Why were British casualties so high on their retreat back into Boston?

British casualties were high during their retreat back into Boston primarily due to the effective tactics employed by American militia forces, who utilized guerrilla warfare and took advantage of the terrain. As the British retreated after the Battles of Lexington and Concord, they were subjected to continuous harassment and sniper fire from colonists hiding behind trees, walls, and other cover. The British soldiers, marching in formation and heavily laden with equipment, were vulnerable to this kind of ambush, leading to significant losses. Additionally, the lack of proper reconnaissance and underestimating the determination of the colonial forces contributed to the high casualty rates.