Response of a free-flying songbird to an experimental shift of the light polarization pattern around sunset

Abstract

SUMMARYThe magnetic field, the sun, the stars and the polarization pattern of visible light during twilight are important cues for orientation in nocturnally migrating songbirds. As these cues change with time and location on Earth, the polarization pattern was put forward as a likely key reference system calibrating the other compass systems. Whether this applies generally to migratory birds is, however, controversially discussed. We used an experimental approach in free-flying birds to study the role of polarization for their departure direction in autumn. Experimental birds experienced a 90deg shift of the band of maximum polarization during sunset, whereas control birds experienced the polarization pattern as under natural conditions. Full view of the sunset cues near the horizon was provided during the cue conflict exposure. Here we show that both the experimental and the control birds being released after nautical twilight departed consistently towards south-southeast. Radiotelemetry allowed tracking of the first 15km of the birdsʼ outward journey, thus the intrinsic migration direction as chosen by the birds was measured. We found no recalibration of the magnetic compass after pre-exposure to a cue conflict between the natural magnetic field and the artificially shifted polarization pattern at sunset. The lacking difference in the departure direction of both groups may suggest that birds did not recalibrate any of the compass systems during the experiment. As free-flying migrants can use all available orientation cues after release, it remains unknown whether our birds might have used the magnetic and/or star compass to determine their departure direction.

Polarized skylight does not calibrate the compass system of a migratory bat

“…Further experiments are necessary and it remains to be tested if P. nathusii depend on polarized light for orientation during the nonmigratory period. It should also be noted that in birds, results are inconsistent, with some data supporting the role of polarization as a primary calibration reference for the magnetic compass during migration [3,13], while others do not [9][10][11][12]. A review of the published literature indicated that methodological differences, namely, access to a view of the horizon at sunset, may explain these differences [24].…”

“…In migrating passerine birds, these cues appear to be used hierarchically, in which one provides an absolute geographical reference that calibrates others, which are then used as a compass to take up the desired direction of orientation [7]. Disagreement persists as to whether the magnetic field [8][9][10][11][12], or polarized light [3,13] provides this absolute geographical reference.…”

Stopover optimization in a long-distance migrant: the role of fuel load and nocturnal take-off time in Alaskan northern wheatears (Oenanthe oenanthe)

“…Alaskan wheatears took off within a small time window shortly after sunset, i.e., before the end of nautical twilight when skylight polarization pattern may be used for calibrating the compass systems [19,82-84]. Our birds departed at higher sun elevations and earlier in relation to the proportion of the night as compared to other studies (GLM with binomial error distribution: P < 0.0001; Figure 5 and Additional file 9; [14-18]).…”

Migratory blackcaps tested in Emlen funnels can orient at 85 but not at 88 degrees magnetic inclination

“…One hour before, the birds were placed outdoors in wooden transportation cages fitted with 7 cm diameter mesh-covered peepholes. This enabled the birds to potentially calibrate their magnetic compass from twilight cues (Cochran et al, 2004;Muheim et al, 2006; but see Chernetsov et al, 2011;Schmaljohann et al, 2013). Directly thereafter, the birds were placed in modified Emlen funnels (Emlen and Emlen, 1966), made of aluminium (35 cm diameter, 15 cm high, walls 45 deg inclined).…”