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.

7.6.1 Emlen 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 ofTipp-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.

7.6.2 Manipulating 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

Fig. 7.6 Adult white-crowned sparrow in orientation cage (Emlen type) at the magnetic North Pole. The sloping walls are covered by type-writer correction paper. Photo: Susanne Akesson.

magnetic field using large magnetic coils, such as modified Helmholz coils (Wiltschko and Wiltschko 1995), the perception of the geomagnetic field can be manipulated. In a similar way sun compass orientation has been investigated by shifting the position of the sun using mirrors, and by using filters to shift the alignment of polarized light and to depolarize the incoming light. Opaque Plexiglas sheets placed on top of the cages are used to screen off visual cues. By shifting the birds' internal time sense relative to the natural dark—light cycle using an artificial dark—light cycle, the function of the birds time-compensated sun compass can be investigated.

In studies of the functional characteristics of the birds' magnetic sense various techniques have been applied, such as exposure to strong (0.5—1 T) magnetic pulses and thereafter observation of the birds' orientation in cages, as well as neurophysiological recordings during magnetic field manipulations (for review see Wiltschko and Wiltschko 1995). In the search for a magnetic sensor containing magnetite, histology techniques, magnetic force microscopy, and a Superconducting Quantum Interference Device (SQUID) are used.

In conditioning experiments a bird is trained to detect a feeder associated with a particular stimulus (magnetic, visual), and then the bird's ability to use this cue is challenged with only the stimuli present at randomized locations.

7.6.3 Displacement experiments

Birds and other animals have been suggested to navigate by using either a combination of two geomagnetic parameters, field intensity and the angle of inclination

(bi-coordinate magnetic navigation) varying across the Earth's surface or by celestial information. Both the geomagnetic and celestial parameters (elevation angle to certain star configurations and sky rotation) can be manipulated in the laboratory by using large magnetic coils (see above) and a planetarium sky. Studies have been performed with passerines where course shifts have been recorded as a response to simulated geomagnetic and geographical displacements.

Large-scale displacement experiments with ringed birds, performed mainly during 1930—70, have been used to study navigation abilities. The main aim is to find whether displaced birds maintain the same heading and end up in the "wrong" place (expected in clock-and-compass orientation), or whether they change their heading in accordance with the displacement and end up in the correct place (i.e. true navigation to a specific goal). Some of the most spectacular experiments involved over 15,000 starlings transported from the Netherlands in autumn to release sites in Switzerland and Spain. In more recent years the orientation of caged passerine migrants has been studied during lateral displacements by ship relative to their intended migration route (e.g. Akesson et al. 2001a), asking the same type of questions.

7.6.4 Selection experiments

Both Zugunruhe (restless behavior shown at migration times) and direction are encoded in the birds' genetic migration program (Berthold 1996), and are probably exposed to strong selection. The length and intensity of nocturnal migratory activity can be studied in cages recording the bird's jumping activity. The inheritance of migratory activity has been studied both by selection- and cross-breeding experiments demonstrating that the phenotypic character can be changed in only a few generations. Furthermore, cross-breeding experiments with migratory Blackcaps Sylvia atricapilla from European populations with different migratory directions (SW- and SE- in autumn) show that the migratory direction is inherited in an intermediate fashion in the offspring (Berthold 1996).

7.6.5 Circular statistics

A special type of statistics called "circular statistics" is required to analyze circular data (e.g. Batschelet 1981), such as departure directions of migrating birds recorded by radio-telemetry or migratory activity recorded in Emlen funnels. Since we deal with directions that can be represented on a circle we cannot use linear statistics to treat these data. This can be illustrated by the angular difference between 10° and 350° being only 20°, with a mean direction of 0° (north), while the arithmetic mean of 180° would erroneously indicate a mean direction toward south. Vector addition is used to calculate the mean vector for a group of

Fig. 7.7 Circular diagram showing orientation of white-crowned sparrows Zonotrichia leucophrys gambelii at the magnetic North Pole. a is the mean direction, r is the length of the mean vector, N is sample size and p is 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 Akesson et al. (2001 a).

Fig. 7.7 Circular diagram showing orientation of white-crowned sparrows Zonotrichia leucophrys gambelii at the magnetic North Pole. a is the mean direction, r is the length of the mean vector, N is sample size and p is 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 Akesson et al. (2001 a).

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.

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