Why do mulberry trees in grass houses make houses for pigeons?

Pigeons have no good shelter. Straw House is a novel written by writer Cao Wenxuan. In this novel, Sang Sang has a good place to live, but Sang Sang's pigeons don't have such a good place to live, so Sang Sang is determined to get a good place for the pigeons and build them a house with her cupboard. The work tells the unforgettable six-year primary school life of the boy Sang Sang.