Male mate choice based on sexually selected traits in female eastern bluebirds, Sialia sialis

1. Male mate choice based on sexually selected traits in female eastern bluebirds, Sialia sialis Summary In many animals, females are considered the choosy sex while males compete for mates. However, in species with biparental care and variation in the quality of females, males would also benefit from being choosy. In several species of birds, females base their mate choice decisions on plumage characteristics such as color and size of a patch. Additionally, plumage coloration and patch size are often indicators of quality, reproductive success, and parental effort. Eastern bluebirds (Sialia sialis) are a socially monogamous passerine. Both males and females provide parental care by defending and feeding the young. As adults, both sexes possess colorful plumage patches, a blue rump patch, a chestnut breast patch, and a blue tail. In males, individuals with brighter blue plumage enjoy a higher reproductive success. In this study, we examined whether coloration of female eastern bluebirds is related to individual quality measured by various metrics of reproductive success. • Preliminary Results • Principal components analysis for rump coloration: • Female Rump Coloration Male Rump Coloration • ~92% of variation ~65% of variation • explained. explained. • Preliminary Conclusions • Eastern bluebirds do not mate assortatively based on either rump or chest coloration. • Female eastern bluebirds that have brighter rumps do not necessarily have darker chests. Suggesting the two patches may convey different information to receivers. • The brightness of female rump plumage is a predictor of average egg volume of her first clutch. Females with brighter rump patches tend to lay earlier clutches and have chicks in higher condition prior to fledging. • Males may be using female coloration to assess individual quality and choose mates. This would lead to a selection pressure on female coloration. • No correlation between male and female rump coloration (r = -0.067, N = 34,p = 0.708), nor between female chest and rump brightness (r = 0.106, N = 41, p = 0.510). • Related Study • Male Mate Preference • Females randomly assigned to one of four plumage manipulationtreatment groups: • - Plumage experimentally altered with non-toxic, permanent markers: • Males went through six pairwise trials • - Each trial used a unique combination of experimental female treatments • Male preference measured by behavior: • - Time spent singing to a female • - Time spent in association with a female • - Number of displays directed toward a female • Mate preference chamber: Joanna K. Hubbard & John P. Swaddle Biology Department, College of William & Mary, Williamsburg, VA Bright Rump Dark Breast Bright Rump Light Breast Dull Rump Dark Breast Dull Rump Light Breast • Methods • Reproductive Data Collection • We collected field data for initial nesting attempts: • - Adult condition - Color of mate • - Date of 1st egg - Clutch size • - Egg Volume - Chick Condition • - # of fledglings - Provisioning rate • We collected plumage samples from adults: • Color Analysis • We measured feathers using spectrometry: • - Arranged nine feathers similar to how they lay on a bird’s body • - Took three readings for each sample • Quantified three descriptors of color: • - Hue: peak wavelength • - Saturation: purity of color • - Brightness: amount of light reflected • Various female characteristics are predictors of reproductive metrics. Below are summaries of backwards stepwise regression models for different metrics. • Females with a higher rump PC2 score (brighter rumps), lay larger eggs and tend to have chicks in higher condition prior to fledging. Rump patch Breast patch Tail feathers Acknowledgments Dr. Dan Cristol, Dr. George Gilchrist, Alex Gunderson, Jake Sequiera, and the Swaddle Lab. This work is supported by NSF IOB-0133795 to JPS, The Charles Center at William & Mary, The Virginia Society of Ornithology, The Coastal Virginia Wildlife Observatory, and The Williamsburg Bird Club.