Fig. 2: Time-compensated sun compass (TCSC) headings and self-correction.

A TCSC migrant clock-synchronized to local conditions (top) maintains its preferred direction (solid black arrow) by adjusting its heading relative to the daily clockwise rotation in sun azimuth (here at sunset, solid red arrow). Following an error-free flight-step (lower left), the longitudinally displaced migrant will be clock-shifted relative to local time. Here, the clock-accelerated shift results in an over-compensation to proximate sun azimuth, i.e., counter-clockwise TCSC offset (dashed red arrow), hence increasingly Southward heading (dashed black arrow). If the migrant’s initial heading is imprecise (dot-dashed grey line), its longitudinal displacement will lead to a contrasting clock-shift. Here, the clock-lagged migrant (lower right) will under-compensate relative to proximate sun azimuth, resulting in a clockwise offset (dashed red arrow) and hence a self-corrected heading (dashed black line). Between-step shifts in proximate sunset azimuth become biologically relevant at multi-day and multi-step scales (Fig. 7). Images from www.dreamstime.com and www.flaticon.com.