Female coloration, sexual selection, and male mate choice in eastern bluebirds, Sialia sialis
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. Additionally, blue plumage appears to be a nutritionally dependent trait, indicating that coloration may convey information about an individual’s quality.
In this study, we examined whether coloration of female eastern bluebirds is related to individual quality measured by various metrics of reproductive success.
- 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. 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 lay earlier clutches and clutches with a larger average egg volume.
- Males may be using female coloration to assess individual quality and choose mates. This would lead to a selection pressure on female coloration.
- Chest coloration, which does not appear to relate to reproductive success, may be used in intrasexual communication.
- Preliminary Results
- Principal components analysis for rump coloration:
- Female Rump ColorationMale Rump Coloration
- ~92% of variation explained~66% of variation explained
- No correlation between male and female rump brightness (r = -0.159, N = 32,p = 0.383), nor between female chest and rump brightness (r = 0.044, N = 36, p = 0.799).
- 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 preference trials
- - Each trial used a unique combination of experimental female treatments
- Male preference measured by various behaviors:
- - Time spent singing to a female
- - Time spent in association with a female
- - Number of sexual displays directed toward a female
- Mate preference chamber:
Joanna K. Hubbard & John P. Swaddle
Biology Department, College of William & Mary, Williamsburg, VA
- Reproductive Data Collection
- We collected field data for initial nesting attempts:
- - Adult body condition- Color of mate
- - Date of first egg- Fledgling condition
- - Clutch size- Number of fledglings
- - Egg Volume- 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 laid eggs earlier in the season
Dr. Dan Cristol, Dr. George Gilchrist, Jake Sequiera, Alex Gunderson, and members of 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