelson's Sharp-tailed Sparrow to subspecies at all. I am satisfied with a simple ID to species (as currently defined), and with the explanation for the riddle of why there would seem to be an off-track Atlantic Nelson's here. Perhaps Sibley's guide, while very good, is not God.

Kevin McGowan adds:

I'm trying to remember what Sibley said when he was here looking at our sharp-tailed sparrows while working on his book. I think he thought that everything collected from Ithaca was alterus, despite other, older IDs as nelsoni. I believe that he wasn't convinced that the two subspecies could be distinguished with any accuracy, and that may be why he didn't try to illustrate them in the book. A couple of birds and small series can lead one to think some identifications are easy, but with large series and lots of experience, things often get murkier.

I believe Sibley gives the explanation for why he didn't use subspecies names in the front of his book.

Tom replies:

It should be noted that in regard to the skins used as comparison in the identification gallery linked to previously, the only birds compared (and presented as racially distinct in the photos) were taken from the breeding grounds of their respective races (i.e. alterus collected in James Bay and nelsoni collected in Prairie Canada), so the particular identification of those individuals is not really in question. Indeed, all the specimens in the CUMV collection of birds historically collected in Ithaca have dull, low-contrast scapulars and tertials and seem consistent with alterus. That is why the bright, high contrast individual labeled as Bird #1 (and consistent with A. n. nelsoni) in the gallery (http://picasaweb.google.com/jaegermaster/ComparisonOfSharpTailedSparrows) was of interest to Mike Harvey and I when we observed it last weekend at Marten's Tract with the Cayuga Bird Club group. Since the CUMV collection has a particular dearth of A. n. nelsoni specimens, I asked a friend at Louisiana State University to contribute some additional known A. n. nelsoni photographs from collection trays. Early this morning, I added a group of photos of specimens of A. n. nelsoni from the LSU Museum of Science collections that were sent to me by Devin Bosler at LSU (I believe these were collected on the Gulf Coast instead of Prairie Canada, but still are representative of A. n. nelsoni, which is far and away the main subspecies that winters along the Gulf Coast).

The reason I compiled the gallery was to investigate the differentiation of A. n. nelsoni (Prairie) and A. n. alterus (James Bay) in the field, a quandary that I feel seems to be addressed somewhat unsatisfactorily in recent publications on sparrow identification (especially in terms of reliance on seemingly variable facial and breast patterns). Other suggestions/ outside information is welcome. I would be especially interested in seeing other folks' photos of Nelson's Sharp-tailed Sparrows from the interior northeast on fall migration (and especially central NY and Pennsylvania) - feel free to email some to me if you would be interested in sharing.

Kevin:

Tom is to be congratulated on putting up such nice images of specimens. It's an interesting challenge to identify these subspecies. Nelsoni nelsoni from n. subvirgatus is pretty straight-forward, but alterus is problematic on both ends.

I found a couple of references that I had been looking for on the topic. David Sibley wrote an article in Birding on the topic, (1996, vol 38: 196-208), and in it he cautions about identifying alterus. He states that all Ithaca specimens looked to be alterus to him (although some had been called nelsoni by others).

A recently published paper in the Wilson Journal of Ornithology addresses this problem too. (Greenlaw and Woolfenden. 2007. Wintering distributions and migration of Saltmarsh and Nelson's sharp-tailed sparrows. WJO 119: 361-377.) They looked at over 1,000 specimens (most of those existing) and came to the conclusion that both nelsoni and alterus should migrate through our area. I believe they accepted at least one Cornell specimen from Ithaca as nelsoni. They give a key to identifying the subspecies in an appendix, stressing the flank streaking, back pattern, auricular patch, crown, and bill length. They caution "We recognize that A. c. caudacutus and A. n. nelsoni are especially variable in some plumage characters, and that differentiation of ... A. n. nelsoni versus A. n. alterus is particularly problematic. As a conservative practice, an investigator should accept only clear-cut examples of ... A. n. alterus in making racial assignments." They also say that the problem will be with bright, strongly marked alterus being assigned as nelsoni. Using their criteria, the two authors independently assigned subspecies to the specimens (ignoring what others had called them) and disagreed on 9%.

Tom's specimen photos, and comparison with field shots, are incredibly helpful. Go Tom!

Let's take a look at my bird for comparison:


One key difference between nelsoni and alterus as Tom points out in this photo is the difference in contrast and bold patterning on the back and scapulars. It almost looks as if my bird is high-contrast, but I can't say for sure. Tom points out differences in the crown patterns in this photo, but details are too lacking to tell for sure what the pattern is in my bird. Basically I can't tell for sure what my bird is, but it does seem similar to this very bold bird that Tom observed. I will stop short of making any sort of claim about my bird, though. I only have these poor photos and no better observations in the field.

The take-home messages here are:
- the difficulty of confident subspecies designations
- the utility of museum collections
- we need many more constructive conversations such as these on our birding listserves!