<p>Kinematics vary with flight speed.</p>

<p><b>(a)</b> The birds tilt their body angle as well as their stroke plane angle as speed decreases. Data used to produce this figure can be found in <a href="plosbiology.org/article/info:doi/10.1371/journal.pbio.3004023#pbio.3004023.s029" target="_blank">S1 Data</a>. <b>(b)</b> Downstroke angular velocity controls the forces generated by the wing, showing a U-shaped pattern with flight speed. Data used to produce this figure can be found in <a href="plosbiology.org/article/info:doi/10.1371/journal.pbio.3004023#pbio.3004023.s029" target="_blank">S1 Data</a>. <b>(c)</b> Hand wing sweep (0 is perpendicular to the flight direction) varies relatively little across speeds and between down- and upstroke.

Data used to produce this figure can be found in <a href="plosbiology.org/article/info:doi/10.1371/journal.pbio.3004023#pbio.3004023.s029" target="_blank">S1 Data</a>. <b>(d)</b> Nightjars show a high span ratio across flight speeds compared to other bird species. The nightjar has a span ratio of around 80%, which is in the same range as in swifts and slightly below hummingbirds. Data from other species are digitized from figures in [<a href="plosbiology.org/article/info:doi/10.1371/journal.pbio.3004023#pbio.3004023.ref015" target="_blank">15</a>] and [<a href="plosbiology.org/article/info:doi/10.1371/journal.pbio.3004023#pbio.3004023.ref016" target="_blank">16</a>].