Complex sentence: However, a common cause of death for these bison continues to be hunting by human beings, when many of the bison leave the park during the winter, heading up into Montana, especially through the Lamar Valley.
Split into simpler sentences: However, a common cause of death for these bison continues to be hunting by human beings. This occurs when many of the bison leave the park during the winter, heading up into Montana, especially through the Lamar Valley.