Prodynorphin and kappa opioid receptor mRNA expression in the brain relates to social status and behavior in male European starlings

•Dominant male starlings had lower OPRK1 mRNA in VTA and mPOA than subordinates.•OPRK1 in VTA related negatively to sexual/agonistic behaviors and gonad volume.•PDYN in mPOA related negatively to nesting behaviors and positively to gonad volume.•Area X PDYN related positively to singing; OPRK1 related positively to gonad volume.•Kappa receptors/dynorphin relate to individual differences in sociosexual behaviors.