USE OF JUVENAL PLUMAGE IN DIAGNOSING SPECIES LIMITS: AN EXAMPLE USING BUNTINGS IN THE GENUS PLECTROPHENAX

Snow Bunting specimens were classified into two currently recognized subspecies, determined by the collection locality and the last AOU Check-list to treat subspecies (AOU 1957). Because our specimens were all in juvenal plumage and not fully grown, we had to exclude measurements and use geographic range to classify the subspecies, which were described on the basis of adult plumage and morphometric characteristics. We examined specimens of P. n. nivalis (n = 16) and P. n. townsendi (n = 14). Levene's test and independent-sample f-tests were again conducted to test for differences. To help visualize the relative difference between species and between subspecies, we plotted the three most significant variables between McKay's and Snow buntings and the mean value per form on a 3-D scatterplot for both the species-level and subspecies-level comparisons using STATISTICA, version 5.5 (StatSoft, Tulsa, Oklahoma).

Because sexual dichromatism, foxing, or both, could have biased our results, we tested for these effects. We used Levene's test and independent-sample f-tests to test for sexual differences within subspecies and species, after which we combined all of the specimens to test for overall sexual differences. Because specimens of some species are known to change color over time, whether through fading or through foxing (for definitions, see Gabrielson and Lincoln 1951), we examined reflectance variables against the year the specimen was collected using linear regression.

We first tested for significant differences between the two forms, then determined how well the reflectance characters separated these taxa using a discriminant analysis conducted with SPSS. Discriminant analysis is a useful statistical tool for determining the ability of overlapping characters to be used to separate groups (e.g., Mayr and Ashlock 1991, Winker 1997, Figuerola et al. 1999). Analyses were conducted using both equal prior probabilities and probabilities calculated using group size.

RESULTS

When compared visually, the juvenal plumages of McKay's and Snow buntings appeared to have clear "average" differences in the shades of throat and back plumage (Fig. 1). Light reflectance spectrophotometry revealed overlap in every measured character (Table 1). However, McKay's Buntings were significantly lighter than Snow Buntings on the back and the throat (variable L in Table 1). They were also a significantly different shade (lighter brown) on one axis of color for the throat (variable a in Table 1), and on one axis of color (brown) on the back (variable b in Table 1). We thus rejected our null hypothesis of no differences; the juvenal plumages of McKay's and Snow buntings were significantly different.

To compare the level of differentiation between McKay's and Snow buntings with the level of differentiation between the two subspecies of Snow Bunting sampled, we hypothesized that nominate nivalis was not significantly different from P. n. townsendi. The subspecies townsendi was based primarily on larger morphological measurements (Ridgway 1887). The six variables examined did not differ, though for both the throat and the back a was significantly different between the two subspecies before multiple test corrections (variable a in Table 2). This difference was not consistently evident visually. Here, we could not reject the null hypothesis that these two subspecies are not different. A multivariate plot of three variables showed that the degree of difference between McKay's and Snow buntings was greater than that between the two subspecies of Snow Bunting (Fig. 2).