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USE OF JUVENAL PLUMAGE IN DIAGNOSING SPECIES LIMITS: AN EXAMPLE USING BUNTINGS IN THE GENUS PLECTROPHENAX
Auk, The, Jul 2007 by Maley, James M, Winker, Kevin
ABSTRACT.-
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Species limits in the genus Plectrophenax have been difficult to assess. McKay's Buntings (Plectrophenax hyperboreus) are very similar both morphologically and behaviorally to Snow Buntings (P. nivalis). However, their breeding ranges are allopatric, and there is limited evidence of gene flow. The juvenal plumage of McKay's Buntings has never been described as different from that of Snow Buntings. Comparison of a series of McKay's Buntings in juvenal plumage with a series of Snow Buntings in juvenal plumage showed clear differences between the two forms. We used color spectrophotometry to quantify the differences between the two taxa in two areas of the body that appeared to be consistently different, the throat and back. The relative magnitude of the difference between McKay's and Snow buntings was greater than homologous differences between two subspecies of Snow Bunting (P. n. nivalis and P. n. townsendi). Four out of six variables were significantly different between McKay's and Snow buntings, whereas none of the variables were significantly different between the two subspecies of Snow Bunting. Bonferroni corrected t-tests of sexual dimorphism and regression of the variables against year of collection showed that these factors were not associated with these differences. Discriminant analysis accurately separated 100% of the specimens into their respective groups. These differences are notable given the evolutionarily conservative nature of juvenal plumage. Our results support continued recognition of McKay's Bunting as a species and reconfirm the use of juvenal plumage to help determine species limits. Received 7 December 2005, accepted 25 July 2006.
Key words: discriminant analysis, juvenal plumage, Plectrophenax, reflectance spectrophotometry, species limits.
Uso del Plumaje Juvenil para Diagnosticar los Límites entre Especies: un Ejemplo en el Género Plectrophenax
Resumen.- Los límites entre especies del género Plectrophenax han sido difíciles de establecer. Plectrophenax hyperboreus es muy similar en morfología y comportamienento a P. nivalis. Sin embargo, sus rangos de cría son alopátricos y la evidencia de flujo genético es limitada. El plumaje juvenil de P. hyperboreus nunca ha sido descrito como diferente del de P. nivalis. Una comparación de series de especímenes de P. hyperboreus y P. nivalis en plumaje juvenil mostró claramente que existen diferencias entre las dos formas. Utilizamos espectrofotometría de color para cuantificar las diferencias entre los dos taxones en dos regiones del cuerpo que parecían diferir consistentemente, la garganta y la espalda. La magnitud relativa de la diferencia entre P. hyperboreus y P. nivalis fue mayor que la de diferencias homólogas observadas entre dos subespecies de P. nivalis (P. ?. nivalis y P. n. townsendi). Cuatro de seis variables fueron significativamente diferentes entre P. hyperboreus y P. nivalis, mientras que ninguna de las variables fue significativamente diferente entre las dos subespecies de P. nivalis. Pruebas de t corregidas mediante Bonferroni para evaluar dimorfismo sexual y análisis de regresión de las variables contra el año de colección, mostraron que esos factores no están asociados con las diferencias. Análisis discriminantes separaron el 100% de los especímenes con exactitud en sus grupos respectivos. Estas diferencias son notables dada la naturaleza evolutivamente conservada del plumaje juvenil. Nuestros resultados apoyan el reconocimiento continuado de P. hyperboreus como una especie, y reconfirman el uso del plumaje juvenil para ayudar a determinar los límites entre especies.
IN CLOSELY RELATED forms that have distinct but subtly different adult characters, juvenal plumage differences can be a useful indicator of species limits. Phillips and Dickerman (1965) argued that in certain instances juvenal plumage is just as useful as adult plumages for diagnosing taxon limits and evolutionary affinities, especially in taxa that have extremely similar adult plumages. In passerines, consistent juvenal plumage differences in the complete absence of adult differences have been used to separate forms into subspecies (Phillips and Dickerman 1965). The primary reason for this utility is the evolutionarily conservative nature of the juvenal plumage. Juvenal plumage is considered more evolutionarily conservative than adult plumages because it is generally worn for a brief period, and selection at this stage is likely to be strong to maintain cryptic plumage for avoiding predator detection. Also, this plumage is probably not subjected to sexual selection, because birds molt the juvenal plumage well before reaching sexual maturity. Passerines offer clear examples of this evolutionary conservatism, such as among the Turdidae, in which spot-breasted juvenal plumage is essentially ubiquitous, even among species in which adults have no breast-spotting. The evolutionarily conservative nature of juvenal plumage makes it a potentially useful source of characters for determining species limits, especially in groups of birds that are closely related or have subtle adult plumage differences.
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