Why did the animals sing beasts of England slowly and mournfully as the gathered at the knoll?

The animals sang "Beasts of England" slowly and mournfully at the knoll as a reflection of their collective sorrow and longing for a better future. The song, which originally symbolized hope and unity, took on a somber tone due to the loss of their leader, Boxer, and the betrayal of their ideals. This melancholic rendition served as a poignant reminder of their struggles and the fading dreams of the Animal Farm rebellion.