A phenetic analysis of the redpolls Carduelis flammea flammea and C. hornemanni exilipes

-A review of the historical treatment of Common and Hoary redpolls (Carduelis flammea flammea and C. hornemanni exilipes) revealed no diagnostic means of distinguishing the two taxa. Interpretations of species limits have varied, resulting in conflicting treatment of intermediate forms. Variability in plumage and skeletal characters of redpolls from 9 breeding locations in Alaska and Canada were examined to determine if a means of discriminating between the two taxa could be found, and to determine if individuals with intermediate plumage characteristics (potential hybrids) were also morphologically intermediate. Because sexes differed in most skeletal dimensions, all analyses were duplicated for the two sexes. A character index was developed based on three plumage characteristics: rump streaking, undertail covert streaking, and lateral (side) streaking. This index characterized redpolls on a darkness gradient from extreme flammea (dark, heavy streaking) to extreme exilipes (light, no streaking). The observed range of scores was divided into thirds to designate flammea, unknown (intermediate), and exilipes classes. Females showed a much narrower range of plumage variability than males. No females were white enough to be considered exilipes on the scale used for males. Most birds from the flammea and exilipes extremes of plumage variability could be distinguished by skeletal criteria (using discriminant function analysis); however, no discontinuity existed, as other birds were intermediate for both plumage and skeletal characteristics. Intermediates were found at all sampled localities where both plumage morphs occurred. Results of analyses based on plumage (character index) and skeletal measurements were very similar and showed that samples containing intermediate plumage birds were also intermediate skeletally. Considering the continuous variability of both plumage and skeletal characters and the lack of any diagnostic criterion for identifying the taxa, the two redpolls should be merged into a single taxon, Carduelis flammea. Received 22 November 1983, accepted 23 May 1984. COMMON and Hoary redpolls (Carduelis flammea and C. hornemanni) are small finches of the subgenus Acanthis (A.O.U. 1983). The taxonomy of redpolls has long been perplexing. Jehl and Smith (1970) considered it to be "one of the most perplexing problems in North American ornithology." Similar concerns were expressed by Bannerman (1953), Todd (1963), and others. Coues (1862) recognized 6 species of redpolls in North America, plus another in Europe; Salomonsen (1950) listed only one worldwide, while the American Ornithologists' Union (1983) recognized two. Of the many problems in our understanding of the relations of the various redpoll taxa, the specific distinction of C. flammea and C. hornemanni is of priI Present address: LGL Alaska Research Associates, Inc., 505 West Northern Lights Boulevard, Suite 201, Anchorage, Alaska 99503 USA. ary importance. The relationship of the most widespread subspecies of each of these species, C. f. flammea and C. hornemanni exilipes, is central to this problem. This study was an attempt to clarify the relationship between these two redpoll taxa using plumage and skeletal characters. Alaska redpolls were considered in detail because published accounts indicate that the greatest taxonomic difficulty in North America occurs in this region (Gabrielson and Lincoln 1959, Baldwin 1961). Smaller samples from across Canada also were analyzed. Characteristics previously used to distinguish exilipes from flammea are listed in Table 1. No single character distinguishes these taxa from each other unequivocally. The generalizations that exist are useful for identification of extreme examples only. Unfortunately, no description of the range of variability in either exilipes or flammea has been made, resulting in 82 The Auk 102: 82-96. January 1985 This content downloaded from 207.46.13.184 on Wed, 10 Aug 2016 06:44:40 UTC All use subject to http://about.jstor.org/terms January 1985] Redpoll Systematics 83 TABLE 1. Differences in bill and plumage characteristics used to distinguish between Carduelis hornemanni exilipes and C. flammea flammea. Character exilipes flammea Source Bill length Small Moderate Coues 1903 Bill shape More conic, less compressed, Opposite Coues 1862 darker colored with dense nasal plumuli Superciliary More distinct Less distinct Coues 1862 stripe Breast color Rosy, many shades lighter than Darker Coues 1862 flammea Sides Paucity of streaks Coues 1862 Strongly streaked flanks Bannerman 1953 Feet Smaller, weaker Opposite Coues 1862 Toes much shorter Tarsus > middle toe plus claw Tarsus = middle toe plus claw General Lighter Much darker Godfrey 1966 confusion as to how to identify specimens that are apparently intermediate in some characters. Because they were originally described using morphological criteria, any attempt to determine whether flammea and exilipes are indeed separate species logically should look for at least one morphological feature to discriminate between them. If a character can be found with a range of states such that the distribution of scores of samples of the pure species is discontinuous (the discontinuity indicating species limits), suspected hybrids should have intermediate scores along this character. Under the strictest definition of the biological species (Mayr 1970), the presence of any hybrids would make the specific distinction of the parental taxa suspect. However, it has been common practice to accept species distinctiveness if hybridization is limited. Hence, if few intermediates exist, whether they be hybrids or extreme variants of parental species, the separation of exilipes and flammea would be justified. For the purpose of this paper, both hybrid and intermediate are used to describe birds that possess characteristics intermediate between "typical" flammea and exilipes. If these two forms are morphologically distinct, then intermediates are probably true hybrids or their offspring. If flammea and exilipes are not distinct, the intermediate birds probably represent a segment of continuous variation rather than hybrids, and it is necessary to conclude that the recognition of two species is unfounded. Two methods often used to detect the presence of hybrid individuals and/or populations are hybrid indices (examples using avian subjects include Sibley and Short 1959a, b; Sibley and West 1959; Short 1963; Szijj 1963; Rising 1970; and Scott et al. 1976) and multivariate analyses of skeletal dimensions (Rohwer 1972). Although frequently used to identify hybrids (or populations containing hybrids), hybrid indices do not directly reflect the hybrid nature of the individual to be scored. I have used the term "character index" because the scale is derived from several characters. Plumage characters used in character indices are often the same variables that give rise to the hypothesis of hybridization in the first place, and their use can lead to a circular argument (Scheuler and Rising 1976). I used the analysis of skeletal characters as an independent test of the findings of the character index analysis. METHODS AND MATERIALS Specimens.-Only adults collected during the breeding season were used in the plumage and skeletal studies. Alaska redpolls were collected in three regions: the Akulik-Inglutalik River delta, Norton Bay; Interior Alaska (no large collections were made, so this sample comprises three smaller samples: Talkeetna Mountains near Hatcher Pass, Denali Highway at the Brushkana River, and Fairbanks); and the Kongakut River, Arctic National Wildlife Refuge. Four redpolls collected at Kaktovik, Barter Island, were added to the Kongakut River sample. These speciThis content downloaded from 207.46.13.184 on Wed, 10 Aug 2016 06:44:40 UTC All use subject to http://about.jstor.org/terms 84 DECLAN M. TROY [Auk, Vol. 102 TABLE 2. Number and location of specimens from which skeletal measurements were taken. Location Males Females Akulik-Inglutalik River delta, 100 51 Alaska Interior Alaska 21 11 Kongakut River, Alaska 67 31 Inuvik, N.W.T. 38 18 Krekovick Landing, N.W.T. 17 10 Coppermine, N.W.T. 16 11 Churchill, Manitoba 8 Winisk, Ontario 8 Fort Chimo, Quebec 37 8