Variation and Adaptive Function of Iridescent Coloration in the Black-billed Magpie (Pica pica)

Abstract

Iridescent coloration, which is produced by coherent scattering of light from keratin nanostructures in feathers, has not been fully investigated in terms of its adaptive function and evolutionary significance. In particular, there have been no studies of the iridescent coloration of avian species that lack distinctive sexual dichromatism. Black-billed Magpies (Pica pica), which are common and abundant throughout Korea, have two types of iridescent coloration on their wings and tail. I examined variation in iridescent coloration of magpies in relation to individual social class, body conditions, and breeding success, at Seoul National University, Seoul, Korea. In addition, I measured the iridescent coloration of magpies from nine local areas in Korea in order to understand links between the coloration and the variation in local climatic factors, in particular, deviation in temperature, humidity, precipitation, and wind speed of the study year from normal years. Adult magpies had higher color scores, characterized by brighter, shorter wavelength-directed, more saturated colors, than did young birds. The results imply that plumage coloration in magpies may act as a signal that enables young magpies to avoid direct competition with strongly territorial breeding adults, or with more experienced non-breeding adults in the same flock. Iridescent coloration was significantly linked with indices of size and body condition in males only, particularly, with their tail characteristics, which are often regarded as quality indicators in breeding males. Breeding males with brighter iridescent coloration started breeding earlier and had better breeding success, even though brighter coloration did not guarantee the earlier fledging of their young. Iridescent tail coloration had a higher deviation value, estimated by the coefficient of variation, than that of other morphological traits, and the coloration of males was mainly related to variation in climatic factors. Based on these results, I suggest that the iridescent tail coloration of male magpies can be an honest signal of body condition and breeding success and is therefore probably a trait selected for by females. Further research on the effect of environmental variation (such as nutritional conditions and climate fluctuations) on the feather structures of magpies, and the relationship between variations in feather structure and consequential variation in iridescent coloration, is necessary.