Editor's Note: Recent mitochondrial genetic data indicate that Carduelis is polyphyletic and that Acanthis spp. belong to different clade. See the 50th supplement to the AOU Check-list of North American Birds for details. Future revisions of this account will reflect this change.
During the summer, Common Redpolls are found in boreal and taiga regions of both the Old and New World Arctic, where they are often among the most common breeding passerines. In North America, their distribution shows significant overlap with human populations only in winter, and then only in alternating irruption years. The irruption cycle displayed by this species is driven by widespread failure in seed-crop production among high-latitude tree species—especially spruce (Picea sp.) and birch (Betula sp.)—which forces these birds to winter farther south ( Bock and Lepthien 1976f , Bock 1982b , Larson and Bock 1986b ). Irruption cycles, documented in Christmas Bird Count (CBC) data, have received attention and analysis. Most studies of breeding biology have been undertaken in Europe (e.g., Pulliainen and Peiponen 1981 ) but observations of more limited nature have been made in North America (e.g., Walkinshaw 1948a ).
Redpolls are busy, acrobatic little finches, spending much of their time flitting about, feeding, and calling with their incessant che, che, che Contact Calls or their plaintive dsooee . Their bouncy flight and rolling feeding flocks in winter fields add to the feeling of constant and exuberant activity.
Recent work on this species has investigated redpoll taxonomy. Are there 1, 2, . . . or 6 species of redpolls ( Troy 1985 , Knox 1988b , Herremans 1990 )? In this account, we follow the conservative approach to redpoll taxonomy that identifies 2 species: Common Redpoll and Hoary Redpoll (Carduelis hornemanni). See Knox and Lowther 2000b for account of similar Hoary Redpoll.