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

Joanna K. Hubbard & John P. SwaddleBiology Department, College of William & Mary,

Williamsburg, VA

AcknowledgmentsDr. 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.

MethodsReproductive 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

Rump patch Breast patch Tail feathers

SummaryIn 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.

Related StudyMale Mate Preference• Females randomly assigned to one of four plumage manipulation treatment 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:

Bright RumpDark Breast

Bright RumpLight Breast

Dull RumpDark Breast

Dull RumpLight Breast

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.

• Various female characteristics are predictors of reproductive metrics. Below are summaries of backwards stepwise regression models for different metrics.

• 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).

• Females with a higher rump PC2 score (brighter rumps), lay larger eggs and tend to have chicks in higher condition prior to fledging.

Female Rump Treatments

05

10152025303540

200 400 600 800

Wavelength (nm)

Ref

lect

ance

(%)

Unmanipulated Dulled Brightened

Female Chest Treatments

0

10

20

30

40

200 400 600 800

Wavelength (nm)

Ref

lect

ance

(%)

Darkened Unmanipulated Lightened

Component Matrix

.307 .903-.742 .613.967 -.171.857 .400

BrightnessHueSaturationUV Chroma

1 2PC

Component Matrix

.757

.924-.612.883

BrightnessHueSaturationUV Chroma

1PC

Reproductive Metric Predictor β Sig.

Date of First Egg PC2 for ♀ rump color -2.845 0.099

Avg. Egg Volume♀ condition 776.465 0.032

PC2 for ♀ rump color 1044.443 0.011

Avg. Chick Condition PC2 for ♀ rump color 0.746 0.077

# of Fledglings PC1 for ♂ rump color 0.475 0.011