Bird Ecology and Conservation A Handbook of Techniques

directions, and the length of this vector indicates the concentration. From the
mean vector and sample size, suitable test statistics can be calculated (Batschelet
1981). Data recorded in Emlen-funnels are usually divided into sectors and the
mean orientation for each bird in a test is calculated. Results from different
individuals of an experimental condition can later be pooled for which the mean
orientation is calculated. Examples of funnel experiments performed at the
Geomagnetic North Pole are given in Figure 7.7.

7.7 Modeling migration

Migration behavior, including stopover duration, optimal fuel loads, the use of
winds and physiological flexibility can be predicted using simple optimality
models (Alerstam and Hedenström 1998 for review). Predictions derived from
different currencies, such as energy or time minimization and maximizing sur-
vival, often differ and which strategy is adopted can be tested. Predictions regard-
ing the optimal flight behavior have also been derived. At present, there are more
theories than there are critical tests of their assumptions and predictions, and so
there are plenty of research opportunities.
Dynamic or state-dependent optimization is used to derive optimal migra-
tion polices and the annual routine programs of migratory birds (Clark and
Mangel 2000; Houston and McNamara 2000), including the scheduling of

Modeling migration| 175

gN

(a) (b)

6 ° gN

= 143°

r= 0.44

N=28

p< 0.001

=58°

r= 0.017

N= 26

p> 0.05

Fig. 7.7Circular diagram showing orientation of white-crowned sparrows Zonotrichia
leucophrys gambeliiat the magnetic North Pole. is the mean direction, ris the length
of the mean vector, Nis sample size and pis level significance of a Rayleigh test
(Batschelet 1981). (a) Visual cues available to the birds, (b) an opaque sheet covers
the orientation cage and restricts the birds to the use of magnetic cues only, and they
become disoriented because the magnetic field lines are vertical and provide no
directional information. Based on Åkesson et al. (2001a).