He described the shape of a bird with his finger in the sand.

described 是动词 describe 的过去式，describe 由前缀 de-（表示"向下、完全"）和词根 scribe（"写"）组成，字面意思是"详细写下"，引申为"描述、描绘"的动作。在句中指"用手指在沙地上描绘出鸟的形状"。