Faculty Sponsor: Dr. Mark Stanback
Eastern Bluebirds (Sialia sialis) are unusual in that 2/3 of females eject model Brown-headed Cowbird (Molothrus ater) eggs from their nests and 1/3 incubate them. We tested the consistency of female behavior by placing a model cowbird egg into the nests of females during the period of egg-laying. For those females who ejected the cowbird egg, we added another one week later (when we checked for the fate of the original model egg). We compared the repeatability of egg ejection during different times in the same breeding attempt as well as over sequential breeding attempts (spring and summer of the same year). We found that 92% females who tossed a model cowbird egg during laying also tossed a second cowbird egg added to their nest and 94% of females were consistent between nests. We tested between-nest repeatability three ways. First, we used a Spearman’s rank-order correlation to calculate between-nest repeatability. Repeatability calculated using Spearman’s rho was 0.546 (95% CI: 0.425, 647). We then used a generalized linear model without covariates to find that repeatability was 0.544. Finally, we used a generalized linear mixed model with year and nest box number as random effects to find that repeatability was 0.537. Repeatability was relatively high within- and between-nests.