Billions of birds are on the march.

Many species chase the sunshine, and therefore food sources, according to the apparent motion of the sun, north and south of the equator.

The arrival and departure of bird species are often symbolic of a change in season, a hint that better weather may be around the corner, or that winter is on the way.

At 03:39 GMT on Friday, the Sun passed over the celestial equator. Around this day, everywhere on the planet gets equal hours of darkness and daylight.

In Poland, that event coincides with the arrival of thousands of storks. A quarter of the world’s white stork population - around 50,000 birds - choose Poland as their nesting ground.

The Polish people, at least those in the countryside, consider the bird to be a sign of luck and that they protect houses from fire and lightning. Many people construct frames on the roofs of their houses to help the birds with their nest building.

In Germanic and Greek culture the stork is seen as a symbol of a mother’s love.

The storks usually stay until September, heading southwards around the time of the autumn equinox, moving towards the Savanna of Kenya and Uganda, southwards to the Cape Province of South Africa.