Black-crowned night herons (Nycticorax nycticorax) are nocturnal wading marsh birds in the family Ardeidae. I studied the departure of black-crowned night herons from their roost in northwestern California, and light intensity, measured in Lux, from September to November 2016. There was a strong, negative correlation between light intensity and roost departure in this species. 976 individuals were observed, in which over 94% did not depart from the roost until ≤ 0.3 Lux. The optimal roost departure light intensity was 0.0 Lux. The first heron left the roost 22 minutes after sunset, on average. Finding that light intensity heavily influences black-crowned night heron roost departure behavior could perhaps propel further more intensive studies to focus on the spectral sensitivity and visual abilities of this species. Management implications include advising developers against constructing light fixtures that give off high intensity near roost sites, to avoid affecting the behavior of black-crowned night heron roost departure flight patterns.