Not all songbirds calibrate their magnetic compass from twilight cues: a telemetry study

Abstract

SUMMARY Migratory birds are able to use the sun and associated polarised light patterns, stellar cues and the geomagnetic field for orientation. No general agreement has been reached regarding the hierarchy of orientation cues. Recent data from naturally migrating North American Catharus thrushes suggests that they calibrate geomagnetic information daily from twilight cues. Similar results have been shown in caged birds in a few studies but not confirmed in others. We report that free-flying European migrants, song thrushes Turdus philomelos, released after pre-exposure to a horizontally rotated magnetic field, do not recalibrate their magnetic compass from solar cues, but rather show a simple domination of either the magnetic or the stellar compass. We suggest that different songbird species possess different hierarchies of orientation cues, depending on the geographic and ecological challenges met by the migrants.

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

“…Our study is in agreement with a number of recent studies that did not find an effect of cue conflict exposures on the orientation of the corresponding study species (Wiltschko et al, 2008a;Wiltschko et al, 2008b;Gaggini et al, 2010;Chernetsov et al, 2011; but see Muheim et al, 2008). Similar to our study, the authors reported no compass calibration, but instead a simple domination of magnetic or stellar cues over the sunset cues (Chernetsov et al, 2011).…”

“…The birds were released from the cages when the elevation of the sun was lower than 13.7deg below the horizon, i.e. after nautical twilight, so that stars, when not cloudy, were visible and the skylight polarization pattern had disappeared (see Cochran et al, 2004;Chernetsov et al, 2011). The birds were then held in the open hand, so that they could decide by themselves when and into which direction to depart.…”

“…The polarization pattern during twilight has been proposed as a candidate cue for the key reference system against which the other compass systems are calibrated (Muheim et al, 2006b;Muheim et al, 2007;Muheim et al, 2008). However, it remains unclear how universal this calibration strategy is (Wiltschko et al, 2008a;Wiltschko et al, 2008b;Gaggini et al, 2010;Chernetsov et al, 2011).…”

“…To our knowledge, only three studies (Sandberg et al, 2000;Cochran et al, 2004;Chernetsov et al, 2011) have monitored freeflying birds to test the orientation of birds, i.e. to track the departure directions after exposure to a cue conflict over the first part of their journey.…”

“…In addition, Emlen and Demong (Emlen and Demong, 1978) found that birds released at migratory altitude under clear sky conditions flew in a straight line towards the seasonally appropriate migration direction, whereas the concentration of birds' tracks was reduced under an overcast situation. The major advantages of tracking free-flying birds with Emlen funnel experiments are that: (1) the drawbacks of the Emlen funnel are overcome, (2) free-flying birds can phenotypically respond to the environmental cues after the experiment during the departure, (3) the intrinsic migration direction as chosen by the birds is detected, and (4) they move through space (Emlen and Emlen, 1966;Nievergelt et al, 1999;Nievergelt and Liechti, 2000;Chernetsov et al, 2011).…”