Egg color polymorphism and morph-ratio variation in Korean populations of the Vinous-throated Parrotbill

Understanding the occurrence of multiple distinct phenotypes in a population of a species, i.e., polymorphism, is one of the challenges encountered in evolutionary biology. Egg color polymorphism in birds is one example of morphological polymorphism and disruptive selection has been proposed as a hypothetical mechanism to explain its occurrence. We studied how polymorphic egg colors (immaculate blue and white) occur in Korean populations of the Vinous-throated Parrotbill (Paradoxornis webbianus). Egg color ratios (the proportion of nests with blue eggs in a population) were monitored over a large spatial scale and egg colors were quantified using a spectrophotometer. We found egg color ratios to vary spatially among populations. Interestingly, there was a latitudinal morph-ratio cline in egg color ratios. The proportion of nests with blue eggs increased considerably with the latitude declined towards the southern part of the Korean peninsula. There were some quantitative variations in egg colors among populations. However, the pattern of variations was not consistent with those of the population egg color ratios. Based on these results, we discuss a potential scenario for the evolution of egg color polymorphism in the Vinous-throated Parrotbill.