Example kinematic traces for takeoff, mid-flight and landing for bird 2.
Arrows indicate the direction of wingtip movement
during downstroke. Top panels show wingtip position
relative to the shoulder for each wingbeat. Gray crosses indicate the
location of the shoulder when the origin is not
shown. Lower panels show wingtip position in the global coordinate
system,
with tick marks indicating distance from the perch.
The traces illustrate the changes in stroke planes from negative angles
during takeoff and mid-flight to positive angles
during landing. The steeply tilted stroke planes during takeoff account
for
the compressed appearance of the wingtip path in
the dorsal view during takeoff. These trends in stroke plane angle (SPA)
are evident for both local and global coordinate
systems.
http://jeb.biologists.org/content/213/10/1651/F5.expansion.html