A New View on an Old Debate: Type of Cue-Conflict Manipulation and Availability of Stars Can Explain the Discrepancies between Cue-Calibration Experiments with Migratory Songbirds

Abstract

Migratory birds use multiple compass systems for orientation, including a magnetic, star and sun/polarized light compass. To keep these compasses in register, birds have to regularly update them with respect to a common reference. However, cue-conflict studies have revealed contradictory results on the compass hierarchy, favoring either celestial or magnetic compass cues as the primary calibration reference. Both the geomagnetic field and polarized light cues present at sunrise and sunset have been shown to play a role in compass cue integration, and evidence suggests that polarized light cues at sunrise and sunset may provide the primary calibration reference for the other compass systems. We tested whether migratory garden warblers recalibrated their compasses when they were exposed to the natural celestial cues at sunset in a shifted magnetic field, which are conditions that have been shown to be necessary for the use of a compass reference based on polarized light cues. We released the birds on the same evening under a starry sky and followed them by radio tracking. We found no evidence of compass recalibration, even though the birds had a full view of polarized light cues near the horizon at sunset during the cue-conflict exposure. Based on a meta-analysis of the available literature, we propose an extended unifying theory on compass cue hierarchy used by migratory birds to calibrate the different compasses. According to this scheme, birds recalibrate their magnetic compass by sunrise/sunset polarized light cues, provided they have access to the vertically aligned band of maximum polarization near the horizon and a view of landmarks. Once the stars appear in the sky, the birds then recalibrate the star compass with respect of the recalibrated magnetic compass. If sunrise and sunset information can be viewed from the same location, the birds average the information to get a true geographic reference. If polarized light information is not available near the horizon at sunrise or sunset, the birds temporarily transfer the previously calibrated magnetic compass information to the available celestial compasses. We conclude that the type of cue-conflict manipulation and the availability of stars can explain the discrepancies between studies.

Keywords: bird orientation; calibration; compass; magnetic field; migration; polarized light; stars; sun.

Figure 1

Orientation of garden warblers followed by radio-tracking in Falsterbo, Sweden. (A–C) Initial orientation of the first 10 min after release from the lighthouse receiver station for (A) birds released immediately after tagging during daytime, and (B,C) birds released at night after cue-conflict exposure under (B) the control condition and (C) the experimental condition. (D–F) Vanishing bearings of garden warblers (D) released immediately after tagging during daytime: (E) control and (F) experimental group.

Figure 2

(A) Mean variation in magnetic declination between 1900 and 2015 for the different continents. (B) Mean variation in magnetic inclination by migration strategy (short- to medium-distance migrants vs. long-distance migrants). For details, see Table S2.

Figure 3

Outcomes of recent cue-calibration experiments. Included are studies that exposed migratory birds to a cue conflict between the magnetic field (MF; blue arrows) and polarized light (PL; orange double arrow) cues at sunrise or sunset with view of the horizon. (A,B) MF artificially shifted relative to the natural celestial cues (exemplified by a MF shifted 90° clockwise relative to its natural alignment). (A) MF shift in the presence of natural PL cues at sunset in the absence of stars; (B) MF shift in the presence of natural PL cues at sunset and stars. (C,D) ±90° shift of an artificial PL pattern relative to the natural MF at sunrise or sunset. (C), Shift of PL pattern in the absence of stars; (D), Shift of PL pattern in the presence of stars. Results of cue-conflict exposures as measured in orientation experiments in funnels (magnetic compass orientation; center column) or in release experiments outdoors at night (right column). The expected orientation is shown for birds tested for magnetic compass (large blue arrow or double arrow) and/or for birds released outdoors at night with access to the magnetic compass and star compass (large yellow arrow). Large double arrows are used when the direction of expected shift is axial (after exposure to a shift in the polarization axis). See Supplemental References and Table S3 for detailed methods on cue-conflict procedures used in the different studies.

Figure 4

Compass hierarchy and calibration strategy used by migratory birds during exposure to conflicting information between celestial and magnetic compass cues. See main text for further explanation.