Self-correcting Sun Compass, Spherical Geometry and Cue-transfers Predict Naïve Migratory Performance

Migratory orientation of many animals is inheritable, enabling naïve migrants to reach remote destinations independently following stepwise (often, nightly) geomagnetic or celestial cues. Which if any such “compass courses” can explain narrow-front trans-continental routes remains unresolved, and evident error-corrections by naïve migrants remain unexplained. We assessed robustness to errors among airborne compass courses and quantified inaugural migration performance globally, accounting for cue transfers (e.g., sun to star compass), in-flight cue maintenance, and previously-overlooked spherical-geometry (longitude) effects. We found (i) sun-compass courses partially self-correct, making them most robust between flight-steps, (ii) within nocturnal flight-steps, geomagnetic or star-compass headings outperform cue-transferred sun-compass steps, (iii) across diverse airborne migration routes, the relative favourability of sun-compass over other courses increases with increasing goal-area, required flight steps and a spherical-geometry factor. Our results can explain enhanced naïve migrant performance, observed diversity in compass-cue hierarchies, and sun-compass orientation being key to many long-distance inaugural migrations.

Ontogeny of the star compass in birds: pied flycatchers (Ficedula hypoleuca) can establish the star compass in spring

ABSTRACTThe star compass of birds, like the sun compass, is not innate. To possess either of them, birds have to observe the rotating sky and determine its centre of rotation (in the case of the star compass) or the sun's movement (for the sun compass). Young birds are believed to learn how to use the star compass before their first migration, even though the evidence of this is lacking. Here, we tested whether hand-raised Pied flycatchers (Ficedula hypoleuca) that had not established the star compass prior to their first autumn migration can gain it later in their ontogeny, in spring. We also attempted to examine whether the observation of diurnal celestial cues (the sun and polarized light) prior to autumn migration would affect the process of star compass learning in spring. When tested in the vertical magnetic field under the natural starry sky, the group of birds that observed the stars in spring as the first celestial cues were able to choose the migratory direction. In contrast, the birds that had never seen the stars were not able to use the nightly celestial cues in the vertical magnetic field. However, birds that had seen the daytime celestial cues till autumn and the stars at spring were disoriented, although this might be due to the small sample size. Our data suggest the possibility that the star compass may be learned in spring and emphasize the necessity for further research into the interaction of celestial compasses.

Migrating songbirds tested in computer-controlled Emlen funnels use stellar cues for a time-independent compass

SUMMARY This paper investigates how young pied flycatchers, Ficedula hypoleuca, and blackcaps, Sylvia atricapilla, interpret and use celestial cues. In order to record these data, we developed a computer-controlled version of the Emlen funnel, which enabled us to make detailed temporal analyses. First, we showed that the birds use a star compass. Then, we tested the birds under a stationary planetarium sky, which simulated the star pattern of the local sky at 02:35 h for 11 consecutive hours of the night, and compared the birds’ directional choices as a function of time with the predictions from five alternative stellar orientation hypotheses. The results supported the hypothesis suggesting that birds use a time-independent star compass based on learned geometrical star configurations to pinpoint the rotational point of the starry sky (north). In contrast, neither hypotheses suggesting that birds use the stars for establishing their global position and then perform true star navigation nor those suggesting the use of a time-compensated star compass were supported.