Final answer:
Historians know a lot about the Pilgrims' first days in America through William Bradford's journal and the oral history passed down by Native Americans.
Step-by-step explanation:
Historians know a lot about the Pilgrims' first days in America through various sources. One important source is William Bradford's journal called "Of Plymouth Plantation." In this journal, Bradford recorded the challenges and experiences of the Pilgrims during their early days in America. Another source of information is the oral history passed down by Native Americans, who interacted with the Pilgrims and shared their perspectives.