North American Birds

VOLUME 68 NO4 2015

A Quarterly Journal of Ornithological Record Published by the American Birding Association. The mission of the journal is to provide a complete overview of the changing panorama of our continent’s birdlife.

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461 V o l u m e 6 8 ( 2 0 1 5 ) • N u m b e r 4 w e s t e r n m a r s h w r e n i n n o va s c ot i a rufous in fresh basic plumage. Interior Western birds have brown crowns with blackish borders, and the supercilium tends to be sandy in tone and rather indis- tinct compared to the stark supercilium of Eastern and Coastal Pacifc birds. • Eastern group subspecies have plumages much brighter and richer in dark tones (rufous, black) than Interior West birds and tend to be a bit more richly colored than Coastal Pacifc birds. Like Coastal Pacifc birds, rufous tones are strongest in uppertail coverts and scapulars, with fanks also washed rufous. Eastern birds have blackish crowns, though many show a narrow central strip of brownish. The blackish crown and rufous auriculars make the white supercilium appear es- pecially stark. One outlier, the distinctive Worthington's Marsh Wren (subspecies griseus) that nests in the coastal Southeast, is quite grayish overall (Brewster 1893). The Coastal Pacifc group and the Inte- rior Western group are thought to form a different evolutionary unit from the Eastern group: Kroodsma (1989) argues that differ- ing songs and singing behaviors act as iso- lating mechanisms. Where Interior Western and Eastern groups come into contact—as in Saskatchewan marshes, where iliacus and laingi co-occur—members of these Eastern and Interior Western groups appear to mate assortatively (Kroodsma and Verner 2014). There has not yet been a formal proposal to the American Ornithologists' Union to split these groups into two species. Owing to both variation in, and intergra- dation between, some subspecies, identi - fcation of extralimital Marsh Wrens to the level of subspecies (or subspecies group), even birds in fresh plumage, might not be possible with certainty; nevertheless, docu- mentation of potential vagrants is of inter- est, and the strongly migratory taxa are the most likely to appear as vagrants. In general, Coastal Pacifc and Eastern birds that breed along the coast are resident, whereas most Marsh Wrens in the continent's interior win - ter south of their breeding grounds (Pyle 1997). With regard to the age of the bird, a bird in juvenal plumage is ruled out, as that plum- age is held only through August (Pyle 1997) and would not be seen in mid-October. In Marsh Wren, juvenal plumage is duller and less strongly patterned than formative or defnitive basic, and a dispersing juvenile of Eastern or Coastal Pacifc groups could pose a very diffcult identifcation problem if found well away from its natal areas. Marsh Wrens have an incomplete preformative molt, completed on the breeding grounds not later than mid-October. This molt in - cludes most or all of the body feathers as well as the rectrices (Pyle 1997). The formative plumage is described as essentially the same as the defnitive basic, except for the pres- ence of molt limits in the wings (Kroodsma and Verner 2014). The Miners Marsh wren showed an apparent molt limit between ter- tials and secondaries and thus appears to be in formative plumage. Marsh Wrens of both Coastal Pacifc and Eastern groups have strong rufous tones, show strong plumage contrasts overall, and have a prominent white supercilium that contrasts with a blackish crown. The Miners Marsh wren's rather plain appearance over- all, lacking extensive black in the tail and crown, was strikingly different from Eastern birds. Thus it seems unlikely that the Nova Scotia birds (Figures 2-4) are of the East- ern group (though an Eastern group bird in dilute plumage is conceivable) and are certainly not Worthington's Marsh Wrens. The Coastal Pacifc subspecies aestuari- nus (includes deserticola of other authors), browningi, clarkae, and paludicola are mostly resident in the breeding range and seem un- likely to appear as vagrants to Nova Scotia and are also rather richly colored, more like Eastern group birds. Among the migratory western taxa, then, one is left to consider the Interior Western group subspecies: pul- Figure 1. Breeding ranges ( blue for resident, purple for summer only) of the three subspecies currently included in the Interior Western group of Marsh Wren, based on Roberson (2015). Map by Eduardo Rother. Interior West pulverius plesius laingi United states canada v e r

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