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Orientation behavior of Garden Warblers (Sylvia borin) under monochromatic light of various wavelengths
Auk, The, Jan 2000 by Rappl, Ralf, Wiltschko, Roswitha, Weindler, Peter, Berthold, Peter, Wiltschko, Wolfgang
A little more than 30 years ago, a study revealed that migratory European Robins (Erithacus rubecula) could use the earth's magnetic field for directional orientation (Merkel and Wiltschko 1965, W. Wiltschko 1968). Today, magnetic compass orientation has been demonstrated in more than 15 other species of migratory birds and in homing pigeons, and it seems to be a rather widespread mechanism among birds (R. Wiltschko and Wiltschko 1995). Yet, it is still unclear how birds perceive the geomagnetic field and obtain directional information.
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One hypothesis under investigation is that magnetoreception occurs via light-dependent processes parallel to the processes of vision in the avian eye (Leask 1977, 1978; Schulten and Windemuth 1986). This hypothesis assumes that incident light elevates certain macromolecules into an excited state, where they may be transferred to an excited triplet state and where further reactions are determined by the direction of the ambient magnetic field. This would mean that light is essential for magnetoreception. Results from early experiments with homing pigeons are in agreement with this prediction. Inexperienced young pigeons derive their home direction from information obtained with their magnetic compass during the outward journey (R. Wiltschko and Wiltschko 1978); pigeons transported to the release site in total darkness respond like birds transported without meaningful magnetic information, which indicates that light is indeed necessary for magnetic compass orientation (W. Wiltschko and Wiltschko 1981).
Similar experiments with migratory birds in total darkness proved impossible. Directional tendencies of migrants can be recorded only when the birds show a certain amount of activity, but Zugunruhe (i.e. migratory activity) is suppressed by darkness (Gwinner 1974). However, another approach allows testing the hypothesis of light-dependent magnetoreception in migrants. Leask (1977) suggested that exposing birds to monochromatic light at various wavelengths might modify the excitation transfer probability to the excited triplet state, allowing one to look for possible effects on orientation behavior as indicators of magnetoreception. Previous experiments with Australian Silvereyes (Zosterops lateralis lateralis; W. Wiltschko et al. 1993, Munro et al. 1997) and European Robins (W. Wiltschko and Wiltschko 1995, 1999) were in accordance with light-dependent processes of magnetoreception: the birds were well oriented in their migratory direction under blue light of 443 nm and under green light of 565 nm, whereas they failed to orient under red light with a peak at 630 nm.
Given the importance of these experiments in understanding magnetoreception, it is desirable to test additional species under various wavelengths of light, in particular species with different migratory behaviors. Because the two species tested previously were short-distance migrants, we chose the Garden Warbler (Sylvia borin), which is a nocturnal migrant that breeds in northern and central Europe and migrates long distances to wintering sites in Africa south of the Sahara. In addition to blue, green, and red light, we used yellow-orange light with a peak wavelength of 590 nrn to narrow the range of wavelengths at which magnetic orientation is no longer possible.
Methods.-Twelve young Garden Warblers hatched near Radolfzell at Lake Constance between 3 and 10 June 1994 were taken from their nests and moved to our Frankfurt laboratory on 17 June 1994, where they were hand raised and kept in closed rooms in a magnetic field that was close to normal. The photoperiod simulated the natural one, with a daylight level of 550 lux produced bV fluorescent lamps.
The test lights included blue light with a peak at 443 nm and lambda/2 at 402 and 472 nm, produced by a cool beam lamp and a glass filter, and green, orangeyellow, and red lights produced by sets of 25 LEDs each (see W. Wiltschko et al. 1993). Green had a peak at 565 nm, with lambda/2 at 550 and 583 nm. The yellow LEDs were somewhat variable with peaks between 584 and 592 nm, but mostly at 590 nm and lambda/2 at 572 and 609 nm (see W. Wiltschko and Wiltschko 1999: fig. 1). The peaks of the red LEDs lay between 626 and 635 nm; most of them had their peak at 630 and 631 nm and lambda/2 at about 613 and 656 nm. Blue, green, and red were the same lights that were used in previous studies (e.g. W. Wiltschko et al. 1993, Munro et al. 1997, R. Wiltschko and Wiltschko 1998). The lights passed through a set of three diffusers before they reached the birds in the test cages (see below); they were adjusted to be of equal quantal flux of 8.7 X 10^sup 15^ quanta per s and m^sup 2^ (see W. Wiltschko et al. 1993). Tests under "white" tight provided by a 15-watt incandescent light bulb served as a control. Each bird was tested under the various lights in a pseudorandom sequence. One bird escaped after the first three tests, and another produced no usable recordings under yellow light and only one under red light.
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