effect of turbulence on flight, turbulence can easily be created by inserting nets
or objects upstream from the test section.
One important role for wind tunnels is the validation or testing of flight
mechanical theory, which among other things, predicts a U-shaped relationship
between power output and flight speed. Measurements of metabolic rate, using
a respirometry mask or doubly labeled water, have often been used for evaluating
flight mechanical theory, but this measures power input. These measurements
are only valid if the conversion efficiency of fuel energy to mechanical work is
constant at all speeds, which might not be true. Direct measurement of mechan-
ical power output is difficult, but can be made by inserting a strain gauge and
measuring the force applied to the humerus by the flight muscle (Dial et al. 1997;
Tobalske et al. 2003), variations in the vertical acceleration of the body com-
bined with wing-beat kinematics (Pennycuick et al. 2000), or from estimating
the impulse associated with vortex wake structures.
The wind tunnel can also be used for studying wing-beat kinematics and flight
style by using high-speed video cameras.
7.6 Orientation and navigation
Orientation refers to compass orientation or directed movement, while the term
navigation is usually restricted to the theory and practice of charting a course to
a distant goal to which the animal has no direct sensory contact.
7.6.1Emlen funnels
Compass orientation has traditionally been studied in passerine migrants by
recording their directed migratory activity (Zugunruhe) in circular cages, so
called Emlen funnels (e.g. Emlen and Emlen 1966; Figure 7.6). The technique is
suitable to record orientation in small passerines, such as the European Robin
Erithacus rubecula. Modified and enlarged cages can be used for other species
as well, such as waders (Sandberg and Gudmundsson 1996). During the
experimental period, lasting between 1 h and a complete night period, the bird’s
activity is recorded by its claw scratches in the pigment of Tipp-Ex paper covering
the sloping walls of the cage, or by having an ink pad in the bottom of the cage
and white paper on the sloping walls to record the bird’s movements. Circular
cages with automatic computer registration are now used in several laboratories.
7.6.2Manipulating sensory input
The main benefit of studying migratory orientation in cages is that the external
information perceived by the bird can be manipulated. By producing an artificial
172 |Migration