Condition Dependence and Status Signaling Function of Structural Coloration in the Eastern Bluebird (Sialia sialis)

Abstract

I examined the relationship between blue feather coloration, nutritional condition, and status signaling in eastern bluebirds (Sialia sialis). I quantified nutritional condition by measuring growth rates of tail feathers for an entire bluebird population to compare different age classes for both males and females. I found that males had wider growth bars than females and that older breeders had wider growth bars than younger breeders. Wider growth bars were correlated with longer tail feathers. Growth bar width was correlated with tail coloration for yearling males but not for males older than two years. Growth bar width was correlated with tail coloration in all females but there was a stronger correlation for second year females. Growth bar width was significantly correlated with number of offspring fledged the prior year as well as the number of offspring produced in the current year. I then compared the behavioral response from territorial males toward artificially brightened and darkened model conspecifics to test the signaling function of bright plumage. Bluebird males attacked models with brighter blue coloration significantly more often than models with darker blue coloration. Bluebird males did not attack models with brighter chestnut coloration significantly more often than models with darker chestnut coloration. The structurally-based blue plumage appears to be condition dependent and a signal of status used during male competition, whereas the melanin-based orange plumage does not.