“…For the predictor variables, we included our two behavioral measures (number of songs and bouts of non-vocal behaviors), and expression in Area X for each neural marker (CB 1 , FABP7, FABP5, FAAH, DAGLα). While, Area X may play a role in reinforcement related to vocal learning (e.g., Hoffman et al, 2016), it does not have a definitive role in reward processes comparable to other brain regions (e.g., PAG, POM, VTA; thus we would not predict a relationship between CPP and Area X mRNA expression, a prediction we confirmed below). However, Area X is critical for song learning (Sohrabji et al, 1990; Scharff and Nottebohm, 1991) and variability in song production (Leblois et al, 2010; Leblois and Perkel, 2012); therefore, we also conducted separate correlation analyses for each neural marker to examine the relationship between the number of songs produced and mRNA expression in Area X.…”