Frequency histograms for wind speed and pressure vertical velocity.

<p>The top graphs show the atmospheric conditions from the North American Regional Reanalysis Model <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002154#pone.0002154-Mesinger1" target="_blank">[21]</a> for 1–31 May, 2003–2005, at the 925 mb level at 0300 UTC (2100 local time) over the site where our thrushes were captured and released. The bottom graphs show the atmospheric conditions encountered by the 15 Swainson's thrushes in this study.</p

Partial regression plots for the cruise phase general linear model.

<p>The dependent variable in the analysis was average heart rate (Hz) during the cruise phase of 14 migratory flights. Heart rate (and thus energy expenditure) increased with increasing wingtip roundedness (C₂), wingloading (gm⁻²), and wind speed aloft (ms⁻¹), independent of wind direction. It decreased with increasing pressure vertical velocity (increasing atmospheric stability, Pas⁻¹), although not significantly. Axes are unstandardized residuals.</p

Frequency histograms for wingtip shape (C₂, lower values indicate more pointed wings [3]) and wingloading.

<p>The top graphs show the wingtip shapes and wingloadings of 93 Swainson's Thrushes captured in Champaign-Urbana, IL in May 2003–2005 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002154#pone.0002154-Bowlin2" target="_blank">[25]</a> while the bottom graphs show the same data for the 15 Swainson's Thrushes followed in this study. Average wingloading is slightly higher in this study than in the larger sample primarily because of the added mass of the transmitter.</p

Determining approximate flight altitude.

<p>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.</p

Frequency histogram for average heart rate.

<p>The graph shows the average heart rates (mean±s.e.m., 13.35±0.18 Hz) of 15 free-flying Swainson's Thrushes migrating over the central United States in spring after initial ascent and prior to final descent. Note the large amount of variation. The histogram for the initial ascent phase (not shown) is virtually identical except the mean is higher: 14.43±0.16 Hz.</p

Migratory Wildebeest in the Serengeti

<p>The migration of large mammals is an increasingly threatened phenomenon around the world. (Photograph: Nathan Gregory)</p

Wingtip shape in Swainson's Thrushes.

<p>To the left is a relatively pointed wing (C₂ = 0.139) and to the right is a relatively round wing (C₂ = 0.493). Grid squares in both pictures are 6.35 mm on each side.</p

Virtual vanishing bearings. Triangles inside the diagram represent the orientation of each bird at 2 km from the release site.

<p>Outer arrows: home direction. Inner arrows: mean vector of the distribution. Mean vector length and direction, and homeward component are reported (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022385#s3" target="_blank">Results</a> for further explanations).</p

Mean vector distributions relative to the sections of the tracks recorded in the first two hours after release.

<p>Further explanations as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022385#pone-0022385-g007" target="_blank">Figure 7</a>.</p

Examples of tracks.

<p>Other explanations as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022385#pone-0022385-g001" target="_blank">Figure 1</a>.</p