Rustling.
Up ahead, about a hundred yards away, the trail branched to the right. Like all others in these hills, the path was red. African-earth red. Cloying and sucking after the rains, choking with dust in the dry season. Damp and springy in between times. Like now: a vibrant leaf-mulch bounce. Except suddenly it wasn't red at all. The path was becoming dark, a roiling mass of blackness pressing towards him.
Fast.
A shoal of black energy driving everything before it. The hairs on his arms rose. As it approached he could make out insects pouring from the front of the column, trying to escape as larger animals might from a forest fire. It was now obvious why the birds had been present, taking advantage of a vast meal being produced by the advancing force. He saw an animal with quills, a small porcupine perhaps, struggling to get out of the way. Within a few seconds the whole body was overtaken by the column. Soon nothing was visible except the tip of a spine.
Ants.