Fancy Females: The Function of Female Coloration in Eastern Bluebirds, Sialia sialis

1. Summary Methods cont. Results cont. • Aviary study: male mate preference • Females randomly assigned to one of four plumage manipulationtreatment groups: • Figure 1: effect of plumage manipulations • Males went through 6 pairwise trials • Figure 2: Preference chamber (top view) • Sexual behaviors used to assay preference: • Number of songs • Number of displays • Number of box visits • Time spent with female • Assortative Mating • Evidence of assortative mating by body condition and tail coloration: • Figure 3: positive associations between pairs, condition: rS = 0.275, n = 93, p = 0.008; tail color: rS = 0.247, n = 91, p = 0.018 Traditionally, females are considered the choosy sex as a result of a high parental investment in terms of egg production and parental care. However, when males provide parental care the parental investment made by each sex is more balanced. Consequently, some degree of choosiness can benefit both sexes. In the socially monogamous eastern bluebird both males and females provide parental care. Additionally, both males and females possess colorful plumage, and within each sex there is among-individual variation in plumage coloration. In this study, we explored whether female coloration is a sexually selected trait that may influence male mate preference. We found that female tail coloration is weakly predictive of reproductive success, however, rump and chest coloration appear to be unrelated to any fitness metric measured. Experimental male mate preference trials revealed that rump and chest coloration do not influence male mate preference; however, preference for tail coloration was not tested. Female rump and chest coloration do not appear to be under sexual selection, yet tail coloration might indicate fitness potential and be used in male and mutual mate choice. Bright Rump Dark Breast Bright Rump Light Breast Dull Rump Dark Breast Dull Rump Light Breast Fancy Females: The Function of Female Coloration in Eastern Bluebirds, Sialia sialis • Male Mate Preference • Female rump and chest coloration does not influence male mate preference: • Figure 4: Average male preference for each treatment (1) bright rump dark chest, (2) bright rump light chest, (3) dull rump dark chest, (4) dull rump light chest. Repeated measures ANOVA: F3,8 = 0.418, p = 0.745 – Natural ± 1 sd – enhanced – reduced Joanna K. Hubbard & John P. Swaddle Institute for Integrative Bird Behavior Studies, Biology Department, College of William & Mary, Williamsburg, VA Conclusions Methods Results • Field study: female coloration and fitness • Collected field data for initial nesting attempts: • - Date of 1st egg - Brood condition • - Clutch size - # of fledglings • - Egg volume - Adult Condition • - Brood size - Provisioning rate • Collected plumage samples from adults for analysis with spectrometer: • Quantified three descriptors of color: • Brightness: amount of light reflected • Hue: wavelength at peak reflectance • Chroma: proportion reflected in specific color range (UV or red) • Compared female color and condition to: • Fitness metrics • Color and condition of mate • Female coloration and fitness • No relationship between female color and adult body condition • Female tail color weakly predicts reproductive output: • Table 1: Multiple regression model: r2 = 0.058, F6,73 = 1.91, p = 0.109 • Female body condition and tail color weakly predict reproductive effort: • Table 2: Multiple regression model: r2 = 0.086, F6,73 2.243, p = 0.048 • Female coloration does not indicate current body condition. However, female body condition somewhat predicts reproductive effort • Female rump and chest coloration do not predict fitness metrics in the field, contrary to a previous study (Siefferman and Hill, Evolution, 59, 1819-1828), and do not influence male mate preference in aviary tests • However, female tail coloration somewhat predicts reproductive output and effort in the field • And there is assortative mating by tail coloration (and body condition), which is consistent with mutual mate choice • Therefore, female tail coloration may be under sexual selection Rump patch Chest patch Tail feathers Acknowledgements We thank A. Gunderson, C. Kight, and J. Sequeira for help with fieldwork, and D. Cristol and G. Gilchrist for comments. This work was supported by the Virginia Society of Ornithology, Coastal Virginia Wildlife Observatory, Williamsburg Bird Club, and Arts & Sciences Graduate Research Grant from the College of William & Mary to J. Hubbard, and NSF grant IOB-0133795 to J. Swaddle.