Birds and Stars

The sheer scale of bird migrations, considering their small size and the vast distances they fly, is a testament to their extraordinary navigational abilities and endurance

*Birds setting off at sunset for their night of flying on their long migration. Image credit South Methodist University*

Bird migrations are incredible feats of nature, with billions of birds flying vast distances across the Earth each year. Driven by the need to find suitable breeding, feeding, and wintering grounds, some of the journeys span entire continents and oceans.

Birds utilise a variety of navigation methods. These include navigating by the sun (and polarised light when the sun is obscured); using landmarks, such as lakes, rivers and mountains, using the Earth’s magnetic field (unlike humans, birds can detect the magnetic field generated by Earth’s molten core and use it to determine their position and direction), and even using smell landmarks. Birds don’t rely on just one navigational method; they integrate various methods to navigate effectively.

However, the majority of migratory birds fly at night and thus their primary navigation method is using the stars as their compass. Nighttime flying has many advantages for birds. Free of daytime thermals, the atmosphere is more stable, making it easier to maintain a steady course, especially for the small birds. Cooler night temperatures also help keep the birds from overheating. And flying under cover of darkness can be a lifesaver for those birds that frequently wind up on the menu of hawks and falcons and other birds of prey.

This tiny little bird, a Common Reed Warbler, is a migratory bird that breeds in Europe and western Asia and winters in western, eastern, and southern Africa; primarily flying at night as it covers thousands of kilometres annually. Image credit Miguel Rouco

How do birds navigate by the stars?

In 1967, ornithologist Stephen Emlen published his study of how Indigo Buntings (a small seed-eating bird that migrates from southern Canada to northern South America and back again each year) learn to navigate. He raised baby Indigo Buntings in a planetarium.

In the northern hemisphere the bright star Polaris (often called the North Star) always points towards the north. It’s located very close to the Earth’s north celestial pole, which is an extension of Earth’s rotational axis into space. Because of this, as the Earth rotates, Polaris appears to stay in the same position in the sky, while other stars seem to circle around it.

However, in the planetarium, instead of circling Polaris, the Pole Star, Emlen’s artificial night sky rotated about Betelgeuse, Orion’s gorgeous red star.

When his baby Indigo Buntings grew up, and the time for migration approached, they felt impelled to fly away from Betelgeuse. They behaved as if they had formed a concept of “south,” as the direction opposite the place that Betelgeuse occupied in the sky. In other words, its north-south axis determination was completely wrong.

Emlen surmised that outdoors in nature, baby Indigo Buntings in their nests spend their nights studying the starry sky. They notice that the stars close to the Pole Star move the slowest, tracing the path of a small circle around that star. From then on, the night sky is map and compass for them. And it will guide them on their migrations. (Who can’t love imagining a nest full of baby birds peeping out from beneath their mother’s warm feathers to imprint the pattern of stars and their apparent rotation around the celestial pole?)

A baby Indigo Bunting in its nest. The little chap must possess an incredibly complex brain in that little head in order to achieve the remarkable feat of imprinting stars and their rotation in the sky. Image credit Carin Duke

The northern hemisphere has bright Polaris… we have very faint Sigma Octantis

In the southern hemisphere we don’t have a bright star like Polaris at our south celestial pole. We have Sigma Octantis which is close to the south celestial pole but extremely difficult to pick out as it is only slightly brighter than 5th magnitude, making it barely visible to the naked eye.

But we do have the bright stars of the Southern Cross (Crux), the southern hemisphere’s most famous constellation!

*Our beautiful and famous constellation, the Southern Cross. Image credit ESO/S. Brunier*

It is circumpolar, meaning it never sets below the horizon, from a latitude of 35 degrees south and all latitudes further south. This means that at 35°S and any point south of that, the Southern Cross is visible throughout the night, all year round. (North of 35°S it can be seen throughout the year at some point during the night.)

The Southern Cross doesn’t point to the South Celestial Pole like Polaris points to the north. Instead, by drawing a line through the long axis of the cross, extending it by about 4.5 times the length between the two main stars, then dropping a line straight down from that point to the horizon will indicate the direction of due south.

*Image credit image Laurent Fournet & Jean-Philippe Olive*

In the long-exposure photograph you can see the star trails, due to the Earth’s rotation, forming concentric circles around the south celestial pole. In the absence of a bright star like Polaris directly above the South Pole, several circumpolar constellations are conspicuously bright, the most prominent being the Southern Cross whose striking star trails are at right centre (along with a couple of Carina’s brightest stars).

Thus, the stars of this most beautiful little constellation revolving around the south celestial pole have been used for navigational purposes for centuries by humans… and for uncounted eons of time by migrating birds, helping them to orient themselves, especially for north-south axis determination.

Helping our feathered friends on their perilous journeys

Whenever you see the Southern Cross in our skies (or any stars for that matter), think of those tiny little brown warblers, flying thousands of kilometres at night and relying on the stars for navigation. Artificial light pollution can disorient them, leading to exhaustion, predation, and mass collisions with structures.

It is not difficult to minimise light pollution’s impact on migrating birds (and indeed all nocturnal animals). Reduce unnecessary nighttime lighting – while security lighting is important, over-illumination of gardens, even in high-crime areas, is unnecessary and obscures the night sky and has a major impact on wildlife.

Good lighting design focuses on using the right amount of light – in the right places – at the right times, to enhance safety without causing light pollution. Use shielded, downward-facing lights, motion sensors, and timers to control light usage. Employ warm-toned, yellow-spectrum lights.

It’s this easy…

Encourage your friends and family to do the same, spreading awareness about these simple yet impactful ways to protect our migrating feathered friends.