The Black Sun in Jutland, Denmark

Come to the Danish marshlands as the sun is beginning its daily retreat, for it's now that more than a million migrating starlings begin to think about bedtime in both the spring and the fall. After a day of grazing on spindly Daddy Long Legs, insect larvae and other creepy crawlies, the flock of speckled black birds starts a synchronized aerial dance as they search for the perfect place to nap. The to-ing and fro-ing flock dominates the horizon, their wave-like dance obscuring the faint light of the fading sun, giving the formation its eerie name.

The Black Sun is a symphony in motion, beginning slowly with smaller flocks in front then growing into a crescendo of larger groups, before slowing down then swelling up again spontaneously. On evenings when buzzards, falcons and other birds of prey prowl, the starlings' patterns in the sky are more dramatic, and the collective rustle of millions of wings resembles the sound of a gushing waterfall.

The dancing starlings are en route from Norway, Sweden and Finland to their breeding grounds in France, Britain, Belgium and the Netherlands. Once the cunning birds have descended to a reed-covered spot, they sometimes trick their predators by swapping sleeping locations.

Other times they're forced to move when the weight of hundreds of birds breaks the reeds. The unlucky ones on top of a reed face more danger than those on the bottom, but they always sleep en masse after their ballet in the sky. That's team spirit for you.

The precise whereabouts of the twilight ritual is impossible to predict. Tønder tourist board recommends visiting with a nature guide or on a group tour to increase your chances of seeing the spectacle.

When: Mid-March-mid April and mid- September-mid-October. The spectacle starts around sunset and lasts about 20 minutes.

Strange but true: Starlings collect up to 25 herb varieties to fight off mites, fleas and other bugs.

Insider tips: Refrain from flash photography, as it disturbs the birds, and bring binoculars and a mat to sit on (don't stand as the starlings will be frightened by your silhouette against the sky). Wrap up warm, especially in spring, but don't wear bright clothes. Wear rubber boots in the sludgy marshes.

Get there: It's a 15-minute to 1-hour drive from Tønder train station, depending on where the starlings are. The nearest airport is Billund, about 110km (68 miles) from Tønder. Another option is Hamburg, Germany, about 220km (137 miles) from Tønder.

Price: Free.

Information: Tønder tourist board (tel. +45 7475 5130; www.visittonder.dk)