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*rare, irruption. Large, dark, and streaky. The cheeks on rostrata are dark and nearly concolor with the face, with prominent streaking extending from the head down onto the breast and sides, eventually forming discernible thick streaks on the flanks. Some apparent nominate Commons can appear as dark and streaky as rostrata, but these individuals are not larger than conspecifics, and often they have thinner streaks on the undertail coverts. Compared to nominate Commons, rostrata consistently are 30-50% larger and their undertail covert streaking is wider with relatively little pale fringing. Based on these individuals and others seen in the area this month, the median covert edging indeed averages thinner on rostrata than flammea, although some rostrata have thicker edging than some flammea. Some flammea have almost no edging at all. As well, the bills on rostrata average thicker, although some flammea have similarly thick bills. Although the undertail covert streaking on rostrata averages thicker and darker than on nominate Commons, this trait seems to vary greatly. The individual that appears regularly in the videos has thin undertail covert streaking relative to other rostrata seen in the area recently. One pale rostrata/islandica-type individual was particularly interesting. This bird was clearly larger in size like a rostrata or hornemanni, except it had a pale gray back, a whitish rump, and broad, diffuse flank streaking. The undertail covert streaking was thick and dark as is typical on rostrata. The bill appeared thick as in rostrata, but it had long nasal bristles covering half of the upper mandible and a steep forehead as would be expected on hornemanni. The suite of traits gave the impression of a dark hornemanni or pale rostrata, or a mix of the two. The phenotypically variable islandica classification traditionally has been reserved for the Common Redpoll population in Iceland; this individual appears similar to those. However, islandica has been suggested to be a hybrid population between A. f. rostrata and A. h. hornemanni. If rostrata and hornemanni hybridize and produce individuals that look like this, then an alternative source population for this individual could be Greenland.
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