A waterhole can exist for a hundred years. But every year it goes through a profound change. The arc spans from decline to rebirth. The water is slowly evaporating and the level of the waterhole in Kenya's Tsavo National Park is beginning to fall. When the seeds of the grasses ripen in the surrounding bushland, a true force of nature appears at the waterhole. The sky darkens as the blood-billed weavers arrive at the start of the dry season. They are considered the most common birds in the world and their flocks consist of tens of thousands of animals. They come from the north, where they raised their young in huge breeding colonies.