Later experiments, in which young birds viewed a planetarium sky rotating around the star Betelgeuse, found that the birds learned this star, and not Polaris, as their true North. These birds, and likely many others, are able to learn the pattern of stars surrounding the center of celestial rotation, allowing them to pinpoint the poleward region of the sky.
Learning to find the center of rotation is a convenient but robust strategy, that, unlike the Sun compass, does not rely on their internal clock. Jetlagged birds viewing the evening Sun are fooled into flying in the wrong direction, choosing their bearing as if the Sun were at its noontime azimuth (apparently ignoring its lower elevation). But those kept overnight in a planetarium projecting an earlier or later sky — a virtual ‘displacement’ East or West of thousands of kilometers — appear unphased by this longitudinal detour.
To adjust for movements of both the Sun and stars with any accuracy, migratory birds would require two internal clocks synchronized to solar time and sidereal time, respectively. But by learning to identify the center of rotation, they avoid this complication. It seems birds forgo sidereal time in favor of their more straightforward strategy.
A starry night in broad brushstrokes
While birds sometimes favor simplicity over sophistication, insects are true masters of minimalism. Their bodies are too small for brains or eyes as large as those carried by birds, so they make do with what they have. What’s more, when it comes to detecting patterns of stars, insects appear to have taken a wrong turn along their evolutionary path.
Unlike the eyes of humans and birds, in which a single lens focusses the light field onto the retina, insects evolved ‘compound eyes’ consisting of multiple facets. Each facet includes a single lens projecting onto a distinct group of light-sensors. This modular design is a good first step from a single light-sensing organ to an eye that detects spatial detail — instead of modifying the organ, just duplicate it to add more image ‘pixels.’
But compound eyes reach an awkward trade-off in dim light. Larger lenses collect more light, and good resolution requires many lenses, enlarging eyes to the limits of what an insect cannot afford. To compensate, most nocturnal insects have removed the tissue between facets, allowing multiple facets to project onto a single set of light-sensors: a ‘superposition’ compound eye. The broader beam reaching each sensor boosts sensitivity, but the optics still rarely bestow enough resolving power to distinguish individual stars. Instead, insects likely see a van Gogh-style starry night: smooth patches of light and dark across the sky. But that does not stop them from following the stars.
As the Sun sets over the South African savannah, nocturnal dung beetles emerge. Dung is an excellent source of water and nutrients in a harsh environment, so they compete with a myriad of dung-eating insects converging on each dung heap. To save themselves a portion of the feast, some beetles sculp themselves a ball and roll it away, escaping their competitors. To achieve this, they must roll their ball in a straight line. While their day-active counterparts use the Sun as their reference point, like the birds’ Sun compass, nocturnal dung beetles instead rely on the Milky Way.
