Table 1. Determining approximate flight altitude.

Pressure surface level (mb)	Predicted heading (°)
1000	77.9
975	59.9
950	25.0
925	9.5
900	24.5
875	8.7
850	15.2
825	36.5

Here we show how we determined the flight altitude of one of our Swainson's Thrushes. We used the bird's groundspeed and track direction as well as data on the winds aloft at the various pressure surface levels to predict the bird's heading, which could then be compared to its actual heading of 60°. It is clear from the table that the bird was flying near the 975 mb pressure surface level, so we used data from the 975 mb pressure surface level in our analysis of the effects of atmospheric conditions on heart rate. We performed this analysis for each of the 15 Swainson's Thrushes in this study to determine their flight altitudes.