Billions of birds undertake long-distance migratory journeys each year, using approximately one-third of their energy budget on flight alone. While we know a great deal about avian physiology and behavior on the ground at stopover sites, we know almost nothing about the physiology and behavior of birds during flight. Because of this, many of the predictions aerodynamic theory makes about energy expenditure during migratory flight remain untested. In particular, aerodynamic theory predicts that pointed wingtips and low wingloading will reduce energy expenditure during long-distance flight. In this thesis, I tested these predictions both indirectly and directly. First, I used phylogenetically independent contrasts to demonstrate that wingtip shape varied with migratory behavior in the twelve-species avian genus Catharus. Next I caught Swainson's Thrushes (Catharus ustulatus ) during spring migration and showed that individuals with more pointed wingtips and lower wingloading arrived earlier in the season, consistent with the hypothesis that these morphological features allow individuals to save energy and migrate faster than conspecifics. Because I could not rule out alternative explanations for these patterns, I measured individual energy expenditure during migratory flight directly using the heart rate method. In order to do this, I performed calibrations between heart rate and energy expenditure for Swainson's Thrushes at rest and during flight, showing that there was a linear relationship between heart rate and energy expenditure during both activities. For the naturally-migrating birds, I found that individual energy expenditure during flight was correlated with wingtip shape, wingloading, wind speed during the flight, and atmospheric stability. Swainson's Thrushes with more pointed wingtips and lower wingloading used less energy than conspecifics; they used more energy than conspectuses when wind speeds were high and atmospheric stability was low. Overall, my thesis contains the best evidence to date that selection on energy efficiency during migratory flight could produce some of the observed diversity in avian wing shape as well as data that could be used to predict how individual passerine migrants will respond to climate change.