On a hike, have you ever been on the opposite side of a fast-moving stream from a friend? It's really hard to hear them. Wind, rocks or trees also make communication difficult for people and birds. One ornithologist studied the effects of different habitats on birds' communication. Birds with high-pitched calls find it more difficult to contact a companion in the forest. That is because the thick vegetation absorbs and scatters higher sounds more than lower ones. The same principle would apply to birds who live in low, dense vegetation compared to those who live in open habitats. The low dense vegetation absorbs and scatters high pitches of birds. The open land will allow birds with high-pitched voices to be heard more easily.