Within-individual changes in developmental stability affect flight performance

Developmental stability, as measured by fluctuating asymmetry, has been purported to be an indicator of individual quality, and low asymmetry can be selected for by sexual selection processes. However, low asymmetry can also arise due to biomechanical constraints operating on trait design, as it is predicted that asymmetry will decrease mechanical efficiency. Specifically, it has been predicted that wing length asymmetry will be negatively related to avian flight performance. To date, empirical investigations have only studied the influence of increasing asymmetry beyond naturally occurring average values. I examined the influence of within-individual changes in primary feather developmental stability on flight performance in European starlings by studying asymmetry and flight before and after wing molt. Individuals that exhibited a decrease in wing asymmetry through molt experienced increased aerodynamic performance in terms of both angle of takeoff and level flapping-flight speed. Birds that increased wing asymmetry suffered a decrease in flight performance. Takeoff speed and the ability to negotiate an aerial obstacle course were unaffected by asymmetry. My data provide empirical support for the predicted influence of wing asymmetry on flight, even though the changes in asymmetry were very small (mean = 0.47% of trait size) and further indicate the importance of biomechanical considerations in any study of developmental stability.