Tuesday, October 30, 2007

Sharp-tailed Sparrow Species Limits

Blogging on Peer-Reviewed Research
Following up on my previous posts on Nelson’s Sharp-tailed Sparrows and the difficulties in their identification to subspecies, I looked into the systematics and taxonomy of the Sharp-tailed Sparrow group. Nelson’s Sharp-tailed Sparrow (consisting of Ammodramus nelsoni nelsoni, A. n. alterus, and A. n. subvirgatus) and Saltmarsh Sharp-tailed Sparrow (A. caudacutus caudacutus and A. c. diversus) were split from the former lump species Sharp-tailed Sparrow (A. caudacutus) in 1995 by the 40th Supplement to the AOU Check-list of North American Birds (Auk 112(3): 819-830. 1995) with the following reasoning (pg 826):

Split Ammodramus caudacutus to A. caudacutus and A. nelsoni, following Greenlaw (1993), who found that the two groups of populations differ in song, morphology and habitat, with limited interbreeding at a secondary contact zone in southern Maine. Although Rising and Avise (1993) suggested retaining these two taxa as subspecies of caudacutus, they do not interbreed freely and should be ranked at the species level.

The two major papers leading to this split are:

Jon S. Greenlaw. 1993. Behavioral and morphological diversification in Sharp-tailed Sparrows (Ammodramus caudacutus) of the Atlantic coast. The Auk. 110(2): 286-303.

And

James D. Rising and John C. Avise. 1993. Application of genealogical-concordance principles to the taxonomy and evolutionary history of the sharp-tailed sparrow (Ammodramus caudacutus). The Auk. 110(4): 844-856.

Before diving into the nuts and bolts of these two papers and the justification for the split, lets review the subspecies (I’ll refer to the split versions for recognition):

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 Sharp-tail) - breeding on the Atlantic coast south to Maine
A. c. caudacutus (Eastern Sharp-tail) – southern Maine to New Jersey
A. c. diversus (Southern Sharp-tail) – southern New Jersey to Virginia

I crudely chopped together this figure from these sources: Rising’s Sparrow Guide and BNA Account. See also the figures in my previous post.

See also these specimen comparisons from Tom Johnson: dorsal and ventral comparisons.

Although they were the later of the two publications, I’ll start with Rising and Avise (1993). Rising and Avise (1993) followed up on the discovery in an earlier paper on the systematics of Ammodramus (Zink and Avise 1990) that there were two distinct mitochondrial genotypes in A. caudacutus (pre-split). They collected a series of 220 specimens from all populations and analyzed the mtDNA of 107 of those with restriction endonucleases (a technique now replaced by sequencing; this paper is old enough that they refer to this as “phylogeography” in quotes). The results indicated two distinct, strongly supported clades of Sharp-tails. The clades divided the populations into what are now Nelson’s and Saltmarsh, with two exceptions: a locale in Maine where the mtDNA from both species occurred with equal frequency, and a locale in Massachusetts where about one bird in five had some Nelson’s mtDNA.

The authors examined the phenotypes of the introgressed individuals:

As you can see, all birds with southern (Saltmarsh) mtDNA had southern plumage, while some of the birds with northern (Nelson’s) mtDNA had southern plumage. mtDNA is passed down by female lineage, so from this discordance in the plumage vs. genetics, the authors conclude that there is differential mate preference in the contact zone: southern females prefer southern males, while northern females show no selectivity. This would allow the introgression of southern males and their plumage characters into the northern mtDNA ‘population’. The authors hypothesize that this is due to larger southern males outcompeting smaller northern males.

In sum, the authors discover a distinct mtDNA split, a distinct division in morphological characters examined, and a contact zone with asymmetrical hybridization. The authors take the taxonomically conservative stance of maintaining one Sharp-tail species, but giving the Nelson’s and Saltmarsh divisions subspecies rank, subsuming the previous subspecific variation. They argue against splitting until stronger evidence of intrinsic reproductive barriers is found.

One pre-zygotic (before mating) reproductive barrier is behavioral differences. Greenlaw (1993) added to the study of Sharp-tails with behavior and morphological examinations across the contact zone. Saltmarsh subspecies sing what he describes as a ‘complex whisper song’ with no display, whereas Nelson’s sing a different song and an accompanying flight display.



His analysis of behavior and morphology characters showed no gradient of change northward up the coast, instead representing an abrupt shift in the same contact zone in Maine (Popham Beach) as sampled in Rising and Avise (1993). Even more interesting, the two types coexist together in that contact zone. The same stepwise transition occurs in habitat type: tidal saltwater marshes in the south to brackish less-tidal marshes in the north.

Combining these two studies, we get populations distinct in: genetics, plumage, morphology, behavior, voice, and habitat, - and coexisting with a small level of introgression in a narrow band in Maine. The exact barriers to reproduction between the two forms are not known, but these studies suggest it is a form of behavioral isolation involving different song types and displays. Indeed, the sympatric presence of the two forms with some asymmetric gene flow is evidence of assortative mating – an indication of species-level differences. Putting this evidence together, despite a more detailed study of the contact zone lacking, was enough for the AOU to consider the two as separate species.

References:

40th Supplement to the AOU Check-list of North American Birds (Auk 112(3): 819-830. 1995)

Jon S. Greenlaw. 1993. Behavioral and morphological diversification in Sharp-tailed Sparrows (Ammodramus caudacutus) of the Atlantic coast. The Auk. 110(2): 286-303.

Greenlaw, Jon S., and James D. Rising. 1994 . Saltmarsh Sharp-tailed Sparrow (Ammodramus caudacutus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu.proxy.library.cornell.edu:2048/bna/species/112 doi:bna.112

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

James D. Rising and John C. Avise. 1993. Application of genealogical-concordance principles to the taxonomy and evolutionary history of the sharp-tailed sparrow (Ammodramus caudacutus). The Auk. 110(4): 844-856.

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