Influence of elevation on the sexual dimorphic gap in mountain white-crowned sparrows (Zonotrichia leucophrys oriantha).

Abstract

This study examined sexual dimorphism and dichromatism in mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) and how it changes over an elevational gradient. Birds living in high elevation habitats are subject to harsh climatic conditions and a shorter breeding season, which affects how many MWCS can inhabit the area, how females select a mate, and how much energy for parental care is required for both males and females to acquire reproductive success. This increased parental effort requirement then has an impact on sexual selection pressures on males and on individual fitness. Mountain white-crowned sparrows were trapped with potter traps and mist nets at low (2962m) and high elevations (3240m) and measurements taken on wing, retrices, tarsus, and culmen length as well as crown stripe width and then compared between three pools of male and female pairs: 1) All birds caught during this study randomly paired from low elevation compared to high elevation, 2) Potentially breeding pairs caught in the same low elevation locations compared to high elevation, and 3) Confirmed nesting pairs compared between low and high elevation sites. Male morphological feature size was also compared across increasing elevation. No significant differences in dimorphism were found for the paired MWCS, but a negative correlation was detected in male crown ratio proportion (R2 = 0.04, P = 0.08). This trend suggests that sexual dichromatism in MWCS males inversely correlates with elevation based on reduced sexual selection pressures and increased parental care requirements at higher elevations