Small-scale divergence in egg color along an elevation gradient in the mexican jay (aphelocoma ultramarina): A condition-dependent response?
The functional significance of bird egg color and patterning is a continuing subject of interest and debate. Extreme polymorphism in eggshell appearance is often thought to be maintained by frequency-dependent selection operating within populations. However, variation could also be explained by small-scale differentiation combined with limited migration. Here, we report the existence of extreme variation in egg color in a population of Mexican Jays (Aphelocoma ultramarina) inhabiting a steep elevation and habitat gradient within a single mountain range, the Sierra del Carmen of Coahuila, Mexico. We quantified egg color of 143 eggs from 54 nests throughout the mountain range, using digital photos. Color was also quantified for a subset of these eggs, using a spectrometer. Results from both methods support the conclusion that egg color has diverged at a remarkably small spatial scale of 3–15 km. Photo color quantification indicated that eggs at high elevation were greener than those at low elevation. Spectrometer results supported this conclusion, with more-pigmented (i.e., less reflectant) eggs occurring at high elevation. Unlike in other species, differences in condition do not seem to drive these divergence patterns in egg color. Further study is needed to determine to what extent differences result from heritable genetic change attributable to divergent selection pressures or from an environmentally induced phenotypic response.
John E. McCormack and Elena C. Berg. "Small-scale divergence in egg color along an elevation gradient in the mexican jay (aphelocoma ultramarina): A condition-dependent response?" The Auk 127.1 (2010): 35-43.