If female birds prefer mates of the highest genetic quality, but are constrained in their choice of partner by the choices of higher quality females, one would expect to observe a positive correlation between male and female condition within pairs. Alternatively, within-pair similarity could be due to the fact that paired birds live on the same territory and share parental duties. To address this, we measured eastern bluebird (Sialia sialis) condition during the chick-rearing phase using indices of current vs. past condition. Mass and hematocrit both reflect nutritional and physiological condition at the time of capture. Size-specific mass and particularly bilateral asymmetry presumably reflect the condition of the bird during feather growth in the prior year. Mass and hematocrit showed significant correlation between male and female social mates. Feather-based indices showed no correlation.
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