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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Figure 2. Common Scoter, Crescent City, California, 25 January 2015. The shape of the yellow-orange patch on the culmen is distinctive for Common Scoter: it occupies the center of the culmen and includes the area around the nostrils. The shape and length of the tail are also evident in this image. Photograph by William A. Bouton. 451 V o l u m e 6 8 ( 2 0 1 5 ) • N u m b e r 4 F I R S T N O R T H A M E R I C A N R E C O R D O F C O M M O N S C OT E R diately compared them to photographs of Black and Common Scoters posted online. Svetich replied that the Common Scoter images he found were perfect matches with the Crescent City scoter. He also pointed out a potentially useful mark evident in photographs online, namely the prominent yellow-orange orbital skin of adult male Common Scoter, a feature lacking in pho- tographs of Black Scoter he found online. Encouraged by Svetich, Bouton then posted the observation to the internet (CALBIRDS email group), with links to his photographs on Flickr. Within several hours, with experts unanimous in their as- sessments, it was clear that the identifca- tion of the bird as an adult male Common Scoter was correct. Birders from many states were able to study the bird at Crescent City 1-13 February (Figures 3-4, 6-8); it was not reported thereafter. Fowler made several visits to Crescent City harbor to study the scoter. On 1 Feb- ruary, Daryl Coldren, Jared Hughey, Greg Chapman, and Fowler observed the bird in fight, noting the contrast in the underwing between blackish lesser and median under- wing coverts and paler, silver-sheened fight feathers, similar to Black Scoter (Behrens and Cox 2013). When the bird took fight, Fowler observed its dark feet and legs and heard a whistling wing noise similar in quality to that of Black and other scoter species. On several occasions, Fowler and others observed the scoter pulling small barnacles and mussels off the boat docks in the boat basin. On later dates, Fowler and many visiting birders noted the Common Scoter associating more often with a large fock of mostly Surf Scoters that included a single Long-tailed Duck (Clangula hyemalis) and a Harlequin Duck (Histrionicus histrion- icus). Local fshermen indicated that Pacifc Herring (Clupea pallasii) were spawning in the harbor at that time. Discussion Field notes from and discussions with other birders were supportive of the identifcation as Common Scoter. On 3 February, several observers heard the bird vocalize, including Curtis Marantz, who heard it give "a whis- tled, pee-peeu call when it raised to fap its wings." The typical call of male Common Scoter is higher in pitch and four times shorter than the mournful, drawn-out call of male Black Scoter (Sangster 2009). Several observers familiar with Black Scoter commented that the Common Sco- ter's bill looked longer. In addition to dif- ferences in the shape and pigmentation of the bill between adult male Common and Black Scoters, bill length also differs. Bord- age and Savard (2010) indicate bill length of 43.7 mm in male Black Scoter (range 40-47 mm); Cramp and Simmons (1977) indicate bill length of 47.5 mm in male Common Scoter (range 43-51 mm). Dean (1989) presents similar data on bill length. The overlap in bill length (cf. also Collinson et al. 2006) probably means that it has lim- ited value for identifcation in the feld. In Common Scoter, the nostrils are set closer to the base of the bill than in Black Scoter (Figures 1-5). Dement'ev et al. (1967) in- dicate that the distance from the anterior end of the nostrils to the tip of the bill in Black Scoter is 25-28 mm; Common, which has nostrils set farther back on the longer bill, would presumably have measurements over 30 mm. Dement'ev et al. (1967), Scott (1957), and older references indicate that Black Scoter has a more protrusive and decurved nail at the end of the bill than Common, and this difference appears to be true of many females as well as males, thus possibly useful for feld identifcation of vagrants. The Crescent City scoter showed a modest nail, not especially thick or strongly decurved. Collinson et al. (2006), however, suggest that the nail of Black Scoter appears more strongly decurved than in Common but that the nail is not necessarily larger or more protrusive. Structural or shape differences between Common and Black Scoters might also be of use in feld identifcation. Alderfer (2014) in- dicates that the wings of Common are more pointed than those of Black. Average wing length does appear to be greater in male Common (228-247 mm) than in male Black (213-241 mm) but only slightly (Collin- son et al. 2006). Alderfer (2014) notes that Common has a thinner head and neck than Black; and Waring (1993) and Blomdahl et al. (2007) illustrate clearly the difference in head/neck shape between the species when seen in fight, Black showing a rather thick neck and heavy head, Common generally showing a slenderer neck and slimmer head (Figures 8-10). Black Scoter overall appears bulkier when on the water among Common Scoters, and part of that difference in appear- ance is attributed to Black's tendency to "ride higher" in the water than Common (Garner

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