“…This review is focused on opioids and the reinforcement of flocking behavior; however, a major focus of current research on group living, including flocking in birds, is on neuropeptides in the vasopressin/oxytocin, “nonapeptide” family (homologs to avian vasotocin/mesotocin) (Goodson, 2008, 2012; Goodson et al, 2009b, 2012a; Goodson and Kingsbury, 2011; Ondrasek et al, 2018; Beery, 2019). Opioids have long been known to alter nonapeptide release (Bicknell and Leng, 1982; Brown et al, 2000), and nonapeptides influence opioid activity to modulate behavioral responses to social and painful stimuli (including the pain of social separation) (Csiffary et al, 1992; Yang et al, 2007, 2011a,b; Moaddab et al, 2015; Amini-Khoei et al, 2017).…”