- Both mass and hematocrit exhibited positive within-pair correlations.
- Neither size-specific mass nor bilateral asymmetry of the tail showed significant correlation within pairs.
- Assuming that feather-based condition indices are valid measures of past condition, our results suggest that similarities within breeding pairs are due to the fact that social mates share a territory and parental duties.
For reference data we thank Ellen Rockwell, Ned Robinson, and Kelly Westbrook. Thanks to the Howard Hughes Medical Institute and Davidson College for financial support.